Respiration in Plants

Topic: Cellular Respiration; Subtopic: Electron Transport Chain (ETC)

Keyword Definitions:

• Electron Transport Chain (ETC): A series of protein complexes in mitochondria that transfer electrons to produce ATP.

• Complex II: Also known as succinate dehydrogenase, it transfers electrons from FADH2 to ubiquinone.

• ATP: Adenosine triphosphate, the main energy currency of the cell.

• Oxidative Phosphorylation: The process by which ATP is produced using energy from electron transport.

Lead Question – 2025

The complex II of mitochondrial electron transport chain is also known as:

1. Cytochrome bc1
2. Succinate dehydrogenase
3. Cytochrome c oxidase
4. NADH dehydrogenase

Explanation: Complex II, known as succinate dehydrogenase, is part of both the citric acid cycle and the electron transport chain. It oxidizes succinate to fumarate and transfers electrons to ubiquinone, forming ubiquinol. Unlike other complexes, it does not pump protons but contributes indirectly to the proton gradient used for ATP synthesis. Hence, the correct answer is succinate dehydrogenase.

1. Which of the following complexes in mitochondria receives electrons from NADH?

1. Complex I
2. Complex II
3. Complex III
4. Complex IV

Complex I (NADH dehydrogenase) accepts electrons from NADH and transfers them to ubiquinone. It also pumps protons across the inner mitochondrial membrane, contributing to the proton gradient. This process is essential for oxidative phosphorylation and ATP generation, marking Complex I as the entry point for NADH electrons.

2. Which molecule acts as the final electron acceptor in the mitochondrial electron transport chain?

1. Oxygen
2. Water
3. NAD+
4. FAD

Oxygen acts as the final electron acceptor in the electron transport chain. It combines with electrons and protons to form water. This step ensures the continuous flow of electrons through the chain. The reduction of oxygen to water is essential for sustaining oxidative phosphorylation and ATP synthesis in aerobic organisms.

3. In mitochondria, which of the following complexes directly participates in both the TCA cycle and the ETC?

1. Complex I
2. Complex II
3. Complex III
4. Complex IV

Complex II (succinate dehydrogenase) is unique as it functions in both the citric acid cycle and the electron transport chain. It catalyzes the oxidation of succinate to fumarate while transferring electrons to FAD, producing FADH2. These electrons then enter the ETC via ubiquinone, linking both processes directly.

4. The proton gradient required for ATP synthesis is formed across which membrane of mitochondria?

1. Outer membrane
2. Inner membrane
3. Matrix membrane
4. Cristae lumen

The proton gradient is established across the inner mitochondrial membrane. As electrons move through the ETC, complexes I, III, and IV pump protons into the intermembrane space. This electrochemical gradient powers ATP synthase, driving phosphorylation of ADP to ATP. The inner membrane’s impermeability maintains this essential gradient.

5. Which coenzyme carries electrons from Complex I and II to Complex III?

1. Coenzyme A
2. Ubiquinone (CoQ)
3. Cytochrome c
4. FAD

Ubiquinone (CoQ) transfers electrons from both Complex I and Complex II to Complex III. It acts as a mobile electron carrier within the inner mitochondrial membrane. Upon accepting electrons, it becomes reduced to ubiquinol (CoQH2), which then donates electrons to the cytochrome bc1 complex, facilitating the proton gradient formation.

6. Which of the following complexes pumps no protons during electron transport?

1. Complex I
2. Complex II
3. Complex III
4. Complex IV

Complex II does not pump protons across the inner mitochondrial membrane. It transfers electrons from FADH2 to ubiquinone but does not contribute directly to the proton gradient. This makes ATP yield from FADH2 lower than that from NADH, which donates electrons via Complex I that pumps protons efficiently.

7. (Assertion-Reason Type)

Assertion (A): Complex IV transfers electrons to oxygen forming water.
Reason (R): Oxygen acts as the terminal electron acceptor in the ETC.

1. Both (A) and (R) are true and (R) explains (A).
2. Both (A) and (R) are true but (R) does not explain (A).
3. (A) is true but (R) is false.
4. (A) is false but (R) is true.

Both (A) and (R) are true, and (R) correctly explains (A). Complex IV (cytochrome c oxidase) facilitates electron transfer to molecular oxygen, reducing it to water. This reaction is the final step in the ETC and ensures the continuous electron flow essential for ATP formation and maintenance of the proton gradient.

8. (Matching Type)

Match the following complexes with their functions:

List I — Complex
A. Complex I
B. Complex II
C. Complex III
D. Complex IV

List II — Function
1. Transfers electrons from FADH2
2. Transfers electrons to oxygen
3. Pumps protons and transfers electrons from ubiquinol to cytochrome c
4. Accepts electrons from NADH

Options:
A-4, B-1, C-3, D-2
A-1, B-2, C-4, D-3
A-4, B-2, C-3, D-1
A-3, B-4, C-2, D-1

The correct matching is A-4, B-1, C-3, D-2. Complex I receives electrons from NADH; Complex II transfers from FADH2; Complex III pumps protons and passes electrons from ubiquinol to cytochrome c; Complex IV transfers electrons to oxygen, forming water. Together, these maintain the flow of electrons and energy conversion.

9. (Fill in the Blanks)

ATP synthesis using energy from electron transport is called ___________.

1. Photophosphorylation
2. Substrate-level phosphorylation
3. Oxidative phosphorylation
4. Chemiosmosis

ATP formed through the electron transport chain process is termed oxidative phosphorylation. It occurs as electrons move through complexes I–IV, creating a proton gradient that drives ATP synthase. This mechanism efficiently converts energy from reduced coenzymes NADH and FADH2 into usable cellular energy in the form of ATP.

10. (Choose the Correct Statements)

Statement I: Complex II contributes to proton pumping across the inner mitochondrial membrane.
Statement II: Complex II transfers electrons from FADH2 to ubiquinone.

1. Only Statement I is correct.
2. Only Statement II is correct.
3. Both I and II are correct.
4. Both are incorrect.

Only Statement II is correct. Complex II, also called succinate dehydrogenase, oxidizes succinate to fumarate and transfers electrons to ubiquinone, forming ubiquinol. However, unlike complexes I, III, and IV, it does not pump protons across the inner mitochondrial membrane, so it contributes indirectly to the proton gradient.

Topic: Aerobic and Anaerobic Respiration; Subtopic: Oxidation-Reduction Reactions and Phosphorylation Mechanisms

Keyword Definitions:
• Oxidation-Reduction: Chemical reaction involving transfer of electrons between two substances.
• Phosphorylation: The process of adding a phosphate group to ADP to form ATP.
• Hydrogen Acceptor: A molecule that accepts electrons and hydrogen during redox reactions.
• Proton Gradient: Difference in proton concentration across a membrane driving ATP synthesis.
• Mitochondria: Organelle responsible for energy production through cellular respiration.

Lead Question – 2024 (Jhajjhar)

Which of the following statements are correct about respiration?
A. Energy of oxidation-reduction is utilised for phosphorylation
B. Oxygen acts as the final hydrogen acceptor
C. The photo-oxidative energy is utilised for production of proton gradient required for phosphorylation
D. The role of oxygen is limited to the terminal stage of the respiration process
E. Protons cross the outer membrane of mitochondria through the channel formed by an integral membrane protein complex

1. A, B, C, E only
2. A, B, D only
3. B, C, D, E only
4. A, C, D only

Explanation:
In cellular respiration, energy released from oxidation-reduction reactions drives phosphorylation to produce ATP. Oxygen serves as the final electron and hydrogen acceptor, forming water at the terminal stage. Statement A, B, and D are correct, as oxygen’s role is crucial in oxidative phosphorylation and occurs specifically during the electron transport chain. (Answer: 2)

1. Which organelle is the site of Krebs cycle?
1. Nucleus
2. Cytoplasm
3. Mitochondria
4. Ribosome

Explanation:
The Krebs cycle occurs in the mitochondrial matrix, where acetyl-CoA is oxidized to CO₂ and high-energy molecules NADH and FADH₂ are produced. These carry electrons to the electron transport chain, enabling ATP synthesis through oxidative phosphorylation. Hence, the correct answer is mitochondria. (Answer: 3)

2. Which of the following is produced during glycolysis?
1. NADPH
2. NADH
3. FADH₂
4. Acetyl-CoA

Explanation:
During glycolysis, glucose is broken down into two molecules of pyruvate with the net gain of 2 ATP and 2 NADH molecules. NADH carries high-energy electrons to the mitochondria for oxidative phosphorylation. Thus, the correct product formed during glycolysis is NADH. (Answer: 2)

3. In anaerobic respiration, pyruvate is converted into:
1. Lactic acid or ethanol
2. Acetyl-CoA
3. Glucose
4. Succinic acid

Explanation:
In the absence of oxygen, pyruvate undergoes fermentation to form lactic acid in animals or ethanol and CO₂ in yeast. This process regenerates NAD⁺ required for glycolysis to continue producing ATP under anaerobic conditions. (Answer: 1)

4. How many molecules of ATP are produced by complete oxidation of one glucose molecule?
1. 12
2. 24
3. 30–32
4. 38–40

Explanation:
Complete aerobic oxidation of one glucose molecule through glycolysis, Krebs cycle, and oxidative phosphorylation yields about 30–32 ATP molecules, depending on the cell type and shuttle system used. This represents the maximum energy efficiency of aerobic respiration. (Answer: 3)

5. The electron transport chain is located in:
1. Cytoplasm
2. Inner mitochondrial membrane
3. Nucleus
4. Outer mitochondrial membrane

Explanation:
The electron transport chain is embedded in the inner mitochondrial membrane. It transfers electrons from NADH and FADH₂ to oxygen via a series of complexes, releasing energy that drives ATP synthesis. This process forms the foundation of oxidative phosphorylation. (Answer: 2)

6. Which of the following is the final product of the Krebs cycle?
1. Pyruvate
2. CO₂
3. Ethanol
4. Acetaldehyde

Explanation:
The Krebs cycle completes the oxidation of acetyl-CoA to carbon dioxide (CO₂). Along with this, high-energy carriers NADH and FADH₂ are produced, which feed electrons into the electron transport chain for ATP synthesis. Therefore, CO₂ is the final product. (Answer: 2)

7. Assertion–Reason Question
Assertion (A): Oxygen acts as the terminal electron acceptor in aerobic respiration.
Reason (R): It combines with hydrogen to form water at the end of the electron transport chain.
1. Both A and R are true and R is the correct explanation of A
2. Both A and R are true but R is not the correct explanation of A
3. A is true but R is false
4. A is false but R is true

Explanation:
In the electron transport chain, oxygen accepts electrons and hydrogen ions to form water, completing aerobic respiration. Both assertion and reason are correct, and the reason accurately explains the assertion. (Answer: 1)

8. Matching Type Question
Match the following:
A. Glycolysis – I. Cytoplasm
B. Krebs cycle – II. Mitochondrial matrix
C. ETC – III. Inner mitochondrial membrane
D. Alcoholic fermentation – IV. Yeast
1. A-I, B-II, C-III, D-IV
2. A-II, B-I, C-III, D-IV
3. A-I, B-III, C-II, D-IV
4. A-II, B-III, C-I, D-IV

Explanation:
Glycolysis occurs in the cytoplasm, the Krebs cycle in the mitochondrial matrix, and the electron transport chain along the inner mitochondrial membrane. Alcoholic fermentation occurs in yeast cells under anaerobic conditions. Thus, A-I, B-II, C-III, D-IV is correct. (Answer: 1)

9. Fill in the Blanks Question
In respiration, the enzyme ________ catalyses the conversion of pyruvate to acetyl-CoA.
1. Lactate dehydrogenase
2. Pyruvate dehydrogenase
3. Succinic dehydrogenase
4. Hexokinase

Explanation:
Pyruvate dehydrogenase catalyzes the decarboxylation of pyruvate to form acetyl-CoA, linking glycolysis to the Krebs cycle. This multienzyme complex plays a key regulatory role in aerobic respiration. (Answer: 2)

10. Choose the Correct Statements (Statement I & II)
Statement I: Oxygen is required for ATP generation during oxidative phosphorylation.
Statement II: ATP synthesis occurs when protons move across the mitochondrial membrane through ATP synthase.
1. Both statements are correct
2. Both statements are incorrect
3. Statement I is correct, Statement II is incorrect
4. Statement I is incorrect, Statement II is correct

Explanation:
Both statements are correct. Oxygen serves as the terminal electron acceptor in oxidative phosphorylation, enabling electron flow, while ATP synthase uses the proton gradient across the inner mitochondrial membrane to generate ATP. (Answer: 1)

Topic: Respiration and Energy Production; Subtopic: Chemiosmotic Mechanism of ATP Formation

Keyword Definitions:

• ATP: Adenosine Triphosphate, the primary energy currency of the cell.

• Proton Gradient: A difference in proton concentration across a membrane, driving ATP synthesis.

• Oxidative Phosphorylation: ATP formation using energy released during electron transport.

• Chemiosmosis: Movement of ions across a semipermeable membrane linked to ATP generation.

• ATP Synthase: Enzyme complex responsible for converting ADP and Pi into ATP using proton motive force.

Lead Question - 2024 (Jhajjhar)
Synthesis of ATP linked to development of a proton gradient across a membrane is:
1. Mass flow hypothesis
2. Wobble hypothesis
3. Chemiosmotic hypothesis
4. Rivet Popper hypothesis
Explanation: The Chemiosmotic hypothesis by Peter Mitchell explains ATP synthesis via proton gradient across membranes in mitochondria and chloroplasts. Electrons passing through the electron transport chain drive protons across the membrane, creating a potential difference. The return flow of protons through ATP synthase provides the energy for ATP formation. This process couples oxidation with phosphorylation efficiently.

1. In photosynthesis, photophosphorylation occurs during:
1. Light-independent reactions
2. Light-dependent reactions
3. Carbon fixation phase
4. Glycolysis
Explanation: ATP is formed during the light-dependent reactions of photosynthesis through photophosphorylation. Sunlight energizes electrons in chlorophyll, which pass through an electron transport chain. The resulting proton gradient across the thylakoid membrane drives ATP synthesis by ATP synthase, forming ATP required for subsequent stages like the Calvin cycle.

2. Which organelle is primarily involved in the chemiosmotic generation of ATP during respiration?
1. Golgi apparatus
2. Lysosome
3. Mitochondrion
4. Endoplasmic reticulum
Explanation: The mitochondrion is the site of oxidative phosphorylation where ATP is synthesized via the chemiosmotic mechanism. The electron transport chain in the inner mitochondrial membrane establishes a proton gradient, and ATP synthase utilizes this proton motive force to produce ATP, supplying energy to various cellular processes efficiently.

3. The proton motive force is generated across which mitochondrial membrane?
1. Outer membrane
2. Inner membrane
3. Matrix membrane
4. Intermembrane space
Explanation: The proton motive force develops across the inner mitochondrial membrane. Electrons moving through the complexes of the electron transport chain pump protons from the matrix into the intermembrane space, creating both electrical and chemical gradients. This energy gradient powers ATP synthase to generate ATP from ADP and inorganic phosphate.

4. The enzyme responsible for ATP synthesis in chemiosmosis is:
1. Cytochrome oxidase
2. ATP synthase
3. Succinate dehydrogenase
4. NADH dehydrogenase
Explanation: ATP synthase is the enzyme that catalyzes ATP synthesis using energy derived from the proton gradient. Located in the inner mitochondrial and thylakoid membranes, it allows protons to re-enter the matrix or stroma. The energy from this flow drives phosphorylation of ADP to form ATP — an essential energy process in all living cells.

5. Which statement best describes the function of the electron transport chain?
1. Synthesizes glucose
2. Pumps protons across a membrane
3. Breaks down ATP
4. Transfers oxygen molecules
Explanation: The electron transport chain functions to pump protons across the inner mitochondrial membrane as electrons move through protein complexes. This creates a proton gradient essential for ATP synthesis. It couples electron transfer from NADH and FADH₂ to oxygen, forming water and conserving released energy as a proton motive force.

6. In chloroplasts, the chemiosmotic gradient is established in:
1. Thylakoid lumen
2. Stroma
3. Outer membrane
4. Intermembrane space
Explanation: In chloroplasts, the thylakoid lumen accumulates protons during light-dependent reactions. Electron transport through photosystems and the cytochrome complex pumps protons into the lumen, creating a gradient. The flow of protons back into the stroma through ATP synthase generates ATP required for carbon fixation in the Calvin cycle.

7. Assertion-Reason Type:
Assertion (A): Proton gradient formation is essential for ATP synthesis.
Reason (R): The flow of protons through ATP synthase provides energy to phosphorylate ADP.
1. Both A and R are true and R explains A.
2. Both A and R are true but R does not explain A.
3. A is true but R is false.
4. A is false but R is true.
Explanation: Both statements are true, and R correctly explains A. Proton gradient formation across membranes provides potential energy. As protons flow back through ATP synthase, the enzyme harnesses this energy to phosphorylate ADP into ATP. This chemiosmotic process is central to energy metabolism in mitochondria and chloroplasts.

8. Matching Type:
Match List-I with List-II
List-I (Structure) — List-II (Location)
A. ATP synthase — I. Thylakoid membrane
B. Cytochrome c oxidase — II. Inner mitochondrial membrane
C. F₀-F₁ complex — III. ATP-generating enzyme
D. Proton channel — IV. Allows H⁺ diffusion
1. A-III, B-II, C-I, D-IV
2. A-I, B-III, C-II, D-IV
3. A-III, B-II, C-IV, D-I
4. A-I, B-IV, C-II, D-III
Explanation: Correct match is A-III, B-II, C-I, D-IV. ATP synthase (A) generates ATP, Cytochrome c oxidase (B) resides in the inner mitochondrial membrane, F₀-F₁ complex (C) is part of the thylakoid membrane, and proton channels (D) permit H⁺ ions to diffuse back, completing the chemiosmotic cycle efficiently.

9. Fill in the Blanks:
The enzyme complex responsible for coupling electron transport with ATP synthesis is ____________.
1. NADH dehydrogenase
2. ATP synthase
3. Cytochrome reductase
4. Succinate dehydrogenase
Explanation: The correct answer is ATP synthase. This enzyme couples the electron transport chain to ATP formation using the proton gradient. It has F₀ and F₁ subunits that facilitate proton flow and ATP generation, respectively. It is the final step in oxidative phosphorylation and photophosphorylation in mitochondria and chloroplasts.

10. Choose the Correct Statements:
Statement I: Electron transport and ATP synthesis are directly coupled by energy transfer.
Statement II: Proton gradient serves as an intermediate coupling mechanism.
1. Only Statement I is correct.
2. Only Statement II is correct.
3. Both Statements I and II are correct.
4. Both Statements I and II are incorrect.
Explanation: The correct choice is Only Statement II is correct. Electron transport and ATP synthesis are indirectly coupled through a proton gradient rather than direct energy transfer. This gradient provides the necessary energy for ATP synthase to produce ATP — the essence of the chemiosmotic theory by Peter Mitchell.

Topic: Glycolysis; Subtopic: Conversion of Sucrose and Role of Enzymes

Keyword Definitions:

• Sucrose: A disaccharide composed of glucose and fructose, serving as the main transport sugar in plants.

• Invertase: An enzyme that hydrolyzes sucrose into glucose and fructose.

• Monosaccharides: Simple sugars like glucose and fructose that enter glycolysis for energy production.

• Glycolysis: A metabolic pathway converting glucose into pyruvate with the release of ATP and NADH.

Lead Question – 2024 (Jhajjhar)

In plants, sucrose is converted into two molecules of monosaccharides due to action of invertase and then both enter the glycolytic pathway. Identify the monosaccharides.

(1) Glucose and glucose
(2) Glucose and fructose
(3) Glucose and galactose
(4) Fructose and galactose

Explanation: The correct answer is (2). In plants, the enzyme invertase catalyzes the hydrolysis of sucrose into glucose and fructose. Both monosaccharides then enter glycolysis, where glucose is directly phosphorylated, while fructose is converted into intermediates. This conversion ensures efficient energy release for plant metabolism during respiration.

1. Which enzyme initiates glycolysis by converting glucose to glucose-6-phosphate?
(1) Hexokinase
(2) Phosphofructokinase
(3) Aldolase
(4) Pyruvate kinase

Explanation: The correct answer is (1). Hexokinase catalyzes the first step of glycolysis, phosphorylating glucose to form glucose-6-phosphate. This reaction traps glucose inside the cell, enabling its conversion into pyruvate through subsequent steps. The enzyme requires ATP, which donates a phosphate group, initiating energy metabolism in cells.

2. Which of the following is the net gain of ATP molecules in glycolysis?
(1) 2 ATP
(2) 4 ATP
(3) 6 ATP
(4) 8 ATP

Explanation: The correct answer is (1). Glycolysis yields a total of 4 ATPs, but 2 ATPs are consumed in early steps, resulting in a net gain of 2 ATP per molecule of glucose. Additionally, glycolysis produces 2 molecules of pyruvate and 2 NADH, contributing to the energy yield under aerobic conditions.

3. Where does glycolysis take place in the cell?
(1) Mitochondria
(2) Cytoplasm
(3) Chloroplast
(4) Endoplasmic reticulum

Explanation: The correct answer is (2). Glycolysis occurs in the cytoplasm of both plant and animal cells. It is an anaerobic process that does not require oxygen and forms pyruvate, which may further enter the mitochondria for aerobic respiration or undergo fermentation under anaerobic conditions to produce energy.

4. Which of the following is the end product of glycolysis under aerobic conditions?
(1) Ethanol
(2) Pyruvate
(3) Lactic acid
(4) Acetyl-CoA

Explanation: The correct answer is (2). Under aerobic conditions, glycolysis produces pyruvate, which is transported into mitochondria. There, it undergoes oxidative decarboxylation to form acetyl-CoA, entering the Krebs cycle. This process ensures the complete oxidation of glucose for maximum ATP generation in aerobic organisms.

5. In which of the following reactions is NAD⁺ reduced to NADH during glycolysis?
(1) Glyceraldehyde-3-phosphate → 1,3-bisphosphoglycerate
(2) Glucose → Glucose-6-phosphate
(3) Phosphoenolpyruvate → Pyruvate
(4) Fructose-6-phosphate → Fructose-1,6-bisphosphate

Explanation: The correct answer is (1). The oxidation of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate involves the reduction of NAD⁺ to NADH. This is the key redox reaction in glycolysis, linking energy capture with substrate oxidation, which later contributes to ATP production in aerobic respiration through the electron transport chain.

6. Which enzyme catalyzes the conversion of phosphoenolpyruvate (PEP) to pyruvate?
(1) Pyruvate dehydrogenase
(2) Pyruvate kinase
(3) Enolase
(4) Aldolase

Explanation: The correct answer is (2). Pyruvate kinase catalyzes the final step of glycolysis, converting phosphoenolpyruvate into pyruvate with the release of one ATP molecule. This reaction is irreversible and plays a crucial role in the regulation of glycolysis, maintaining cellular energy homeostasis through substrate-level phosphorylation.

7. (Assertion-Reason Type)
Assertion (A): Glycolysis can occur in both aerobic and anaerobic conditions.
Reason (R): The process requires oxygen for the breakdown of glucose.
(1) Both A and R are true, and R is the correct explanation of A
(2) Both A and R are true, but R is not the correct explanation of A
(3) A is true, but R is false
(4) A is false, but R is true

Explanation: The correct answer is (3). Glycolysis can proceed without oxygen, making it an anaerobic process. It converts glucose to pyruvate, yielding ATP and NADH. Oxygen becomes essential only for subsequent aerobic steps like the Krebs cycle and oxidative phosphorylation in mitochondria, not for glycolysis itself.

8. (Matching Type)
Match List I with List II:

List I (Enzyme) – List II (Function)
A. Phosphofructokinase – I. Converts fructose-6-phosphate to fructose-1,6-bisphosphate
B. Enolase – II. Removes water from 2-phosphoglycerate
C. Aldolase – III. Splits fructose-1,6-bisphosphate into two triose phosphates
D. Pyruvate kinase – IV. Converts PEP to pyruvate

(1) A-I, B-II, C-III, D-IV
(2) A-II, B-I, C-IV, D-III
(3) A-III, B-IV, C-I, D-II
(4) A-IV, B-I, C-II, D-III

Explanation: The correct answer is (1). Each enzyme has a specific role in glycolysis. Phosphofructokinase phosphorylates fructose-6-phosphate, enolase dehydrates 2-phosphoglycerate, aldolase splits fructose-1,6-bisphosphate, and pyruvate kinase catalyzes the last step converting PEP into pyruvate, ensuring efficient glucose metabolism and ATP production.

9. (Fill in the Blank)
The conversion of glucose into pyruvate without the involvement of oxygen is called ________.
(1) Fermentation
(2) Anaerobic respiration
(3) Glycolysis
(4) Oxidative phosphorylation

Explanation: The correct answer is (3). Glycolysis is the enzymatic breakdown of glucose to pyruvate under both aerobic and anaerobic conditions. It occurs in the cytoplasm and involves a sequence of ten enzyme-catalyzed reactions that generate ATP and NADH, providing immediate energy even in the absence of oxygen.

10. (Choose the Correct Statements)
Statement I: Glycolysis takes place inside the mitochondria.
Statement II: Glycolysis does not require oxygen.
(1) Both statements are true
(2) Both statements are false
(3) Statement I is true but Statement II is false
(4) Statement I is false but Statement II is true

Explanation: The correct answer is (4). Glycolysis occurs in the cytoplasm and is an anaerobic process. It does not depend on oxygen availability and is therefore fundamental for both aerobic and anaerobic organisms, forming the initial stage of glucose oxidation in all living cells.

Chapter: Cellular Respiration and Energy Production; Topic: Mitochondrial Metabolism; Subtopic: Cellular Respiration Pathways

Keyword Definitions:

• Citric Acid Cycle: Also called the Krebs cycle; it occurs in the mitochondrial matrix and generates ATP, NADH, and FADH2 from acetyl-CoA.

• Glycolysis: The first step in cellular respiration that occurs in the cytoplasm, converting glucose into pyruvate and yielding ATP and NADH.

• Electron Transport System (ETS): A chain of proteins located in the inner mitochondrial membrane that transfers electrons to generate ATP.

• Proton Gradient: The difference in proton concentration across the inner mitochondrial membrane that drives ATP synthesis.

Lead Question - 2024

Match List I with List II
List I          List II
A. Citric Acid Cycle    I. Cytoplasm
B. Glycolysis                          II. Mitochondrial matrix
C. Electron Transport System of mitochondria    III. Inner mitochondrial membrane
D. Proton Gradient                     IV. Intermembrane space

Choose the correct answer from the options given below:
(1) A–II, B–I, C–IV, D–III
(2) A–III, B–IV, C–I, D–II
(3) A–IV, B–III, C–II, D–I
(4) A–II, B–I, C–III, D–IV

Explanation: The Citric Acid Cycle occurs in the mitochondrial matrix (A–II), Glycolysis occurs in the cytoplasm (B–I), the Electron Transport System is located in the inner mitochondrial membrane (C–III), and the Proton Gradient forms across the intermembrane space (D–IV). Therefore, the correct answer is (4). These processes together generate ATP by oxidative phosphorylation, providing energy essential for cellular activities and metabolic reactions in aerobic organisms.

Guessed Questions:

1. Which part of the mitochondrion contains enzymes for the citric acid cycle?
(1) Inner membrane (2) Matrix (3) Outer membrane (4) Intermembrane space
Explanation: The enzymes for the citric acid cycle are located in the mitochondrial matrix, where acetyl-CoA is oxidized to CO2. This generates NADH and FADH2, which transfer electrons to the ETS for ATP production, linking the cycle to oxidative phosphorylation and energy generation.

2. The enzyme ATP synthase is located in which part of the mitochondrion?
(1) Inner membrane (2) Matrix (3) Outer membrane (4) Cytoplasm
Explanation: ATP synthase is embedded in the inner mitochondrial membrane. It utilizes the proton gradient generated by the ETS to convert ADP into ATP. As protons move back into the matrix through ATP synthase, the energy released drives phosphorylation, producing most of the cell’s ATP yield.

3. The first step of glycolysis involves which reaction?
(1) Glucose phosphorylation (2) Glucose oxidation (3) Pyruvate reduction (4) ATP hydrolysis
Explanation: Glycolysis begins with phosphorylation of glucose by hexokinase to form glucose-6-phosphate, using one ATP molecule. This traps glucose inside the cell and prepares it for further breakdown into pyruvate, which enters mitochondria for aerobic energy production through the Krebs cycle and electron transport chain.

4. Which of the following statements is true about oxidative phosphorylation?
(1) Occurs in cytoplasm (2) Requires oxygen (3) Produces CO2 (4) Does not use electron transport
Explanation: Oxidative phosphorylation requires oxygen, as it acts as the final electron acceptor in the ETS. This process occurs in the inner mitochondrial membrane, generating ATP through the movement of protons down their gradient. The absence of oxygen halts ATP formation and leads to anaerobic respiration.

5. The intermembrane space of mitochondria is important for which process?
(1) Proton accumulation (2) Glycolysis (3) Krebs cycle (4) Fermentation
Explanation: The intermembrane space is crucial for proton accumulation during electron transport. As electrons move through the ETS, protons are pumped into this space, creating an electrochemical gradient. This gradient drives ATP synthesis through chemiosmosis, linking electron flow with energy generation in cells.

6. During aerobic respiration, most of the ATP is produced by
(1) Glycolysis (2) Krebs cycle (3) Electron Transport System (4) Fermentation
Explanation: Most ATP is produced by the Electron Transport System through oxidative phosphorylation. NADH and FADH2 from glycolysis and the Krebs cycle donate electrons to the chain, powering proton pumps and creating a gradient that drives ATP synthase to form ATP from ADP and inorganic phosphate.

7. -Reason Question
Assertion (A): The inner mitochondrial membrane is selectively permeable.
Reason (R): It allows free passage of all molecules and ions.
(1) Both A and R are true, and R is the correct explanation of A.
(2) Both A and R are true, but R is not the correct explanation of A.
(3) A is true, but R is false.
(4) A is false, but R is true.
Explanation: The inner mitochondrial membrane is selectively permeable (A is true), but it does not allow free passage of all molecules and ions (R is false). It contains specific transport proteins that regulate the movement of ions and metabolites essential for maintaining proton gradient and ATP synthesis.

8. Matching Type Question
Match the enzyme with its function:
A. Hexokinase – I. Converts glucose to glucose-6-phosphate
B. Citrate synthase – II. Combines acetyl-CoA and oxaloacetate
C. Cytochrome oxidase – III. Transfers electrons to oxygen
(1) A–I, B–II, C–III (2) A–III, B–I, C–II (3) A–II, B–III, C–I (4) A–I, B–III, C–II
Explanation: Hexokinase catalyzes phosphorylation of glucose to glucose-6-phosphate (A–I), citrate synthase catalyzes the condensation of acetyl-CoA with oxaloacetate (B–II), and cytochrome oxidase transfers electrons to oxygen, the terminal acceptor (C–III). Therefore, the correct matching is (1), which reflects the stepwise energy conversion processes in respiration.

9. Fill in the Blanks
In mitochondria, ATP is synthesized by the enzyme ________ located in the inner membrane.
(1) ATP synthase (2) Hexokinase (3) Rubisco (4) Peptidase
Explanation: ATP is synthesized by the enzyme ATP synthase located in the inner mitochondrial membrane. It utilizes the proton motive force created by the ETS to phosphorylate ADP into ATP. This process, known as chemiosmosis, forms the key energy-producing stage in aerobic respiration and energy metabolism.

10. Choose the Correct Statements
Statement I: Glycolysis requires oxygen to proceed.
Statement II: The Citric Acid Cycle occurs only in the presence of oxygen.
(1) Both statements are true. (2) Both statements are false.
(3) Statement I is false, Statement II is true. (4) Statement I is true, Statement II is false.
Explanation: Glycolysis can occur in the absence of oxygen, so Statement I is false. The Citric Acid Cycle requires oxygen indirectly because NADH and FADH2 must donate electrons to oxygen through the ETS; hence, Statement II is true. The correct answer is (3).

Topic: Tricarboxylic Acid (TCA) Cycle; Subtopic: Oxidation and Energy Yield

Keyword Definitions:

• TCA Cycle: A cyclic series of reactions in mitochondria that oxidize acetyl-CoA to CO₂ and generate NADH, FADH₂, and ATP.

• Oxidation: Loss of electrons or hydrogen during biochemical reactions producing energy equivalents.

• Succinyl-CoA: A high-energy intermediate that forms succinate while generating GTP.

• Isocitrate: An intermediate oxidized to α-ketoglutarate, releasing CO₂ and NADH.

• Malic Acid: Oxidized to oxaloacetate with NAD⁺ as cofactor in the final step.

Lead Question – 2024

Identify the step in tricarboxylic acid cycle, which does not involve oxidation of substrate.
(1) Succinic acid → Malic acid
(2) Succinyl–CoA → Succinic acid
(3) Isocitrate → α-Ketoglutaric acid
(4) Malic acid → Oxaloacetic acid

Explanation: The conversion of Succinyl-CoA to Succinic acid does not involve oxidation; it is a substrate-level phosphorylation producing GTP. Other steps like isocitrate to α-ketoglutarate, succinate to fumarate, and malate to oxaloacetate involve oxidation reactions coupled with reduction of NAD⁺ or FAD. Hence, option (2) Succinyl-CoA → Succinic acid is correct.

1. In the TCA cycle, which step involves substrate-level phosphorylation?
(1) Succinyl-CoA → Succinic acid
(2) Fumarate → Malate
(3) Citrate → Isocitrate
(4) Malate → Oxaloacetate

During the conversion of Succinyl-CoA to Succinic acid, a high-energy thioester bond is cleaved, leading to GTP synthesis. This represents substrate-level phosphorylation as it directly forms an energy molecule without electron transport chain involvement. Hence, option (1) Succinyl-CoA → Succinic acid is correct.

2. Which coenzyme acts as hydrogen acceptor in oxidation of succinic acid?
(1) NAD⁺
(2) FAD
(3) CoA
(4) ATP

In the conversion of succinate to fumarate, flavin adenine dinucleotide (FAD) acts as the hydrogen acceptor forming FADH₂. This enzyme-linked oxidation is catalyzed by succinate dehydrogenase, a mitochondrial inner membrane-bound enzyme. Hence, option (2) FAD is correct.

3. Number of NADH molecules produced per acetyl-CoA oxidized in one TCA cycle is:
(1) 2
(2) 3
(3) 4
(4) 1

Each acetyl-CoA entering the TCA cycle produces 3 NADH through oxidation at isocitrate, α-ketoglutarate, and malate steps. Along with 1 FADH₂ and 1 GTP, this ensures efficient ATP generation during oxidative phosphorylation. Hence, option (2) 3 is correct.

4. Which of the following enzymes catalyzes oxidative decarboxylation in the TCA cycle?
(1) Citrate synthase
(2) Isocitrate dehydrogenase
(3) Aconitase
(4) Fumarase

Isocitrate dehydrogenase catalyzes the oxidative decarboxylation of isocitrate to α-ketoglutarate, producing NADH and CO₂. It is one of the key regulatory enzymes of the TCA cycle, stimulated by ADP and inhibited by ATP and NADH. Hence, option (2) Isocitrate dehydrogenase is correct.

5. Which intermediate connects glycolysis and the TCA cycle?
(1) Pyruvate
(2) Acetyl-CoA
(3) Oxaloacetate
(4) Citrate

Pyruvate formed in glycolysis enters mitochondria and is converted into Acetyl-CoA by pyruvate dehydrogenase complex, linking glycolysis with the TCA cycle. This oxidative decarboxylation produces NADH and releases CO₂. Hence, option (2) Acetyl-CoA is correct.

6. In TCA cycle, how many ATP equivalents are produced per turn?
(1) 10
(2) 8
(3) 12
(4) 4

Each acetyl-CoA oxidation in the TCA cycle generates 3 NADH (9 ATP), 1 FADH₂ (2 ATP), and 1 GTP (1 ATP equivalent), giving a total of 12 ATP equivalents. Hence, option (3) 12 is correct.

Assertion–Reason Type Question

7. Assertion (A): TCA cycle operates only under aerobic conditions.
Reason (R): NADH and FADH₂ produced in the cycle are reoxidized only through electron transport chain.

(1) Both A and R are true and R is the correct explanation of A
(2) Both A and R are true but R is not the correct explanation of A
(3) A is true but R is false
(4) A is false but R is true

TCA cycle needs continuous oxidation of NADH and FADH₂ by electron transport chain, which requires oxygen as terminal electron acceptor. Hence, both statements are true and R correctly explains A. Correct option is (1).

8. Matching Type Question

Match the enzymes with their cofactors:
A. Pyruvate dehydrogenase – I. FAD
B. Succinate dehydrogenase – II. NAD⁺
C. Malate dehydrogenase – III. Thiamine pyrophosphate
D. α-Ketoglutarate dehydrogenase – IV. CoA

(1) A–III, B–I, C–II, D–IV
(2) A–II, B–III, C–I, D–IV
(3) A–IV, B–II, C–III, D–I
(4) A–I, B–IV, C–II, D–III

Pyruvate dehydrogenase uses thiamine pyrophosphate, succinate dehydrogenase requires FAD, malate dehydrogenase uses NAD⁺, and α-ketoglutarate dehydrogenase is linked to CoA. Hence, the correct match is option (1) A–III, B–I, C–II, D–IV.

9. Fill in the Blanks Question

Each glucose molecule yields ______ turns of the TCA cycle.

(1) One
(2) Two
(3) Three
(4) Four

Each glucose forms two molecules of acetyl-CoA after glycolysis, each entering one turn of the TCA cycle. Thus, two complete cycles occur per glucose molecule oxidized. Hence, option (2) Two is correct.

10. Choose the Correct Statements (Statement I & II)

Statement I: Oxaloacetate condenses with acetyl-CoA to form citrate.
Statement II: This reaction is catalyzed by isocitrate dehydrogenase.

(1) Both statements are true
(2) Statement I true, Statement II false
(3) Statement I false, Statement II true
(4) Both statements are false

The condensation of oxaloacetate and acetyl-CoA forming citrate is catalyzed by citrate synthase, not by isocitrate dehydrogenase. Thus, Statement I is true but Statement II is false. Correct option is (2).

Topic: Stomatal Structure; Subtopic: Guard Cells and Stomata Components

Keyword Definitions:

• Stoma: Pore present on the leaf surface that allows gas exchange and transpiration.

• Guard Cells: Specialized cells surrounding the stoma; regulate opening and closing of the stomatal pore.

• Epidermal Cells: Outermost layer of cells in leaves, stems, and roots providing protection.

• Subsidiary Cells: Cells adjacent to guard cells that provide structural support and assist in stomatal movement.

• Thickened Inner Walls: Inner walls of guard cells which are rigid and help in stomatal opening.

• Thin Outer Walls: Flexible outer walls of guard cells that allow expansion when turgid.

• Transpiration: Loss of water vapor from plant surfaces, mainly through stomata.

• Gas Exchange: Movement of O₂ and CO₂ through stomata for photosynthesis and respiration.

• Turgor Pressure: Pressure of water inside cells that maintains cell shape and drives stomatal movement.

• Plant Anatomy: Study of internal structure and organization of plant tissues and organs.

• Stomatal Pore: Opening formed between guard cells facilitating gas exchange.

Lead Question - 2024:

In the given figure, which component of stomata has thin outer walls and highly thickened inner walls?
(1) Stoma
(2) Epidermal Cells
(3) Guard Cells
(4) Subsidiary Cells

Explanation: Correct answer is (3) Guard Cells. Guard cells are specialized epidermal cells that flank the stomatal pore. They have thin outer walls and thickened inner walls facing the stoma, which enables them to bend and open the stomatal pore when turgid. This structural arrangement facilitates controlled gas exchange and transpiration in plants. Subsidiary cells support guard cells, while epidermal cells form the general leaf surface, and stoma is the pore itself, not a cell. The unique wall thickness and turgor-driven movement of guard cells are essential for stomatal function, photosynthesis, and water regulation in plants.

Guessed MCQs:

1. Single Correct Answer MCQ: Which cells regulate stomatal opening?
(A) Epidermal cells
(B) Guard cells
(C) Subsidiary cells
(D) Parenchyma cells
Explanation: Correct answer is (B). Guard cells control stomatal aperture by changing turgor pressure, facilitating gas exchange and transpiration. Epidermal cells form leaf surface, subsidiary cells support, and parenchyma cells perform photosynthesis.

2. Single Correct Answer MCQ: Thickened inner walls of guard cells help in:
(A) Strengthening epidermis
(B) Opening of stomatal pore
(C) Water absorption
(D) Photosynthesis
Explanation: Correct answer is (B). Thickened inner walls create a hinge effect, allowing guard cells to bow outward under turgor pressure, thus opening the stomatal pore for gas exchange and transpiration.

3. Single Correct Answer MCQ: Subsidiary cells in stomata primarily:
(A) Form stomatal pore
(B) Assist guard cells in movement
(C) Perform photosynthesis
(D) Store water
Explanation: Correct answer is (B). Subsidiary cells flank guard cells and provide mechanical support during opening and closing of stomata, helping maintain turgor-driven stomatal function.

4. Single Correct Answer MCQ: Stoma refers to:
(A) Epidermal cell
(B) Pore between guard cells
(C) Subsidiary cell
(D) Chloroplast
Explanation: Correct answer is (B). Stoma is the opening between guard cells that facilitates gas exchange and transpiration. It is not a cell but the aperture regulated by guard cells.

5. Single Correct Answer MCQ: Which property enables guard cells to bend and open stomata?
(A) Thin outer walls and thick inner walls
(B) Photosynthetic pigments
(C) Epidermal support
(D) Random growth
Explanation: Correct answer is (A). The differential wall thickness of guard cells allows bending under turgor pressure, creating stomatal opening essential for gas exchange and transpiration regulation.

6. Single Correct Answer MCQ: Turgor pressure in guard cells is influenced by:
(A) Water absorption
(B) Sunlight
(C) Temperature
(D) All of the above
Explanation: Correct answer is (D). Guard cell turgor depends on water uptake, which is influenced by environmental factors like sunlight (photosynthesis), temperature, and humidity, regulating stomatal opening and closing.

7. Assertion-Reason MCQ:
Assertion (A): Guard cells have thickened inner walls.
Reason (R): Thickened inner walls allow guard cells to bend and open the stomatal pore.
(A) Both A and R are true and R explains A
(B) Both A and R are true but R does not explain A
(C) A is true, R is false
(D) A is false, R is true
Explanation: Correct answer is (A). The thickened inner walls create rigidity on the inner side of guard cells. When turgid, the cells bend outward, opening the stomatal pore for gas exchange and transpiration, making R the correct explanation for A.

8. Matching Type MCQ: Match List-I with List-II:
List-I: (A) Guard Cells (B) Subsidiary Cells (C) Stoma (D) Epidermal Cells
List-II: (I) Pore for gas exchange (II) Provide support to guard cells (III) Thin outer, thick inner walls (IV) General leaf covering
Options: (A) (B) (C) (D)
1. III II I IV
2. II III IV I
3. I II III IV
4. IV III II I
Explanation: Correct answer is 1. Guard cells have thin outer and thick inner walls (III), subsidiary cells provide mechanical support (II), stoma is the pore (I), and epidermal cells form the leaf surface (IV).

9. Fill in the Blanks / Completion MCQ: The stomatal pore is flanked by _______ which control its opening and closing.
(A) Epidermal cells
(B) Guard cells
(C) Subsidiary cells
(D) Mesophyll cells
Explanation: Correct answer is (B). Guard cells regulate the stomatal aperture through turgor changes, facilitating gas exchange and transpiration, while other cells play supportive or protective roles.

10. Choose the correct statements MCQ:
Statement I: Guard cells control gas exchange by opening and closing stomata.
Statement II: Epidermal cells have thickened inner walls for stomatal movement.
Options:
(A) Both I and II are correct
(B) Only I is correct
(C) Only II is correct
(D) Both are incorrect
Explanation: Correct answer is (B). Statement I is true as guard cells regulate stomatal opening for gas exchange. Statement II is false; epidermal cells provide surface protection but do not control stomatal movement or have thickened inner walls.

Topic: Metabolism; Subtopic: Fatty Acid Oxidation and Respiratory Pathway

• Fatty acids: Long-chain carboxylic acids used as energy storage molecules and substrates for β-oxidation.

• Acetyl CoA: A two-carbon molecule that links carbohydrate, fat, and protein metabolism to the citric acid cycle.

• α-Ketoglutaric acid: A key intermediate of the citric acid cycle involved in amino acid metabolism.

• Dihydroxyacetone phosphate (DHAP): An intermediate in glycolysis and lipid metabolism, involved in triglyceride synthesis.

• Pyruvic acid: The end product of glycolysis, which can enter the citric acid cycle as Acetyl CoA.

• Respiratory pathway: Series of biochemical reactions including glycolysis, citric acid cycle, and electron transport chain that produce ATP.

• β-oxidation: Process of breaking down fatty acids in mitochondria to generate Acetyl CoA.

• Citric acid cycle: Also called TCA cycle, oxidizes Acetyl CoA to produce NADH, FADH₂, and ATP.

• Coenzyme A (CoA): A carrier molecule that forms thioester bonds with acyl groups, central to metabolism.

• Electron transport chain: Series of protein complexes in mitochondria that transfer electrons and synthesize ATP.

• Energy metabolism: Biochemical pathways converting nutrients into usable cellular energy (ATP).

Lead Question - 2023 (Manipur)

Fatty acids are connected with the respiratory pathway through:

• 1. Acetyl CoA

• 2. α-Ketoglutaric acid

• 3. Dihydroxy acetone phosphate

• 4. Pyruvic acid

Explanation: Fatty acids undergo β-oxidation in the mitochondria to produce Acetyl CoA, which enters the citric acid cycle, linking lipid metabolism to the respiratory pathway. α-Ketoglutaric acid and pyruvic acid are intermediates in the citric acid cycle or glycolysis, while DHAP is part of glycolysis and triglyceride synthesis. The correct answer is 1, Acetyl CoA, as it is the central metabolite connecting fatty acid degradation to energy production through the respiratory pathway.

1. During β-oxidation of fatty acids, which molecule is repeatedly cleaved from the fatty acid chain?

• a) Acetyl CoA

• b) Pyruvate

• c) Glucose

• d) NADH

Explanation: In β-oxidation, two-carbon units are sequentially removed from the fatty acid chain as Acetyl CoA. Each cleavage produces one Acetyl CoA molecule that enters the citric acid cycle for ATP production. NADH and FADH₂ are generated as reducing equivalents, but the cleaved unit is Acetyl CoA. Correct answer: a.

2. Which enzyme catalyzes the first step in β-oxidation of fatty acids?

• a) Acyl-CoA dehydrogenase

• b) Citrate synthase

• c) Hexokinase

• d) Lactate dehydrogenase

Explanation: The first step of β-oxidation is catalyzed by Acyl-CoA dehydrogenase, which introduces a double bond between the α and β carbons of the fatty acyl-CoA. This initiates the sequential cleavage of fatty acids to Acetyl CoA. Citrate synthase is part of the TCA cycle, hexokinase in glycolysis, and lactate dehydrogenase in fermentation. Correct answer: a.

3. How many Acetyl CoA molecules are produced from the complete oxidation of palmitic acid (C16:0)?

• a) 8

• b) 16

• c) 7

• d) 4

Explanation: Palmitic acid (16 carbons) undergoes β-oxidation, producing one Acetyl CoA per two-carbon unit removed. Therefore, 16/2 = 8 Acetyl CoA molecules are generated. Each Acetyl CoA then enters the citric acid cycle to produce ATP. Correct answer: a.

4. Which coenzyme carries acyl groups during fatty acid metabolism?

• a) Coenzyme A

• b) NAD+

• c) FAD

• d) ATP

Explanation: Coenzyme A binds acyl groups via thioester bonds to form acyl-CoA, which is central to fatty acid metabolism and links to the citric acid cycle. NAD+ and FAD serve as electron carriers, while ATP provides energy, but acyl group transport is mediated by CoA. Correct answer: a.

5. Which molecule is the main entry point of carbohydrate and fat metabolism into the citric acid cycle?

• a) Acetyl CoA

• b) Pyruvate

• c) α-Ketoglutarate

• d) Oxaloacetate

Explanation: Both carbohydrate (via pyruvate) and fatty acid metabolism (via β-oxidation) converge at Acetyl CoA, which enters the citric acid cycle to produce reducing equivalents and energy. α-Ketoglutarate and oxaloacetate are intermediates of the cycle but not the main entry point. Correct answer: a.

6. Dihydroxyacetone phosphate (DHAP) links glycolysis with:

• a) Triglyceride synthesis

• b) Citric acid cycle

• c) β-oxidation

• d) Electron transport chain

Explanation: DHAP is a glycolytic intermediate that can be converted to glycerol-3-phosphate for triglyceride synthesis. It does not directly enter the citric acid cycle or β-oxidation, and it is not part of the electron transport chain. Correct answer: a.

7. Assertion (A): Fatty acids contribute to ATP production through the citric acid cycle.
Reason (R): β-Oxidation produces Acetyl CoA, NADH, and FADH₂ that enter the respiratory pathway.

• a) Both A and R are true, R explains A

• b) Both A and R are true, R does not explain A

• c) A is true, R is false

• d) A is false, R is true

Explanation: Fatty acids undergo β-oxidation to generate Acetyl CoA, NADH, and FADH₂. Acetyl CoA enters the citric acid cycle, producing more reducing equivalents that feed the electron transport chain for ATP synthesis. Therefore, both assertion and reason are correct, and reason correctly explains the assertion. Correct answer: a.

8. Match the metabolic intermediates with their source:

List-I: (A) Acetyl CoA, (B) Pyruvate, (C) DHAP, (D) α-Ketoglutarate
List-II: (I) Glycolysis, (II) β-Oxidation of fatty acids, (III) Citric acid cycle, (IV) Triglyceride synthesis

• a) A-II, B-I, C-IV, D-III

• b) A-I, B-II, C-III, D-IV

• c) A-II, B-IV, C-I, D-III

• d) A-III, B-I, C-II, D-IV

Explanation: Acetyl CoA comes from β-oxidation of fatty acids (II), pyruvate from glycolysis (I), DHAP is used for triglyceride synthesis (IV), and α-Ketoglutarate is an intermediate of the citric acid cycle (III). Correct answer: a.

9. Fill in the blank: In mitochondrial fatty acid oxidation, each cycle shortens the fatty acid by two carbons and generates ______ and ______.

• a) Acetyl CoA, NADH & FADH₂

• b) Pyruvate, ATP

• c) Glucose, NAD+

• d) Lactate, FADH₂

Explanation: Each cycle of β-oxidation removes a two-carbon unit as Acetyl CoA and generates one NADH and one FADH₂ per cycle, which enter the electron transport chain to produce ATP. Pyruvate, glucose, and lactate are unrelated to β-oxidation. Correct answer: a.

10. Statement I: Fatty acids are converted into Acetyl CoA before entering the citric acid cycle.
Statement II: Acetyl CoA can be used for ketone body synthesis in the liver.

• a) Both I and II are correct

• b) Only I is correct

• c) Only II is correct

• d) Both I and II are incorrect

Explanation: Fatty acids undergo β-oxidation to produce Acetyl CoA, which enters the citric acid cycle or serves as a precursor for ketone body synthesis during fasting. Both statements are accurate and consistent with human metabolism. Correct answer: a.

Topic: Cellular Respiration; Subtopic: Tricarboxylic Acid Cycle (TCA Cycle) / Krebs Cycle

Keyword Definitions:
• TCA cycle: Series of enzymatic reactions in mitochondria oxidizing acetyl-CoA to CO2, producing NADH, FADH2, and ATP.
• Decarboxylation: Removal of a carboxyl group (-COOH) as CO2 from a molecule.
• Acetyl-CoA: Two-carbon molecule derived from pyruvate, entering the TCA cycle.
• NADH and FADH2: Reduced coenzymes carrying electrons to the electron transport chain.
• Electron transport chain: Series of protein complexes in mitochondria generating ATP from electrons transferred by NADH and FADH2.
• Substrate-level phosphorylation: Direct synthesis of ATP from ADP during enzymatic reactions.
• Oxaloacetate: Four-carbon molecule regenerated at the end of TCA cycle, combining with acetyl-CoA.
• Citric acid: First six-carbon intermediate formed in TCA cycle after acetyl-CoA combines with oxaloacetate.

Lead Question - 2023 (Manipur)
How many times decarboxylation occurs during each TCA cycle?
Options:
1. Thrice
2. Many
3. Once
4. Twice
Answer & Explanation: Option 1. During each turn of the TCA cycle, decarboxylation occurs three times. Pyruvate is first converted to acetyl-CoA releasing one CO2, and during the cycle two additional decarboxylation steps release CO2 from isocitrate and α-ketoglutarate. These decarboxylation reactions produce NADH and CO2 and are essential for energy production. Understanding TCA decarboxylation is fundamental for cellular respiration, energy metabolism, and ATP generation. These steps connect carbohydrate, lipid, and protein metabolism, and their regulation is crucial for maintaining metabolic homeostasis in cells.

1. Single Correct Answer MCQ
Which enzyme catalyzes the first decarboxylation in the TCA cycle?
Options:
A. Citrate synthase
B. Isocitrate dehydrogenase
C. α-Ketoglutarate dehydrogenase
D. Malate dehydrogenase
Answer & Explanation: Option B. Isocitrate dehydrogenase catalyzes the oxidative decarboxylation of isocitrate to α-ketoglutarate, releasing CO2 and producing NADH. This is the first decarboxylation within the TCA cycle proper (excluding pyruvate to acetyl-CoA). Understanding enzyme-specific decarboxylation steps is essential for studying energy production, regulation, and metabolic flux. Isocitrate dehydrogenase also serves as a regulatory point controlled by NADH/NAD+ ratio and ATP levels. Knowledge of this reaction is vital for understanding mitochondrial energy metabolism, enzymology, and coordination of the TCA cycle with other metabolic pathways.

2. Single Correct Answer MCQ
Which TCA cycle step generates FADH2?
Options:
A. Isocitrate → α-Ketoglutarate
B. α-Ketoglutarate → Succinyl-CoA
C. Succinate → Fumarate
D. Malate → Oxaloacetate
Answer & Explanation: Option C. Succinate dehydrogenase catalyzes oxidation of succinate to fumarate, producing FADH2. FADH2 donates electrons to the electron transport chain, generating ATP. Recognizing FADH2 production steps is critical for understanding energy yield, mitochondrial electron transport, and TCA cycle function. This reaction links TCA to the inner mitochondrial membrane where succinate dehydrogenase participates in both TCA and electron transport. FADH2 generation complements NADH, and together they provide the reducing equivalents necessary for oxidative phosphorylation and cellular energy metabolism.

3. Single Correct Answer MCQ
How many NADH molecules are produced per TCA cycle?
Options:
A. One
B. Two
C. Three
D. Four
Answer & Explanation: Option C. Each TCA cycle produces three NADH molecules: one from isocitrate → α-ketoglutarate, one from α-ketoglutarate → succinyl-CoA, and one from malate → oxaloacetate. NADH carries electrons to the electron transport chain, generating ATP via oxidative phosphorylation. Understanding NADH production allows calculation of total energy yield per acetyl-CoA. It also highlights the importance of decarboxylation reactions and redox balance in mitochondria. NADH production is central to energy metabolism, linking carbohydrate, lipid, and protein oxidation to ATP synthesis in eukaryotic cells.

4. Single Correct Answer MCQ
Which TCA intermediate is regenerated at the end of the cycle?
Options:
A. Citrate
B. Oxaloacetate
C. α-Ketoglutarate
D. Succinyl-CoA
Answer & Explanation: Option B. Oxaloacetate is regenerated at the end of the TCA cycle, ready to combine with a new acetyl-CoA to continue the cycle. This regeneration ensures continuity of the cycle and constant production of energy intermediates like NADH, FADH2, and GTP/ATP. Understanding oxaloacetate regeneration is essential for studying energy metabolism, substrate cycling, and integration of TCA with gluconeogenesis and amino acid metabolism. Its availability regulates the entry of acetyl-CoA and overall cycle flux, influencing ATP yield and metabolic balance in mitochondria.

5. Single Correct Answer MCQ
During TCA cycle, which reaction produces GTP/ATP?
Options:
A. Succinyl-CoA → Succinate
B. Citrate → Isocitrate
C. Malate → Oxaloacetate
D. α-Ketoglutarate → Succinyl-CoA
Answer & Explanation: Option A. Conversion of succinyl-CoA to succinate catalyzed by succinyl-CoA synthetase produces GTP (or ATP in some tissues) via substrate-level phosphorylation. This is the only substrate-level ATP/GTP generation in TCA, complementing NADH/FADH2-driven oxidative phosphorylation. Understanding this step is crucial for energy metabolism, ATP yield calculations, and mitochondrial function. It illustrates direct energy capture from metabolic intermediates and coordination of enzyme activity with overall TCA flux. The reaction also links catabolic pathways to energy homeostasis in cells.

6. Single Correct Answer MCQ
Which molecule links glycolysis to TCA cycle?
Options:
A. Pyruvate
B. Oxaloacetate
C. Citrate
D. Acetyl-CoA
Answer & Explanation: Option D. Acetyl-CoA, formed from pyruvate decarboxylation, enters the TCA cycle by condensing with oxaloacetate to form citrate. This link connects glycolysis, fatty acid oxidation, and amino acid catabolism to the TCA cycle. Understanding acetyl-CoA’s role is essential for integrating energy metabolism, carbon flux, and ATP production. It highlights the coordination of metabolic pathways, regulation of entry points, and control of energy homeostasis. Acetyl-CoA acts as a central hub in cellular respiration, bridging cytosolic and mitochondrial metabolism.

7. Assertion-Reason MCQ
Assertion (A): Two decarboxylation steps occur inside the TCA cycle itself.
Reason (R): CO2 is released from isocitrate and α-ketoglutarate.
Options:
A. Both A and R are true, R is the correct explanation of A
B. Both A and R are true, R is not the correct explanation of A
C. A is true, R is false
D. A is false, R is true
Answer & Explanation: Option A. Inside the TCA cycle, oxidative decarboxylation occurs at isocitrate → α-ketoglutarate and α-ketoglutarate → succinyl-CoA, releasing two CO2 molecules. The first decarboxylation of pyruvate occurs before entry into the cycle. Recognizing these steps is essential for understanding CO2 production, energy metabolism, and TCA regulation. These reactions produce NADH, provide electrons for oxidative phosphorylation, and illustrate how decarboxylation contributes to the carbon flow and energy balance in mitochondria. Accurate knowledge of decarboxylation is vital for bioenergetics studies.

8. Matching Type MCQ
Match TCA intermediate with its product:
List-I List-II
(A) Isocitrate (I) α-Ketoglutarate + CO2 + NADH
(B) α-Ketoglutarate (II) Succinyl-CoA + CO2 + NADH
(C) Succinate (III) Fumarate + FADH2
(D) Malate (IV) Oxaloacetate + NADH
Options:
1. A-I, B-II, C-III, D-IV
2. A-II, B-I, C-IV, D-III
3. A-I, B-III, C-II, D-IV
4. A-IV, B-III, C-I, D-II
Answer & Explanation: Option 1. Isocitrate → α-ketoglutarate + CO2 + NADH, α-ketoglutarate → Succinyl-CoA + CO2 + NADH, Succinate → Fumarate + FADH2, Malate → Oxaloacetate + NADH. Matching intermediates to products helps understand TCA energy generation, CO2 release, and electron carrier formation. This knowledge is vital for studying mitochondrial metabolism, bioenergetics, and the integration of carbohydrate, lipid, and protein catabolism.

9. Fill in the Blanks / Completion MCQ
During TCA cycle, decarboxylation occurs _______ times per turn.
Options:
A. Once
B. Twice
C. Thrice
D. Four times
Answer & Explanation: Option C. Decarboxylation occurs thrice per acetyl-CoA in TCA: once during pyruvate → acetyl-CoA, and twice inside the cycle at isocitrate and α-ketoglutarate steps. Each releases CO2 and generates NADH. Understanding these steps is essential for energy yield calculation, bioenergetics, and regulation of the cycle. These decarboxylations link carbohydrate catabolism to electron transport, ATP generation, and metabolic integration. Knowledge of decarboxylation frequency informs metabolic studies, physiological understanding, and enzyme regulation in mitochondrial respiration.

10. Choose the Correct Statements MCQ
Statement I: TCA cycle generates NADH, FADH2, and GTP/ATP.
Statement II: Decarboxylation occurs three times including pyruvate → acetyl-CoA.
Options:
A. Both I and II are correct
B. Only I is correct
C. Only II is correct
D. Both I and II are incorrect
Answer & Explanation: Option A. The TCA cycle produces three NADH, one FADH2, and one GTP/ATP per acetyl-CoA. Decarboxylation occurs three times: once in pyruvate conversion to acetyl-CoA and twice within the cycle. Understanding energy generation and CO2 release is crucial for metabolism studies. These concepts connect glycolysis, TCA, and electron transport, informing ATP yield calculations, metabolic flux analysis, and cellular bioenergetics.

Chapter: Cellular Respiration; Topic: Energy Metabolism; Subtopic: Pathways and Enzymes

Keyword Definitions:

• Oxidative decarboxylation: Process by which a carboxyl group is removed from a molecule and CO₂ is released along with electron transfer to NAD⁺.

• Glycolysis: Metabolic pathway converting glucose to pyruvate, producing ATP and NADH.

• Oxidative phosphorylation: Production of ATP using energy from electron transport chain and proton gradient across mitochondria.

• Tricarboxylic acid (TCA) cycle: Series of enzymatic reactions oxidizing acetyl-CoA to CO₂, producing NADH, FADH₂, and GTP/ATP.

• Citrate synthase: Enzyme catalyzing first step of TCA cycle, condensing acetyl-CoA with oxaloacetate.

• Pyruvate dehydrogenase: Enzyme complex converting pyruvate to acetyl-CoA and CO₂, linking glycolysis and TCA cycle.

• Electron transport system (ETS): Series of protein complexes in mitochondria transferring electrons from NADH/FADH₂ to oxygen, generating ATP.

• EMP pathway: Embden-Meyerhof-Parnas pathway, the classical glycolysis route.

• ATP synthesis: Formation of ATP molecules via substrate-level or oxidative phosphorylation.

• NADH/FADH₂: Electron carriers produced in glycolysis, TCA cycle, and oxidation reactions.

• Energy metabolism: The process of converting nutrients into cellular energy (ATP) through biochemical pathways.

Lead Question - 2023:

Match List - I with List -II:
A. Oxidative decarboxylation I. Citrate synthase
B. Glycolysis II. Pyruvate dehydrogenase
C. Oxidative phosphorylation III. Electron transport system
D. Tricarboxylic acid cycle IV. EMP pathway
Choose the correct answer from the options given below:
(1) A-III, B-I, C-II, D-IV
(2) A-II, B-IV, C-III, D-I
(3) A-III, B-IV, C-II, D-I
(4) A-II, B-IV, C-I, D-III

Answer & Explanation: (2) A-II, B-IV, C-III, D-I. Oxidative decarboxylation (A) is catalyzed by Pyruvate dehydrogenase (II), converting pyruvate to acetyl-CoA. Glycolysis (B) follows the EMP pathway (IV). Oxidative phosphorylation (C) is carried out by the electron transport system (III), generating ATP. Tricarboxylic acid cycle (D) begins with citrate synthase (I). These steps are sequentially interconnected in cellular respiration: glucose is converted to pyruvate, then acetyl-CoA enters TCA cycle, producing NADH/FADH₂, which feed electrons into ETS for ATP synthesis. Understanding these enzyme-pathway relationships is crucial for studying bioenergetics, metabolism, and regulation of cellular energy.

1. The first step of glycolysis involves:
(1) Phosphorylation of glucose
(2) Formation of pyruvate
(3) Conversion of fructose-1,6-bisphosphate
(4) Reduction of NAD⁺
Explanation: Glycolysis begins with phosphorylation of glucose to glucose-6-phosphate using ATP. This primes glucose for subsequent breakdown to pyruvate. Correct answer is (1).

2. Pyruvate dehydrogenase complex produces:
(1) ATP
(2) Acetyl-CoA, CO₂, and NADH
(3) Pyruvate
(4) FADH₂
Explanation: Pyruvate dehydrogenase converts pyruvate to acetyl-CoA, releasing CO₂ and generating NADH. This links glycolysis to TCA cycle. Correct answer is (2).

3. The main site of oxidative phosphorylation is:
(1) Cytoplasm
(2) Mitochondrial matrix
(3) Mitochondrial inner membrane
(4) Nucleus
Explanation: Oxidative phosphorylation occurs in the inner mitochondrial membrane, where ETS complexes generate a proton gradient used by ATP synthase to produce ATP. Correct answer is (3).

4. TCA cycle is also known as:
(1) Glycolysis
(2) Krebs cycle
(3) Calvin cycle
(4) Fermentation
Explanation: The tricarboxylic acid (TCA) cycle, also called Krebs cycle, oxidizes acetyl-CoA to CO₂ while producing NADH, FADH₂, and ATP/GTP. Correct answer is (2).

5. Electron transport chain generates ATP by:
(1) Substrate-level phosphorylation
(2) Oxidative phosphorylation
(3) Glycolysis
(4) Fermentation
Explanation: ATP is produced via oxidative phosphorylation, where electrons from NADH/FADH₂ pass through ETS creating a proton gradient that drives ATP synthase. Correct answer is (2).

6. Citrate synthase catalyzes:
(1) Pyruvate to acetyl-CoA
(2) Acetyl-CoA + oxaloacetate to citrate
(3) NADH oxidation
(4) ATP synthesis
Explanation: Citrate synthase catalyzes condensation of acetyl-CoA with oxaloacetate to form citrate, the first step of TCA cycle. Correct answer is (2).

Assertion-Reason Type Question
7. Assertion (A): Glycolysis occurs in cytoplasm.
Reason (R): It does not require mitochondria.
(1) Both A and R true, R explains A
(2) Both A and R true, R does not explain A
(3) A true, R false
(4) A false, R true
Explanation: Glycolysis occurs in cytoplasm and does not require mitochondria, making both statements true and R correctly explains A. Correct answer is (1).

Matching Type Question
8. Match pathway with product:
A. Glycolysis – i. ATP and NADH
B. TCA cycle – ii. CO₂, NADH, FADH₂
C. Oxidative phosphorylation – iii. ATP
D. Pyruvate dehydrogenase – iv. Acetyl-CoA and NADH
(1) A-i, B-ii, C-iii, D-iv
(2) A-ii, B-i, C-iv, D-iii
(3) A-iii, B-iv, C-i, D-ii
(4) A-iv, B-iii, C-ii, D-i
Explanation: Glycolysis produces ATP and NADH; TCA cycle produces CO₂, NADH, FADH₂; oxidative phosphorylation produces ATP; pyruvate dehydrogenase produces acetyl-CoA and NADH. Correct answer is (1).

Fill in the Blanks Question
9. The final electron acceptor in oxidative phosphorylation is ________.
(1) NAD⁺
(2) FAD
(3) O₂
(4) ATP
Explanation: Oxygen serves as the terminal electron acceptor in the electron transport chain, forming water after receiving electrons. Correct answer is (3).

Choose the Correct Statements Question
10. Statement I: Pyruvate dehydrogenase links

Chapter: Cellular Respiration; Topic: Glycolysis; Subtopic: ATP Utilization

Keyword Definitions:

• Glycolysis: Metabolic pathway in cytoplasm converting glucose into pyruvate with net ATP and NADH production.

• ATP (Adenosine Triphosphate): Energy currency of the cell used in biosynthetic and metabolic reactions.

• Glucose-6-phosphate: Phosphorylated form of glucose, formed by hexokinase enzyme using ATP.

• Fructose-6-phosphate: Intermediate in glycolysis converted to fructose-1,6-bisphosphate by phosphofructokinase.

• Fructose-1,6-bisphosphate: Product of second ATP-utilizing step in glycolysis, split into two triose phosphates.

• Hexokinase: Enzyme catalyzing first phosphorylation of glucose in glycolysis.

• Phosphofructokinase: Key regulatory enzyme in glycolysis, catalyzing fructose-6-phosphate phosphorylation.

• Energy Investment Phase: Initial steps of glycolysis where ATP is consumed.

• Triose Phosphates: Glyceraldehyde-3-phosphate and dihydroxyacetone phosphate formed from cleavage of fructose-1,6-bisphosphate.

• Net ATP: Difference between ATP generated and consumed in glycolysis.

• Substrate-Level Phosphorylation: Direct ATP synthesis from phosphorylated intermediates during glycolysis.

Lead Question - 2023:

Given below are two statements: One is labelled as Assertion A and the other as Reason R:
Assertion A: ATP is used at two steps in glycolysis.
Reason R: First ATP is used in converting glucose into glucose-6-phosphate and second ATP is used in conversion of fructose-6-phosphate into fructose-1,6-diphosphate.
Choose the correct answer:
(1) A is true but R is false
(2) A is false but R is true
(3) Both A and R are true and R is the correct explanation of A
(4) Both A and R are true but R is not the correct explanation of A

Answer & Explanation: (3) Both A and R are true and R is the correct explanation of A. In glycolysis, the energy investment phase requires two ATP molecules. The first ATP phosphorylates glucose to glucose-6-phosphate catalyzed by hexokinase. The second ATP phosphorylates fructose-6-phosphate to fructose-1,6-bisphosphate via phosphofructokinase. These steps trap glucose in the cell and commit it to glycolysis. Thus, ATP is indeed used at two distinct steps, and the provided reason accurately describes these phosphorylations, making R a correct explanation of the assertion.

1. In glycolysis, which enzyme catalyzes the first ATP-consuming step?
(1) Phosphofructokinase
(2) Hexokinase
(3) Pyruvate kinase
(4) Aldolase
Explanation: Hexokinase phosphorylates glucose to glucose-6-phosphate using ATP, marking the first ATP-consuming step in glycolysis. Correct answer is (2).

2. The second ATP is consumed during conversion of:
(1) Glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate
(2) Fructose-6-phosphate to fructose-1,6-bisphosphate
(3) Glucose to glucose-6-phosphate
(4) Phosphoenolpyruvate to pyruvate
Explanation: Phosphofructokinase catalyzes phosphorylation of fructose-6-phosphate to fructose-1,6-bisphosphate using ATP. Correct answer is (2).

3. The energy-investment phase of glycolysis consumes how many ATP molecules?
(1) 1
(2) 2
(3) 3
(4) 4
Explanation: Two ATP molecules are invested initially in glycolysis to phosphorylate glucose and fructose-6-phosphate. Correct answer is (2).

4. What is the product after fructose-6-phosphate phosphorylation?
(1) Glucose-6-phosphate
(2) Fructose-1,6-bisphosphate
(3) Glyceraldehyde-3-phosphate
(4) Pyruvate
Explanation: Fructose-6-phosphate is phosphorylated by phosphofructokinase to form fructose-1,6-bisphosphate. Correct answer is (2).

5. Which phase of glycolysis involves ATP consumption?
(1) Energy pay-off phase
(2) Energy investment phase
(3) Oxidative phosphorylation phase
(4) Fermentation phase
Explanation: The initial energy-investment phase consumes two ATP molecules to phosphorylate glucose intermediates. Correct answer is (2).

6. Which enzyme is key regulatory step in glycolysis consuming ATP?
(1) Pyruvate kinase
(2) Phosphoglycerate kinase
(3) Phosphofructokinase
(4) Triose phosphate isomerase
Explanation: Phosphofructokinase is the key regulatory enzyme consuming ATP during phosphorylation of fructose-6-phosphate. Correct answer is (3).

Assertion-Reason Type Question
7. Assertion (A): ATP is required in the initial steps of glycolysis.
Reason (R): Phosphorylation of intermediates traps glucose in the cell and commits it to glycolysis.
(1) Both A and R are true, and R is correct explanation of A
(2) Both A and R are true, but R is not correct explanation of A
(3) A is true, R is false
(4) A is false, R is true
Explanation: ATP phosphorylates glucose and fructose-6-phosphate, trapping glucose in the cytoplasm. Both statements are true, R correctly explains A. Correct answer is (1).

Matching Type Question
8. Match the enzyme with its ATP-related function:
A. Hexokinase – (i) First ATP-consuming step
B. Phosphofructokinase – (ii) Second ATP-consuming step
C. Glyceraldehyde-3-phosphate dehydrogenase – (iii) ATP generating step
D. Pyruvate kinase – (iv) ATP generating step
(1) A-(i), B-(ii), C-(iii), D-(iv)
(2) A-(ii), B-(i), C-(iv), D-(iii)
(3) A-(i), B-(ii), C-(iv), D-(iii)
(4) A-(iii), B-(iv), C-(i), D-(ii)
Explanation: Hexokinase and phosphofructokinase consume ATP; GAPDH indirectly leads to substrate-level phosphorylation; pyruvate kinase generates ATP. Correct answer is (1).

Fill in the Blanks Question
9. In glycolysis, ATP is first used by _______ enzyme.
(1) Hexokinase
(2) Phosphofructokinase
(3) Pyruvate kinase
(4) Aldolase
Explanation: Hexokinase catalyzes the first ATP-consuming phosphorylation of glucose. Correct answer is (1).

Choose the Correct Statements Question
10. Statement I: Two ATP molecules are consumed in the energy-investment phase of glycolysis.
Statement II: ATP is only generated, not consumed, in glycolysis.
(1) Both statements are true
(2) Statement I true, Statement II false
(3) Statement I false, Statement II true
(4) Both statements are false
Explanation: Two ATP molecules are consumed initially, while later steps generate ATP. Statement I is true, Statement II is false. Correct answer is (2).

Topic: Cellular Respiration; Subtopic: Tricarboxylic Acid Cycle (TCA Cycle)

Keyword Definitions:

• TCA Cycle: Also called Krebs cycle or citric acid cycle; central metabolic pathway producing energy through oxidation of acetyl-CoA.

• Ketoglutaric acid: 5-carbon intermediate of TCA cycle, also known as α-ketoglutarate.

• Oxalosuccinic acid: 6-carbon intermediate formed during TCA cycle before decarboxylation to α-ketoglutarate.

• Succinic acid: 4-carbon intermediate of TCA cycle.

• Fumaric acid: 4-carbon intermediate formed from succinate oxidation.

• Decarboxylation: Removal of a carboxyl group as CO₂ during metabolic reactions.

• Oxidation: Loss of electrons or hydrogen during metabolism.

• Coenzyme A (CoA): Carrier molecule that activates acetyl groups for entry into TCA cycle.

• ATP: Primary energy currency generated in cellular respiration.

• NAD⁺/FAD: Electron carriers in TCA cycle.

• Intermediates: Compounds formed during steps of a metabolic pathway.

Lead Question - 2022 (Ganganagar)

The 5-C compound formed during TCA cycle is:

1. ketoglutaric acid

2. Oxalo succinic acid

3. Succinic acid

4. Fumaric acid

Explanation: During the TCA cycle, isocitrate (6-C) undergoes oxidative decarboxylation to form α-ketoglutarate, a 5-carbon compound. Oxalosuccinic acid is the transient 6-C intermediate before decarboxylation. Succinic acid and fumaric acid are 4-C intermediates formed later. α-Ketoglutarate is crucial for amino acid synthesis and energy production. The correct answer is 1. ketoglutaric acid. This step is catalyzed by isocitrate dehydrogenase, generating NADH and CO₂, linking energy production with metabolic intermediates for biosynthetic pathways.

1. Single Correct Answer MCQ:

Which enzyme catalyzes the formation of α-ketoglutarate from isocitrate?

a) Citrate synthase

b) Isocitrate dehydrogenase

c) α-Ketoglutarate dehydrogenase

d) Succinate dehydrogenase

Explanation: Isocitrate dehydrogenase catalyzes oxidative decarboxylation of isocitrate to α-ketoglutarate, producing NADH and CO₂. Citrate synthase forms citrate, α-ketoglutarate dehydrogenase converts α-ketoglutarate to succinyl-CoA, and succinate dehydrogenase acts later. Correct answer is b) Isocitrate dehydrogenase. This step is rate-limiting and regulates TCA cycle flux.

2. Single Correct Answer MCQ:

How many carbon atoms does α-ketoglutarate have?

a) 4

b) 5

c) 6

d) 3

Explanation: α-Ketoglutarate contains 5 carbon atoms, formed by decarboxylation of 6-C isocitrate in the TCA cycle. Succinyl-CoA and subsequent intermediates are 4-C compounds. Proper carbon counting is essential for understanding metabolic flux. Correct answer is b) 5.

3. Single Correct Answer MCQ:

Which coenzyme is required for the conversion of isocitrate to α-ketoglutarate?

a) NAD⁺

b) FAD

c) Coenzyme A

d) NADP⁺

Explanation: NAD⁺ acts as an electron acceptor during oxidative decarboxylation of isocitrate to α-ketoglutarate, producing NADH. FAD participates in later steps, CoA activates succinyl intermediates, and NADP⁺ is not involved in this TCA step. Correct answer is a) NAD⁺.

4. Single Correct Answer MCQ:

Which intermediate precedes α-ketoglutarate in TCA cycle?

a) Citrate

b) Oxalosuccinate

c) Succinyl-CoA

d) Fumarate

Explanation: Oxalosuccinate is the unstable 6-carbon intermediate formed from isocitrate before decarboxylation to α-ketoglutarate. Citrate is converted to isocitrate, Succinyl-CoA follows α-ketoglutarate, and fumarate comes later. Correct answer is b) Oxalosuccinate.

5. Single Correct Answer MCQ:

Which 4-C intermediate is formed after α-ketoglutarate?

a) Succinic acid

b) Fumaric acid

c) Oxaloacetate

d) Malic acid

Explanation: α-Ketoglutarate undergoes oxidative decarboxylation to form succinyl-CoA, which converts to succinic acid, a 4-carbon intermediate. Fumarate, malate, and oxaloacetate appear later. Correct answer is a) Succinic acid.

6. Single Correct Answer MCQ:

Which TCA cycle step produces CO₂ during α-ketoglutarate formation?

a) Citrate synthase reaction

b) Isocitrate to α-ketoglutarate

c) Succinyl-CoA to succinate

d) Malate to oxaloacetate

Explanation: Oxidative decarboxylation of isocitrate to α-ketoglutarate releases CO₂. Citrate synthase does not release CO₂, Succinyl-CoA to succinate generates GTP, and malate to oxaloacetate produces NADH but no CO₂. Correct answer is b) Isocitrate to α-ketoglutarate.

7. Assertion-Reason MCQ:

Assertion (A): α-Ketoglutarate is a 5-carbon intermediate in TCA cycle.

Reason (R): Formed by oxidative decarboxylation of isocitrate catalyzed by isocitrate dehydrogenase.

a) Both A and R are true, R explains A

b) Both A and R are true, R does not explain A

c) A is true, R is false

d) A is false, R is true

Explanation: α-Ketoglutarate contains 5 carbons and is formed from isocitrate via oxidative decarboxylation catalyzed by isocitrate dehydrogenase. This step produces NADH and CO₂. Both the assertion and reason are correct, and the reason explains the assertion. Correct answer is a) Both A and R are true, R explains A.

8. Matching Type MCQ:

Match the intermediates with their carbon numbers:

Column I

A) Citrate

B) α-Ketoglutarate

C) Succinic acid

D) Oxaloacetate

Column II

1) 4-C

2) 6-C

3) 5-C

4) 4-C

Choices:

A-__ B-__ C-__ D-__

Explanation: Citrate is 6-C (A-2), α-Ketoglutarate is 5-C (B-3), Succinic acid is 4-C (C-1), and Oxaloacetate is 4-C (D-4). Understanding carbon numbering is crucial for mapping TCA cycle steps. Correct matches: A-2, B-3, C-1, D-4.

9. Fill in the Blanks / Completion MCQ:

The enzyme catalyzing the formation of succinyl-CoA from α-ketoglutarate is __________.

a) Isocitrate dehydrogenase

b) α-Ketoglutarate dehydrogenase

c) Succinyl-CoA synthetase

d) Malate dehydrogenase

Explanation: α-Ketoglutarate dehydrogenase catalyzes oxidative decarboxylation of α-ketoglutarate to succinyl-CoA, producing NADH and CO₂. Isocitrate dehydrogenase acts earlier, succinyl-CoA synthetase converts succinyl-CoA to succinate, and malate dehydrogenase catalyzes malate to oxaloacetate. Correct answer is b) α-Ketoglutarate dehydrogenase.

10. Choose the correct statements MCQ (Statement I & II):

Statement I: α-Ketoglutarate is a 5-carbon TCA intermediate.

Statement II: Its formation involves oxidative decarboxylation producing NADH and CO₂.

a) Both I and II are correct

b) Only I is correct

c) Only II is correct

d) Both are incorrect

Explanation: α-Ketoglutarate contains 5 carbons and is produced by oxidative decarboxylation of isocitrate, generating NADH and CO₂. Both statements accurately describe this TCA cycle step. Hence, correct answer is a) Both I and II are correct. Subtopic: Electron Transport Chain

• Cytochrome: Heme-containing protein involved in electron transport within mitochondria or chloroplasts.

• Electron Transport Chain (ETC): Series of protein complexes that transfer electrons to generate a proton gradient for ATP synthesis.

• Complex III: Also called cytochrome bc1 complex, transfers electrons from ubiquinol to cytochrome c.

• Complex IV: Also called cytochrome oxidase, transfers electrons to oxygen forming water.

• Mobile Carrier: Molecule that shuttles electrons between protein complexes in the ETC.

• Proton Gradient: Electrochemical gradient formed by movement of protons across a membrane during electron transport.

• ATP Synthesis: Production of ATP by ATP synthase utilizing proton gradient.

• Cytochrome c: Small soluble heme protein that transfers electrons between complex III and IV.

• Redox Reaction: Reaction involving transfer of electrons from one molecule to another.

• Oxidative Phosphorylation: ATP production coupled to electron transport in mitochondria.

• Heme Group: Iron-containing prosthetic group in cytochromes responsible for electron transfer.

Lead Question - 2022 (Abroad)

Identify the cytochrome which acts as a mobile carrier for the transfer of electrons between complex III and IV?

1. Cytochrome a

2. Cytochrome a3

3. Cytochrome b c1

4. Cytochrome c

Explanation: Cytochrome c is a small, soluble heme protein that acts as a mobile carrier transferring electrons from complex III (cytochrome bc1) to complex IV (cytochrome oxidase) in the mitochondrial electron transport chain. This step is essential for proton gradient formation and ATP synthesis. Answer: Cytochrome c. Answer: 4

Q1: Which complex in the ETC reduces oxygen to water?

1. Complex I

2. Complex II

3. Complex III

4. Complex IV

Explanation: Complex IV, also called cytochrome oxidase, accepts electrons from cytochrome c and reduces molecular oxygen to water. This step is the final electron transfer in the mitochondrial ETC and is crucial for maintaining the proton gradient required for ATP synthesis. Answer: Complex IV. Answer: 4

Q2: The primary function of cytochrome b c1 complex is:

1. Electron transfer from NADH to oxygen

2. Electron transfer from ubiquinol to cytochrome c

3. Proton pumping from matrix to cytosol

4. ATP synthesis

Explanation: Cytochrome bc1 complex (Complex III) transfers electrons from ubiquinol to cytochrome c while pumping protons across the inner mitochondrial membrane. This creates an electrochemical gradient utilized for ATP synthesis. Answer: Electron transfer from ubiquinol to cytochrome c. Answer: 2

Q3: Which cytochromes are part of Complex IV?

1. Cytochrome a and a3

2. Cytochrome b and c1

3. Cytochrome c only

4. Cytochrome a and c

Explanation: Complex IV contains cytochrome a and cytochrome a3 as electron carriers. They facilitate the final electron transfer to oxygen, forming water. Cytochrome c is the mobile carrier delivering electrons from Complex III to IV. Answer: Cytochrome a and a3. Answer: 1

Q4: Which of the following is soluble in the intermembrane space?

1. Cytochrome b

2. Cytochrome c

3. Cytochrome a

4. Ubiquinone

Explanation: Cytochrome c is a small, soluble protein in the intermembrane space of mitochondria. It acts as a mobile electron carrier between Complex III and Complex IV, facilitating electron transport while maintaining proton gradient formation for ATP synthesis. Answer: Cytochrome c. Answer: 2

Q5: Ubiquinone transfers electrons between which complexes?

1. Complex I and II to III

2. Complex III to IV

3. Complex IV to oxygen

4. Complex II to IV

Explanation: Ubiquinone (coenzyme Q) transfers electrons from Complex I and II to Complex III in the ETC. It is a lipid-soluble molecule embedded in the inner mitochondrial membrane and participates in proton pumping. Answer: Complex I and II to III. Answer: 1

Q6: What is the role of proton pumping in ETC?

1. Generate ATP directly

2. Create a proton gradient across inner membrane

3. Reduce oxygen to water

4. Transport electrons to cytochrome c

Explanation: Proton pumping by Complexes I, III, and IV generates an electrochemical proton gradient across the inner mitochondrial membrane. This proton motive force drives ATP synthesis via ATP synthase. It does not directly reduce oxygen or transport electrons. Answer: Create a proton gradient across inner membrane. Answer: 2

Q7: Assertion (A): Cytochrome c is water-soluble.
Reason (R): It acts as a mobile carrier in the intermembrane space.

1. A is correct but R is not correct

2. A is not correct but R is correct

3. Both A and R are correct and R explains A

4. Both A and R are correct but R does not explain A

Explanation: Cytochrome c is a small water-soluble protein located in the intermembrane space. It functions as a mobile electron carrier between Complex III and IV. Both assertion and reason are correct, and the reason explains why its solubility is important. Answer: Both A and R are correct and R explains A. Answer: 3

Q8: Match the complex with its main components:

1. Complex I    A. NADH dehydrogenase
2. Complex II    B. Succinate dehydrogenase
3. Complex III    C. Cytochrome b c1
4. Complex IV    D. Cytochrome a, a3

1. 1-A, 2-B, 3-C, 4-D

2. 1-B, 2-A, 3-D, 4-C

3. 1-C, 2-D, 3-A, 4-B

4. 1-D, 2-C, 3-B, 4-A

Explanation: Complex I contains NADH dehydrogenase, Complex II has succinate dehydrogenase, Complex III contains cytochrome bc1, and Complex IV contains cytochrome a and a3. This correct matching shows component distribution in mitochondrial ETC. Answer: 1-A, 2-B, 3-C, 4-D. Answer: 1

Q9: The final electron acceptor in mitochondrial ETC is _______.

1. NAD+

2. FAD

3. Oxygen

4. Water

Explanation: Oxygen acts as the final electron acceptor in the mitochondrial electron transport chain. It accepts electrons from Complex IV via cytochrome a and a3 and combines with protons to form water, completing oxidative phosphorylation. Answer: Oxygen. Answer: 3

Q10: Select correct statements about cytochrome c:

1. It is water-soluble

2. Transfers electrons from Complex III to IV

3. Contains heme group

4. Located in intermembrane space

Explanation: Cytochrome c is a water-soluble protein in the intermembrane

Subtopic: Electron Transport System (ETS)

• ETS Complex: Electron Transport System complexes in mitochondria facilitating oxidative phosphorylation.

• Complex I (NADH dehydrogenase): First complex accepting electrons from NADH.

• Complex II (Succinate dehydrogenase): Accepts electrons from FADH2 and passes to ubiquinone.

• Complex III (Cytochrome bc1): Transfers electrons from ubiquinol to cytochrome c, containing Cyt b and Cyt c1.

• Complex IV (Cytochrome oxidase): Transfers electrons to oxygen, contains Cyt a, a3 and copper centres.

• Ubiquinone: Mobile electron carrier between complexes I/II and III.

• Cytochrome c: Mobile protein transferring electrons from complex III to IV.

• Oxidative Phosphorylation: ATP synthesis coupled to electron transport in mitochondria.

• NADH: Reduced coenzyme donating electrons to complex I.

• FADH2: Reduced coenzyme donating electrons to complex II.

• Mitochondrial Respiration: Process generating ATP via ETS and chemiosmosis.

Lead Question - 2022 (Abroad)

Match List-I with List-II

List-I
(a) ETS complex-I
(b) ETS complex-II
(c) ETS complex-III
(d) ETS complex-IV

List-II
(i) Cyt b1 dehydrogenase
(ii) Cyt a, a3 and 2 copper centres
(iii) NADH dehydrogenase
(iv) Ubiquinone and FADH dehydrogenase

Choose the correct answer from the options given below:

1. (a)-(ii), (b)-(i), (c)-(iv), (d)-(iii)

2. (a)-(iv), (b)-(iii), (c)-(ii), (d)-(i)

3. (a)-(iii), (b)-(iv), (c)-(i), (d)-(ii)

4. (a)-(iii), (b)-(iv), (c)-(ii), (d)-(i)

Explanation: In mitochondria, ETS complex-I is NADH dehydrogenase, accepting electrons from NADH. Complex-II contains FADH and ubiquinone. Complex-III contains cytochrome b1, transferring electrons to cytochrome c. Complex-IV contains cytochrome a, a3 and copper centres, transferring electrons to oxygen. Correct matching: a-III, b-IV, c-I, d-II. Answer: 4

Q1: The primary function of ETS is:

1. Glycolysis

2. ATP synthesis

3. Protein synthesis

4. Lipid metabolism

Explanation: Electron Transport System transfers electrons through complexes I-IV to generate proton gradient, driving ATP synthesis via ATP synthase. It is central to aerobic respiration. Other options like glycolysis, protein synthesis, or lipid metabolism are not direct functions of ETS. Answer: ATP synthesis. Answer: 2

Q2: Electrons from FADH2 enter ETS through:

1. Complex I

2. Complex II

3. Complex III

4. Complex IV

Explanation: FADH2 donates electrons directly to complex II (succinate dehydrogenase). This bypasses complex I, producing fewer ATP molecules per FADH2 compared to NADH. Complex III and IV receive electrons downstream. Answer: Complex II. Answer: 2

Q3: Cytochrome c transfers electrons between:

1. Complex I to II

2. Complex II to IV

3. Complex III to IV

4. Complex I to III

Explanation: Cytochrome c is a mobile electron carrier transferring electrons from complex III (cytochrome bc1) to complex IV (cytochrome oxidase). It is not involved in transfers from complex I or II directly. Answer: Complex III to IV. Answer: 3

Q4: Ubiquinone is:

1. Protein carrier

2. Lipid-soluble electron carrier

3. Enzyme complex

4. ATP synthase subunit

Explanation: Ubiquinone (coenzyme Q) is lipid-soluble, carrying electrons from complexes I and II to complex III within the inner mitochondrial membrane. It is not a protein, enzyme complex, or ATP synthase component. Answer: Lipid-soluble electron carrier. Answer: 2

Q5: Complex IV contains which key prosthetic groups?

1. Cyt b, Fe-S

2. Cyt a, a3, Cu

3. FADH2, Ubiquinone

4. ATP, ADP

Explanation: Complex IV (cytochrome oxidase) contains cytochromes a and a3 and copper centers which transfer electrons to molecular oxygen forming water. FADH2, ubiquinone, or ATP/ADP are not prosthetic groups of complex IV. Answer: Cyt a, a3, Cu. Answer: 2

Q6: NADH dehydrogenase is also called:

1. Complex I

2. Complex II

3. Complex III

4. Complex IV

Explanation: NADH dehydrogenase is complex I of ETS, receiving electrons from NADH and passing them to ubiquinone. Complex II handles FADH2; III and IV are downstream electron carriers. Answer: Complex I. Answer: 1

Q7: Assertion (A): Complex II contributes to proton gradient.
Reason (R): Electrons from FADH2 enter complex II.

1. A is correct but R is not correct

2. A is not correct but R is correct

3. Both A and R are correct and R explains A

4. Both A and R are correct but R does not explain A

Explanation: Complex II transfers electrons from FADH2 to ubiquinone but does not pump protons across the membrane, thus contributing minimally to proton gradient. Assertion is incorrect, Reason is correct. Answer: A is not correct but R is correct. Answer: 2

Q8: Match ETS complexes with primary electron donor:

1. Complex I    A. FADH2
2. Complex II    B. NADH
3. Complex III    C. Ubiquinol
4. Complex IV    D. Cytochrome c

1. 1-B, 2-A, 3-C, 4-D

2. 1-A, 2-B, 3-D, 4-C

3. 1-B, 2-A, 3-D, 4-C

4. 1-C, 2-D, 3-A, 4-B

Explanation: Complex I receives electrons from NADH, complex II from FADH2, complex III from ubiquinol, and complex IV from cytochrome c. Matching is: 1-B, 2-A, 3-C, 4-D. Answer: 1

Q9: Final electron acceptor in ETS is ______.

1. NAD+

2. FAD

3. Oxygen

4. Ubiquinone

Explanation: In mitochondrial respiration, oxygen is the final electron acceptor, combining with electrons and protons to form water. NAD+, FAD, and ubiquinone are intermediate carriers. Answer: Oxygen. Answer: 3

Q10: Choose correct statements about ETS:

1. Complex I oxidizes NADH

2. Complex II oxidizes FADH2

3. Complex IV reduces oxygen

4. ETS directly synthesizes ATP without gradient

Explanation: ETS complexes oxidize NADH/FADH2 and reduce oxygen at complex IV. Proton gradient generated drives ATP synthesis indirectly via ATP synthase. Direct ATP synthesis without gradient is incorrect. Correct statements: 1, 2, 3. Answer: 1, 2, 3

Topic: Glycolysis
Subtopic: ATP Yield from Glucose

Keyword Definitions:

• ATP (Adenosine Triphosphate): Primary energy currency of the cell used in metabolic processes.

• Glucose: Six-carbon monosaccharide that is the starting molecule in glycolysis.

• Pyruvic acid (Pyruvate): Three-carbon molecule formed at the end of glycolysis from glucose.

• Glycolysis: Cytoplasmic pathway that breaks one molecule of glucose into two molecules of pyruvate, producing ATP.

• Net gain: Total ATP produced minus ATP consumed during glycolysis.

• NADH: Reduced electron carrier produced during glycolysis, used in oxidative phosphorylation.

• Substrate-level phosphorylation: Direct synthesis of ATP by transferring phosphate groups from intermediates to ADP.

• Energy investment phase: First phase of glycolysis consuming 2 ATP molecules.

• Energy payoff phase: Second phase of glycolysis producing 4 ATP molecules per glucose.

• Anaerobic conditions: Glycolysis occurs without oxygen, producing ATP and pyruvate or lactate.

Lead Question (2022)
What is the net gain of ATP when each molecule of glucose is converted to two molecules of pyruvic acid?
(1) Six
(2) Two
(3) Eight
(4) Four

Explanation:
During glycolysis, one glucose molecule is converted into two pyruvate molecules. Two ATP molecules are consumed initially, while four ATP molecules are produced later. Therefore, the net gain is 4 – 2 = 2 ATP molecules per glucose. Correct answer is (2).

1. Single Correct Answer MCQ:
Which of the following occurs in the cytoplasm of a cell?
(1) Krebs cycle
(2) Glycolysis
(3) Electron transport chain
(4) Oxidative phosphorylation

Explanation:
Glycolysis occurs in the cytoplasm, converting glucose to pyruvate and producing ATP. Krebs cycle and electron transport chain occur in mitochondria. Oxidative phosphorylation occurs at the inner mitochondrial membrane. Correct answer is (2).

2. Single Correct Answer MCQ:
During glycolysis, how many NADH molecules are produced per glucose?
(1) One
(2) Two
(3) Three
(4) Four

Explanation:
Each glucose molecule generates two molecules of NADH during glycolysis, which later enter the mitochondria for oxidative phosphorylation. Correct answer is (2).

3. Single Correct Answer MCQ:
Glycolysis is considered an anaerobic process because:
(1) It requires oxygen
(2) It occurs without oxygen
(3) It produces carbon dioxide
(4) It occurs in mitochondria

Explanation:
Glycolysis does not require oxygen and occurs in the cytoplasm. It can produce ATP under anaerobic conditions. Oxygen is required later in mitochondrial respiration. Correct answer is (2).

4. Single Correct Answer MCQ:
Which enzyme catalyzes the conversion of glucose to glucose-6-phosphate?
(1) Hexokinase
(2) Phosphofructokinase
(3) Pyruvate kinase
(4) Aldolase

Explanation:
Hexokinase catalyzes the first step of glycolysis, phosphorylating glucose to glucose-6-phosphate, using one ATP molecule. Other enzymes act in later steps. Correct answer is (1).

5. Single Correct Answer MCQ:
The net ATP gain by substrate-level phosphorylation in glycolysis per glucose is:
(1) 1
(2) 2
(3) 3
(4) 4

Explanation:
During glycolysis, 4 ATP molecules are produced by substrate-level phosphorylation, but 2 ATP are used in the energy investment phase. Net ATP gain is 4 – 2 = 2. Correct answer is (2).

6. Single Correct Answer MCQ:
Which glycolysis product can enter mitochondria for further energy extraction?
(1) NAD+
(2) Pyruvate
(3) ADP
(4) Glucose-6-phosphate

Explanation:
Pyruvate generated in glycolysis is transported into mitochondria for the Krebs cycle and oxidative phosphorylation. NAD+ and ADP are cofactors, and glucose-6-phosphate remains in the cytoplasm. Correct answer is (2).

7. Assertion-Reason MCQ:
Assertion (A): Glycolysis produces a net gain of 2 ATP per glucose.
Reason (R): Four ATP are produced and two ATP are consumed during glycolysis.
Options:
(1) Both A and R are correct, R explains A
(2) A correct, R incorrect
(3) A incorrect, R correct
(4) Both A and R incorrect

Explanation:
Glycolysis consumes 2 ATP and produces 4 ATP, giving a net gain of 2 ATP per glucose. Reason accurately explains the assertion. Correct answer is (1).

8. Matching Type MCQ:
Match glycolysis intermediates with ATP usage:
A. Glucose → Glucose-6-phosphate — 1. ATP consumed
B. Fructose-6-phosphate → Fructose-1,6-bisphosphate — 2. ATP consumed
C. 1,3-Bisphosphoglycerate → 3-Phosphoglycerate — 3. ATP produced
D. Phosphoenolpyruvate → Pyruvate — 4. ATP produced
Options:
(1) A–1, B–2, C–3, D–4
(2) A–2, B–1, C–4, D–3
(3) A–1, B–2, C–4, D–3
(4) A–3, B–4, C–1, D–2

Explanation:
ATP is consumed during phosphorylation steps (A–1, B–2) and produced during substrate-level phosphorylation (C–3, D–4). Correct answer is (1).

9. Fill in the Blanks MCQ:
The final product of glycolysis is ________, which can undergo fermentation or enter mitochondria.
(1) Lactate
(2) Pyruvate
(3) Acetyl-CoA
(4) Glucose-6-phosphate

Explanation:
Glycolysis ends with pyruvate. Under anaerobic conditions, pyruvate undergoes fermentation; under aerobic conditions, it enters mitochondria as acetyl-CoA. Lactate is a fermentation product, not direct glycolysis product. Correct answer is (2).

10. Choose the correct statements MCQ:
(a) Glycolysis occurs in cytoplasm
(b) Net ATP gain per glucose is 2
(c) Oxygen is required
(d) NADH is produced
Options:
(1) a, b, d only
(2) a, c only
(3) b, c, d only
(4) a, b, c, d

Explanation:
Glycolysis occurs in cytoplasm (a), produces net 2 ATP (b), and generates NADH (d). It does not require oxygen (c). Correct answer is (1).

Topic: Fermentation
Subtopic: Lactic Acid Fermentation

Keyword Definitions:

• Glucose: A simple sugar (C6H12O6) and primary energy source for cellular respiration.

• Lactic Acid Fermentation: Anaerobic conversion of glucose into lactic acid, yielding small amounts of energy.

• Anaerobic: Metabolic processes occurring without oxygen.

• ATP: Adenosine triphosphate, the energy currency of the cell.

• Glycolysis: First step in glucose breakdown, producing pyruvate and ATP.

• Energy Yield: Amount of usable chemical energy released during metabolic reactions.

Lead Question (2022)
What amount of energy is released from glucose during lactic acid fermentation?
(1) More than 18%
(2) About 10%
(3) Less than 7%
(4) Approximately 15%

Explanation:
During lactic acid fermentation, glucose is partially oxidized to lactic acid. Only a small fraction of energy stored in glucose is released as ATP, roughly 7%. Most energy remains in lactic acid molecules. Hence, the correct answer is (3).

1. Which step occurs first in lactic acid fermentation?
(1) Krebs cycle
(2) Glycolysis
(3) Electron transport chain
(4) Oxidative phosphorylation

Explanation:
Glycolysis is the initial step in lactic acid fermentation, breaking glucose into two molecules of pyruvate and generating ATP. Pyruvate is then reduced to lactic acid. Hence, the correct answer is (2).

2. The end product of lactic acid fermentation in muscles is:
(1) Ethanol
(2) Carbon dioxide
(3) Lactic acid
(4) Acetyl CoA

Explanation:
In anaerobic conditions, muscle cells convert pyruvate into lactic acid to regenerate NAD+, allowing glycolysis to continue producing ATP. Hence, the correct answer is (3).

3. Lactic acid fermentation occurs in:
(1) Yeast
(2) Animal muscles
(3) Plant roots
(4) Both (2) and (3)

Explanation:
Lactic acid fermentation occurs in animal muscles during oxygen deficit and in some plant tissues like waterlogged roots. Yeast performs alcoholic fermentation. Hence, the correct answer is (4).

4. NAD+ is regenerated during lactic acid fermentation by:
(1) Reducing pyruvate to lactate
(2) Oxidizing glucose
(3) ATP hydrolysis
(4) Electron transport chain

Explanation:
Pyruvate acts as an electron acceptor, converting NADH back to NAD+ while forming lactate. This ensures glycolysis continues under anaerobic conditions. Hence, the correct answer is (1).

5. The ATP yield per glucose in lactic acid fermentation is approximately:
(1) 2 ATP
(2) 30 ATP
(3) 36 ATP
(4) 0 ATP

Explanation:
Lactic acid fermentation generates only 2 ATP molecules per glucose via glycolysis. Most energy remains in lactic acid. Hence, the correct answer is (1).

6. Which enzyme catalyzes the conversion of pyruvate to lactate?
(1) Lactate dehydrogenase
(2) Pyruvate kinase
(3) Hexokinase
(4) Phosphofructokinase

Explanation:
Lactate dehydrogenase reduces pyruvate to lactate using NADH, regenerating NAD+ required for glycolysis. Hence, the correct answer is (1).

7. Assertion-Reason Type:
Assertion (A): Lactic acid fermentation releases little energy.
Reason (R): Pyruvate is not fully oxidized to CO2 and water.
(1) Both A and R are correct, and R explains A
(2) Both A and R are correct, but R does not explain A
(3) A is false but R is true
(4) Both A and R are false

Explanation:
In lactic acid fermentation, pyruvate is partially reduced to lactate, not fully oxidized, resulting in low energy release (~7% of glucose). Hence, both A and R are correct, and R explains A. Correct answer is (1).

8. Matching Type:
Match the organism with fermentation type:
A. Muscle cells — 1. Alcoholic
B. Yeast — 2. Lactic acid
C. Bacteria (Lactobacillus) — 2. Lactic acid
D. Plant root — 2. Lactic acid
Options:
(1) A–2, B–1, C–2, D–2
(2) A–1, B–2, C–1, D–2
(3) A–2, B–2, C–1, D–1
(4) A–1, B–1, C–2, D–2

Explanation:
Muscle cells, Lactobacillus, and some plant roots perform lactic acid fermentation, while yeast undergoes alcoholic fermentation. Correct answer is (1).

9. Fill in the Blanks:
During anaerobic glycolysis, _______ is converted to lactate.
(1) Glucose
(2) Pyruvate
(3) NADH
(4) Acetyl CoA

Explanation:
Pyruvate, formed from glucose via glycolysis, is reduced to lactate in anaerobic conditions to regenerate NAD+. Hence, the correct answer is (2).

10. Choose the Correct Statements:
(a) Lactic acid fermentation is anaerobic.
(b) It produces 2 ATP per glucose.
(c) It converts pyruvate fully to CO2 and H2O.
(d) NAD+ is regenerated during the process.
Options:
(1) (a), (b), (d) only
(2) (a) and (c) only
(3) (b) and (c) only
(4) All statements are correct

Explanation:
Lactic acid fermentation is anaerobic, yields 2 ATP per glucose, and regenerates NAD+. Pyruvate is not fully oxidized. Hence, statements (a), (b), and (d) are correct. Correct answer is (1).

Topic: Aerobic Respiration
Subtopic: Pyruvate Dehydrogenase Complex (PDC)

Keyword Definitions:

• Pyruvate dehydrogenase complex (PDC): Multienzyme complex converting pyruvate into acetyl-CoA for Krebs cycle.

• Aerobic respiration: Energy-yielding process using oxygen to oxidize glucose into CO2, generating ATP.

• Calcium (Ca2+): Cofactor that activates pyruvate dehydrogenase by stimulating phosphatase activity.

• Iron (Fe): Component of cytochromes in electron transport chain, not direct PDC cofactor.

• Cobalt (Co): Component of vitamin B12, not directly required for PDC function.

• Magnesium (Mg2+): Cofactor stabilizing ATP and enzyme-substrate complexes in PDC reactions.

• Acetyl-CoA: Product of pyruvate decarboxylation entering Krebs cycle.

• NAD+ and FAD: Electron carriers accepting electrons during pyruvate oxidation.

• Thiamine pyrophosphate (TPP): Vitamin B1-derived coenzyme essential for PDC decarboxylation of pyruvate.

• Lipoamide: Coenzyme that transfers acyl groups in PDC.

• Aerobic energy metabolism: Oxidation of pyruvate in mitochondria to produce ATP efficiently.

Lead Question - 2020 (COVID Reexam)

Pyruvate dehydrogenase activity during aerobic respiration requires :-

1. Calcium
2. Iron
3. Cobalt
4. Magnesium

Explanation: Pyruvate dehydrogenase is activated by calcium ions, which stimulate the dephosphorylation of its E1 component. Magnesium stabilizes ATP and enzyme interactions but calcium is essential for PDC regulation during aerobic respiration. Correct answer is option 1: Calcium. This ensures efficient pyruvate conversion to acetyl-CoA. (50 words)

Guessed Question 1. Single Correct Answer MCQ: Which cofactor activates pyruvate dehydrogenase in mitochondria?

1. Calcium
2. Iron
3. Magnesium
4. Cobalt

Explanation: Calcium ions activate pyruvate dehydrogenase by promoting dephosphorylation of the E1 enzyme. This activation occurs during high-energy demand in aerobic respiration. Correct answer is option 1: Calcium. It regulates flux from pyruvate to acetyl-CoA in mitochondria. (50 words)

Guessed Question 2. Single Correct Answer MCQ: Magnesium in PDC primarily:

1. Accepts electrons
2. Stabilizes ATP and enzyme complexes
3. Releases CO2
4. Activates phosphatase

Explanation: Magnesium stabilizes ATP and enzyme-substrate interactions within PDC, aiding proper enzymatic function. It does not directly activate pyruvate dehydrogenase. Correct answer is option 2: Stabilizes ATP and enzyme complexes. Mg2+ supports the catalytic efficiency of the complex during aerobic respiration. (50 words)

Guessed Question 3. Single Correct Answer MCQ: Product of PDC is:

1. Pyruvate
2. Acetyl-CoA
3. Lactate
4. Oxaloacetate

Explanation: Pyruvate dehydrogenase converts pyruvate into acetyl-CoA, which enters the Krebs cycle for aerobic energy production. Correct answer is option 2: Acetyl-CoA. This step links glycolysis and aerobic respiration efficiently. (50 words)

Guessed Question 4. Single Correct Answer MCQ: Vitamin-derived cofactor in PDC is:

1. Thiamine pyrophosphate
2. NAD+
3. FAD
4. Coenzyme A

Explanation: Thiamine pyrophosphate (TPP), derived from vitamin B1, is essential for the decarboxylation of pyruvate in PDC. Correct answer is option 1: Thiamine pyrophosphate. It participates directly in catalysis, forming hydroxyethyl-TPP intermediate. (50 words)

Guessed Question 5. Assertion-Reason MCQ:

Assertion (A): Pyruvate dehydrogenase requires calcium for activation.
Reason (R): Calcium stimulates phosphatase that dephosphorylates E1 component of PDC.
1. Both A and R true, R explains A
2. Both A and R true, R not correct explanation
3. A true, R false
4. A false, R true

Explanation: Calcium activates pyruvate dehydrogenase by stimulating the phosphatase that removes inhibitory phosphate from the E1 component. Both assertion and reason are true, and R correctly explains A. Correct answer is option 1. This regulation ensures efficient acetyl-CoA production in aerobic respiration. (50 words)

Guessed Question 6. Matching Type MCQ:

Column I - PDC Component
(a) E1 (i) Pyruvate decarboxylation
(b) E2 (ii) Lipoamide-mediated acyl transfer
(c) E3 (iii) FAD-mediated oxidation
(d) Coenzyme A (iv) Accepts acetyl group
Options:
1. (a)-(i), (b)-(ii), (c)-(iii), (d)-(iv)
2. (a)-(ii), (b)-(iii), (c)-(i), (d)-(iv)
3. (a)-(i), (b)-(iii), (c)-(ii), (d)-(iv)
4. (a)-(iii), (b)-(ii), (c)-(i), (d)-(iv)

Explanation: E1 catalyzes pyruvate decarboxylation, E2 transfers acetyl group via lipoamide, E3 oxidizes lipoamide with FAD, and CoA accepts acetyl group to form acetyl-CoA. Correct answer is option 1. This coordinated mechanism ensures efficient pyruvate oxidation in aerobic respiration. (50 words)

Guessed Question 7. Fill in the blank:

Activation of pyruvate dehydrogenase requires _______ ions.

1. Magnesium
2. Calcium
3. Iron
4. Zinc

Explanation: Calcium ions activate pyruvate dehydrogenase by stimulating phosphatase-mediated dephosphorylation of E1 component. Correct answer is option 2: Calcium. This regulation ensures efficient flux from pyruvate to acetyl-CoA during aerobic respiration. (50 words)

Guessed Question 8. Single Correct Answer MCQ: PDC is located in:

1. Cytoplasm
2. Mitochondrial matrix
3. Chloroplast stroma
4. Nucleus

Explanation: Pyruvate dehydrogenase complex is located in the mitochondrial matrix, where it converts pyruvate to acetyl-CoA for Krebs cycle. Correct answer is option 2: Mitochondrial matrix. Matrix localization allows direct integration with aerobic respiration and NADH/FADH2 production. (50 words)

Guessed Question 9. Single Correct Answer MCQ: Essential cofactor for acyl group transfer in PDC is:

1. Lipoamide
2. NAD+
3. FAD
4. Coenzyme Q

Explanation: Lipoamide acts as swinging arm transferring the acetyl group from E2 active site to CoA. Correct answer is option 1: Lipoamide. This is critical for the enzymatic mechanism of pyruvate dehydrogenase complex during aerobic respiration. (50 words)

Guessed Question 10. Choose the correct statements MCQ:

(a) PDC converts pyruvate to acetyl-CoA
(b) Calcium activates PDC
(c) CoA accepts acetyl group
(d) Iron directly activates PDC
Options:
1. a, b, c
2. a, c, d
3. b, c, d
4. a, b, d

Explanation: PDC converts pyruvate to acetyl-CoA, calcium activates PDC, and CoA accepts the acetyl group. Iron is not a direct activator. Correct answer is option 1: a, b, c. These components ensure efficient aerobic pyruvate oxidation. (50 words)

Topic: Citric Acid Cycle
Subtopic: Substrate Level Phosphorylation

Keyword Definitions:

• Citric Acid Cycle: A series of enzyme-catalyzed reactions in mitochondria that oxidize acetyl-CoA to CO₂ while producing NADH, FADH₂, and ATP/GTP.

• Substrate Level Phosphorylation: Direct synthesis of ATP (or GTP) from ADP (or GDP) using energy released in a chemical reaction.

• Oxidative Phosphorylation: ATP synthesis driven by electron transport and proton gradient in mitochondria.

• NADH: Reduced form of nicotinamide adenine dinucleotide, a carrier of electrons and hydrogen ions.

• FADH₂: Reduced form of flavin adenine dinucleotide, another electron carrier in respiration.

• Succinyl-CoA: An intermediate of the citric acid cycle that generates GTP/ATP via substrate level phosphorylation.

Lead Question - 2020

The number of substrate level phosphorylations in one turn of citric acid cycle is :

(1) Two
(2) Three
(3) Zero
(4) One

Explanation: In one turn of the citric acid cycle, only one substrate level phosphorylation occurs during the conversion of succinyl-CoA to succinate, generating GTP or ATP. All other ATP molecules are produced later via oxidative phosphorylation. Hence, the correct answer is option (4).

Guessed Questions:

1) How many NADH molecules are formed in one turn of citric acid cycle?
(1) Two
(2) Three
(3) Four
(4) One

Explanation: In one turn of the citric acid cycle, three NADH molecules are produced at the steps catalyzed by isocitrate dehydrogenase, α-ketoglutarate dehydrogenase, and malate dehydrogenase. These are later used in oxidative phosphorylation to generate ATP. Hence, the correct answer is option (2).

2) Which intermediate links glycolysis to citric acid cycle?
(1) Pyruvate
(2) Acetyl-CoA
(3) Citrate
(4) Oxaloacetate

Explanation: Pyruvate from glycolysis is converted into acetyl-CoA by the pyruvate dehydrogenase complex before entering the citric acid cycle. Thus, acetyl-CoA serves as the connecting link between glycolysis and citric acid cycle. Hence, the correct answer is option (2).

3) Assertion (A): FADH₂ is produced in the citric acid cycle.
Reason (R): It is formed during the oxidation of succinate to fumarate.
(1) Both A and R are true, and R is the correct explanation of A
(2) Both A and R are true, but R is not the correct explanation of A
(3) A is true, R is false
(4) A is false, R is true

Explanation: In the citric acid cycle, FADH₂ is formed when succinate is oxidized to fumarate by succinate dehydrogenase. Both A and R are true, and R correctly explains A. Hence, the correct answer is option (1).

4) Which enzyme catalyzes the substrate level phosphorylation in citric acid cycle?
(1) Succinyl-CoA synthetase
(2) Malate dehydrogenase
(3) Citrate synthase
(4) Aconitase

Explanation: Succinyl-CoA synthetase catalyzes the conversion of succinyl-CoA to succinate with the simultaneous synthesis of ATP or GTP through substrate level phosphorylation. Hence, the correct answer is option (1).

5) Match the products with the corresponding reactions in the citric acid cycle:
(a) Isocitrate dehydrogenase (i) FADH₂
(b) α-ketoglutarate dehydrogenase (ii) NADH
(c) Succinate dehydrogenase (iii) NADH
(d) Malate dehydrogenase (iv) NADH
(1) a-(ii), b-(iii), c-(i), d-(iv)
(2) a-(i), b-(ii), c-(iii), d-(iv)
(3) a-(ii), b-(iv), c-(iii), d-(i)
(4) a-(iii), b-(ii), c-(i), d-(iv)

Explanation: Isocitrate dehydrogenase produces NADH, α-ketoglutarate dehydrogenase produces NADH, succinate dehydrogenase produces FADH₂, and malate dehydrogenase produces NADH. Hence, the correct answer is option (1).

6) Fill in the blank: In one complete oxidation of acetyl-CoA in citric acid cycle, ______ ATP equivalents are formed.
(1) 10
(2) 12
(3) 6
(4) 8

Explanation: Complete oxidation of one acetyl-CoA yields 3 NADH (9 ATP), 1 FADH₂ (2 ATP), and 1 GTP (1 ATP), totaling 12 ATP equivalents. Hence, the correct answer is option (2).

7) Which of the following statements are correct?
(i) Citric acid cycle occurs in cytoplasm
(ii) One FADH₂ is produced per turn
(iii) Three NADH molecules are produced per turn
(1) i and ii only
(2) ii and iii only
(3) i and iii only
(4) i, ii and iii

Explanation: The citric acid cycle occurs in the mitochondrial matrix, not in cytoplasm. Each turn produces one FADH₂ and three NADH molecules. Hence, statements ii and iii are correct. The correct answer is option (2).

8) Which reaction produces CO₂ in citric acid cycle?
(1) Isocitrate to α-ketoglutarate
(2) Succinyl-CoA to succinate
(3) Malate to oxaloacetate
(4) Citrate to isocitrate

Explanation: The decarboxylation of isocitrate to α-ketoglutarate by isocitrate dehydrogenase releases CO₂. Another CO₂ is released in the conversion of α-ketoglutarate to succinyl-CoA. Hence, option (1) is correct.

9) How many ATP equivalents are formed by complete oxidation of one glucose via glycolysis, citric acid cycle, and oxidative phosphorylation?
(1) 30
(2) 36
(3) 38
(4) 40

Explanation: One glucose yields about 36-38 ATP depending on shuttle systems used for NADH transport. Classical calculation gives 38 ATP: 2 from glycolysis, 2 from citric acid cycle, and 34 from oxidative phosphorylation. Hence, the correct answer is option (3).

10) Which coenzyme is required for the conversion of α-ketoglutarate to succinyl-CoA?
(1) Biotin
(2) Thiamine pyrophosphate (TPP)
(3) Pyridoxal phosphate
(4) Coenzyme Q

Explanation: The α-ketoglutarate dehydrogenase complex requires thiamine pyrophosphate (TPP), lipoic acid, FAD, NAD⁺, and CoA as cofactors. TPP plays a central role in this reaction. Hence, the correct answer is option (2).

Subtopic: Glycolysis

Keyword Definitions:

• Glucose: A six-carbon monosaccharide that serves as primary energy source.

• Glucose-6-phosphate: Phosphorylated form of glucose formed during the first step of glycolysis.

• Glycolysis: Metabolic pathway that breaks down glucose to pyruvate, generating ATP.

• Hexokinase: Enzyme that catalyzes phosphorylation of glucose to glucose-6-phosphate.

• Aldolase: Enzyme in glycolysis that splits fructose-1,6-bisphosphate into two three-carbon molecules.

• Phosphofructokinase: Key regulatory enzyme of glycolysis catalyzing phosphorylation of fructose-6-phosphate.

• Enolase: Enzyme that converts 2-phosphoglycerate to phosphoenolpyruvate in glycolysis.

Lead Question (2019):

Conversion of glucose to glucose-6-phosphate, the first irreversible reaction of glycolysis, is catalyzed by:
(1) Aldolase
(2) Hexokinase
(3) Enolase
(4) Phosphofructokinase

Explanation: Correct answer is (2). Hexokinase catalyzes the phosphorylation of glucose to glucose-6-phosphate, the first irreversible step in glycolysis. This reaction traps glucose inside the cell and primes it for further breakdown, ensuring energy extraction and regulation of the glycolytic pathway efficiently.

1) Single Correct Answer MCQ:
Which enzyme catalyzes the formation of fructose-1,6-bisphosphate in glycolysis?
(1) Hexokinase
(2) Phosphofructokinase
(3) Aldolase
(4) Enolase

Explanation: Correct answer is (2). Phosphofructokinase catalyzes phosphorylation of fructose-6-phosphate to fructose-1,6-bisphosphate, a key irreversible regulatory step in glycolysis controlling pathway flux.

2) Single Correct Answer MCQ:
Which enzyme splits fructose-1,6-bisphosphate into two three-carbon molecules?
(1) Aldolase
(2) Hexokinase
(3) Enolase
(4) Phosphofructokinase

Explanation: Correct answer is (1). Aldolase cleaves fructose-1,6-bisphosphate into dihydroxyacetone phosphate and glyceraldehyde-3-phosphate during glycolysis, facilitating energy extraction from glucose.

3) Single Correct Answer MCQ:
Which glycolytic enzyme converts 2-phosphoglycerate to phosphoenolpyruvate?
(1) Aldolase
(2) Hexokinase
(3) Enolase
(4) Phosphofructokinase

Explanation: Correct answer is (3). Enolase catalyzes the dehydration of 2-phosphoglycerate to phosphoenolpyruvate, producing a high-energy intermediate necessary for ATP generation in glycolysis.

4) Single Correct Answer MCQ:
Hexokinase requires which cofactor for activity?
(1) NAD+
(2) ATP
(3) FAD
(4) GTP

Explanation: Correct answer is (2). Hexokinase uses ATP to phosphorylate glucose to glucose-6-phosphate. ATP provides the phosphate group, and this step is irreversible, committing glucose to metabolism within the cell.

5) Single Correct Answer MCQ:
Which step of glycolysis is irreversible?
(1) Glucose to glucose-6-phosphate
(2) Fructose-6-phosphate to fructose-1,6-bisphosphate
(3) Phosphoenolpyruvate to pyruvate
(4) All of the above

Explanation: Correct answer is (4). The irreversible steps of glycolysis are catalyzed by hexokinase, phosphofructokinase, and pyruvate kinase, ensuring directional flow and regulation of the pathway.

6) Single Correct Answer MCQ:
The main purpose of glucose phosphorylation in glycolysis is:
(1) ATP production
(2) Trapping glucose inside cell
(3) Reducing NAD+
(4) Forming pyruvate directly

Explanation: Correct answer is (2). Phosphorylation of glucose to glucose-6-phosphate prevents it from diffusing out of the cell and prepares it for further metabolism in glycolysis.

7) Assertion-Reason MCQ:
Assertion (A): Hexokinase catalyzes the first step of glycolysis.
Reason (R): Hexokinase generates glucose-6-phosphate from glucose using ATP.
Options:
(1) A true, R true, R correct explanation
(2) A true, R true, R not correct explanation
(3) A true, R false
(4) A false, R true

Explanation: Correct answer is (1). The assertion is true; hexokinase catalyzes the first irreversible step. The reason is also correct, describing ATP-dependent formation of glucose-6-phosphate.

8) Matching Type MCQ:
Match the enzyme with its glycolytic function:
(a) Hexokinase - (i) Splits fructose-1,6-bisphosphate
(b) Aldolase - (ii) Phosphorylates glucose
(c) Phosphofructokinase - (iii) Phosphorylates fructose-6-phosphate
(d) Enolase - (iv) Converts 2-phosphoglycerate to phosphoenolpyruvate
Options:
(1) a-ii, b-i, c-iii, d-iv
(2) a-i, b-ii, c-iii, d-iv
(3) a-iii, b-i, c-ii, d-iv
(4) a-ii, b-iii, c-i, d-iv

Explanation: Correct answer is (1). Hexokinase phosphorylates glucose, aldolase cleaves fructose-1,6-bisphosphate, phosphofructokinase phosphorylates fructose-6-phosphate, and enolase dehydrates 2-phosphoglycerate to phosphoenolpyruvate.

9) Fill in the Blanks MCQ:
The first irreversible step of glycolysis is catalyzed by ________.
(1) Hexokinase
(2) Phosphofructokinase
(3) Aldolase
(4) Enolase

Explanation: Correct answer is (1). Hexokinase catalyzes phosphorylation of glucose to glucose-6-phosphate, the first irreversible step in glycolysis ensuring glucose is committed to metabolism.

10) Choose the correct statements MCQ:
(1) Hexokinase catalyzes first glycolytic step
(2) Aldolase splits fructose-1,6-bisphosphate
(3) Enolase forms phosphoenolpyruvate
(4) Phosphofructokinase phosphorylates glucose</

Subtopic: Respiratory Quotient

Keyword Definitions:

• Respiratory Quotient (RQ): Ratio of CO₂ produced to O₂ consumed during respiration.

• Tripalmitin: A triglyceride consisting of three palmitic acid molecules, a lipid.

• Carbohydrate RQ: Typically 1, as equal O₂ is consumed and CO₂ released.

• Lipid RQ: Usually 0.7, less CO₂ produced per O₂ consumed due to high H content.

• Protein RQ: Approximately 0.8, intermediate between carbohydrate and lipid values.

Lead Question (September 2019):

Respiratory Quotient (RQ) value of tripalmitin is:
(1) 0.9
(2) 0.7
(3) 0.07
(4) 0.09

Explanation: The correct answer is (2) 0.7. Tripalmitin is a lipid, and the RQ of lipids is typically 0.7, indicating less CO₂ produced per unit O₂ consumed. NEET UG tests understanding of RQ values for carbohydrates, fats, and proteins, which helps determine the type of substrate utilized in respiration.

1) RQ of carbohydrate is:
(1) 1
(2) 0.7
(3) 0.8
(4) 0.9

Explanation: The correct answer is (1) 1. Carbohydrates produce equal CO₂ per O₂ consumed. NEET UG tests recognition of RQ values to identify the type of nutrient metabolized during respiration.

2) Protein RQ is approximately:
(1) 0.7
(2) 0.8
(3) 1
(4) 0.9

Explanation: The correct answer is (2) 0.8. Proteins yield intermediate CO₂ per O₂ consumed. NEET UG emphasizes the distinction of RQ values among macronutrients for metabolism questions.

3) High RQ (>1) indicates:
(1) Lipid metabolism
(2) Protein metabolism
(3) Excess carbohydrate metabolism
(4) Starvation

Explanation: The correct answer is (3) Excess carbohydrate metabolism. When carbohydrates are metabolized in surplus, RQ can exceed 1. NEET UG often tests interpretation of RQ in physiological and clinical contexts.

4) RQ indicates:
(1) Oxygen consumption only
(2) Carbon dioxide production only
(3) Ratio of CO₂ produced to O₂ consumed
(4) Energy expenditure

Explanation: The correct answer is (3). RQ = CO₂ produced / O₂ consumed. NEET UG tests students’ ability to calculate or interpret respiratory quotient to determine substrate utilization.

5) Tripalmitin is classified as:
(1) Carbohydrate
(2) Protein
(3) Lipid
(4) Nucleic acid

Explanation: The correct answer is (3) Lipid. Tripalmitin is a triglyceride composed of palmitic acid molecules. NEET UG tests chemical classification and its link to RQ values.

6) RQ value is used to determine:
(1) Type of substrate oxidized
(2) Respiratory rate
(3) Blood oxygen levels
(4) ATP production rate

Explanation: The correct answer is (1) Type of substrate oxidized. RQ varies with carbohydrates, fats, and proteins, allowing assessment of which nutrient is metabolized. NEET UG often uses RQ to interpret energy source during metabolism.

7) Assertion-Reason Type:
Assertion (A): Lipid metabolism has RQ = 0.7.
Reason (R): Lipids produce less CO₂ per O₂ consumed due to high hydrogen content.
(1) A true, R true, R correct explanation
(2) A true, R true, R not explanation
(3) A true, R false
(4) A false, R true

Explanation: The correct answer is (1). Lipids, such as tripalmitin, have RQ = 0.7 because their oxidation consumes more oxygen relative to CO₂ released, due to high hydrogen content. NEET UG tests understanding of chemical basis of RQ.

8) Matching Type:
Match substrate with approximate RQ:
(a) Carbohydrate - (i) 0.7
(b) Lipid - (ii) 1
(c) Protein - (iii) 0.8
Options:
(1) a-ii, b-i, c-iii
(2) a-i, b-ii, c-iii
(3) a-iii, b-i, c-ii
(4) a-i, b-iii, c-ii

Explanation: The correct answer is (1). Carbohydrate – 1, Lipid – 0.7, Protein – 0.8. NEET UG often tests students’ ability to correlate substrate type with RQ for energy metabolism analysis.

9) Fill in the Blanks:
The RQ value of tripalmitin, a lipid, is ______.
(1) 0.7
(2) 0.8
(3) 1
(4) 0.9

Explanation: The correct answer is (1) 0.7. Tripalmitin is a triglyceride; oxidation produces less CO₂ per O₂ consumed, giving an RQ of 0.7. NEET UG questions often require students to calculate or recall RQ values for different macronutrients.

10) Choose the correct statements:
(1) Carbohydrate RQ = 1
(2) Lipid RQ = 0.7
(3) Protein RQ = 0.8
(4) Tripalmitin RQ = 0.09
Options:
(1) 1, 2, 3
(2) 2 and 4
(3) 1 and 4
(4) All of the above

Explanation: The correct answer is (1) 1, 2, 3. RQ values correctly reflect substrate metabolism: carbohydrate 1, lipid 0.7, protein 0.8. Tripalmitin has RQ = 0.7, not 0.09. NEET UG tests interpretation of RQ in metabolic studies.

Topic: Stomata Types

Subtopic: Grass Leaf Stomata

Keyword Definitions:

• Stomata: Pores in the epidermis of leaves and stems for gas exchange.

• Grass leaf: Monocot leaf with parallel venation, often having specialized stomata.

• Dumb-bell shaped stomata: Stomata with elongated guard cells and central pore, typical of grasses.

• Barrel shaped stomata: Common in dicots, kidney-shaped guard cells.

• Gas exchange: Process of CO2 uptake and O2 release during photosynthesis.

• Guard cells: Cells surrounding stomatal pore controlling opening and closing.

Lead Question (2018):

Stomata in grass leaf are:

(A) Barrel shaped

(B) Dumb-bell shaped

(C) Rectangular

(D) Kidney shaped

Explanation:

The correct answer is (B) Dumb-bell shaped. Grass, being a monocot, has dumb-bell shaped stomata with narrow ends and central pore, which allow efficient opening and closing. Barrel or kidney-shaped stomata are typically found in dicots. This structure helps in water conservation and gas exchange in grasses.

1. The main function of stomata is:

(A) Photosynthesis

(B) Gas exchange

(C) Transport of water

(D) Nutrient absorption

Explanation:

The correct answer is (B) Gas exchange. Stomata allow CO2 uptake for photosynthesis and O2 release. While stomata influence transpiration and water balance, their primary function is gas exchange, not nutrient absorption or direct photosynthesis.

2. Guard cells of grass stomata contain:

(A) Chloroplasts

(B) No chloroplasts

(C) Only mitochondria

(D) Only vacuoles

Explanation:

The correct answer is (A) Chloroplasts. Guard cells in grass have chloroplasts that help in sensing light and producing ATP for stomatal opening. They are metabolically active and contribute to opening and closing mechanisms.

3. Monocot stomata differ from dicot stomata in being:

(A) Kidney-shaped

(B) Dumb-bell shaped

(C) Circular

(D) Irregular

Explanation:

The correct answer is (B) Dumb-bell shaped. Monocots like grasses have dumb-bell shaped stomata for efficient opening. Dicots usually have kidney or barrel-shaped stomata. This adaptation helps monocots conserve water and respond rapidly to environmental changes.

4. Stomatal opening is regulated by:

(A) Light and potassium ions

(B) CO2 only

(C) Temperature only

(D) Oxygen only

Explanation:

The correct answer is (A) Light and potassium ions. Light triggers photosynthesis in guard cells, leading to K+ ion accumulation, osmotic water influx, and stomatal opening. CO2, temperature, and humidity also influence stomata, but K+ ions and light are primary regulators.

5. In which plant type are dumb-bell shaped stomata common?

(A) Dicots

(B) Monocots

(C) Gymnosperms

(D) Ferns

Explanation:

The correct answer is (B) Monocots. Dumb-bell shaped stomata are characteristic of monocot leaves such as grasses, aiding in rapid opening and closing. Dicots and gymnosperms have kidney or barrel-shaped stomata.

6. Stomatal density is generally higher on:

(A) Upper epidermis

(B) Lower epidermis

(C) Both equally

(D) Stem only

Explanation:

The correct answer is (B) Lower epidermis. Most leaves have higher stomatal density on the lower epidermis to minimize water loss while allowing gas exchange. Upper epidermis may have fewer stomata or none.

7. Assertion-Reason Question:

Assertion (A): Grass stomata are dumb-bell shaped.

Reason (R): This shape allows efficient opening and closing.

(A) Both A and R true, R explains A

(B) Both A and R true, R does not explain A

(C) A true, R false

(D) A false, R true

Explanation:

Correct answer is (A). The dumb-bell shape of guard cells in grasses enables rapid and efficient opening and closing of stomata, conserving water while ensuring adequate gas exchange. Both assertion and reason are true, with the reason explaining the assertion.

8. Matching Type Question:

Match stomatal types with plant examples:

(i) Dumb-bell shaped – (a) Grass

(ii) Kidney-shaped – (b) Sunflower

(iii) Barrel-shaped – (c) Bean

(iv) Circular – (d) Some algae

(A) i-a, ii-b, iii-c, iv-d

(B) i-b, ii-a, iii-d, iv-c

(C) i-c, ii-d, iii-b, iv-a

(D) i-d, ii-c, iii-a, iv-b

Explanation:

Correct answer is (A). Dumb-bell shaped stomata occur in grasses, kidney-shaped in sunflower, barrel-shaped in beans, and circular in some algae. Matching highlights structural diversity in stomata across plant groups.

9. Fill in the Blanks:

Stomata in monocots like grass are ______ and help in ______.

(A) Barrel-shaped, photosynthesis

(B) Dumb-bell shaped, efficient gas exchange

(C) Kidney-shaped, water storage

(D) Rectangular, transpiration only

Explanation:

Correct answer is (B) Dumb-bell shaped, efficient gas exchange. Grass stomata are dumb-bell shaped, allowing rapid opening and closing for gas exchange and water conservation. Barrel-shaped and kidney-shaped stomata are common in dicots.

10. Choose the correct statements:

(A) Grass stomata are dumb-bell shaped

(B) Stomata allow gas exchange

(C) Guard cells contain chloroplasts

(D) Monocots have kidney-shaped stomata

Options:

(1) A, B, C

(2) A, C, D

(3) B, C, D

(4) A, B, D

Explanation:

Correct answer is (1) A, B, C. Grass stomata are dumb-bell shaped, facilitate gas exchange, and guard cells contain chloroplasts. Monocots do not have kidney-shaped stomata, which are typical of dicots.

Topic: Electron Transport and Redox Reactions

Subtopic: Role of NAD+ in Energy Metabolism

Keyword Definitions:

• NAD+: Nicotinamide adenine dinucleotide, a coenzyme that functions as an electron carrier in redox reactions.

• Cellular respiration: Metabolic process converting glucose into ATP with the help of oxygen and electron carriers.

• Electron carrier: Molecule that transfers electrons between biochemical reactions in metabolism.

• Anaerobic respiration: Energy-producing process occurring without oxygen, using alternative electron acceptors.

• ATP synthesis: Production of adenosine triphosphate, the cellular energy currency, through substrate-level or oxidative phosphorylation.

Lead Question (2018):

What is the role of NAD+ in cellular respiration?

(A) It is the final electron acceptor for anaerobic respiration

(B) It functions as an enzyme

(C) It is a nucleotide source for ATP synthesis

(D) It functions as an electron carrier

Explanation:

The correct answer is (D) It functions as an electron carrier. NAD+ accepts electrons and hydrogen ions during glycolysis and the Krebs cycle, forming NADH. It transports these electrons to the electron transport chain, facilitating ATP production. It does not act as an enzyme, nucleotide source, or final electron acceptor in anaerobic respiration.

1. During glycolysis, NAD+ is reduced to:

(A) NADH

(B) FADH2

(C) ATP

(D) ADP

Explanation:

The correct answer is (A) NADH. NAD+ accepts electrons and hydrogen ions from glucose intermediates to form NADH. NADH later donates electrons to the electron transport chain to generate ATP. FADH2, ATP, or ADP are not formed directly from NAD+ in glycolysis.

2. In aerobic respiration, NADH transfers electrons to:

(A) Oxygen directly

(B) Electron transport chain

(C) Pyruvate

(D) Glucose

Explanation:

The correct answer is (B) Electron transport chain. NADH carries electrons from glycolysis and Krebs cycle to the mitochondrial electron transport chain, where they ultimately reduce oxygen to water. This electron transfer drives ATP synthesis. NADH does not transfer electrons directly to pyruvate or glucose.

3. Which process regenerates NAD+ under anaerobic conditions?

(A) Glycolysis

(B) Fermentation

(C) Krebs cycle

(D) Electron transport chain

Explanation:

The correct answer is (B) Fermentation. Fermentation regenerates NAD+ by oxidizing NADH while converting pyruvate to lactate or ethanol, allowing glycolysis to continue. In aerobic respiration, NAD+ is regenerated in the electron transport chain, but under anaerobic conditions, fermentation is essential for NAD+ availability.

4. NAD+ is classified as a:

(A) Coenzyme

(B) Enzyme

(C) Substrate

(D) Hormone

Explanation:

The correct answer is (A) Coenzyme. NAD+ is a non-protein coenzyme that assists enzymes in redox reactions by accepting and donating electrons. It is not an enzyme, substrate, or hormone, but a crucial electron carrier in energy metabolism.

5. NADH contributes to ATP production by:

(A) Directly forming ATP

(B) Donating electrons to electron transport chain

(C) Acting as a substrate for glycolysis

(D) Binding oxygen

Explanation:

The correct answer is (B) Donating electrons to electron transport chain. NADH transfers electrons to complexes in the mitochondrial electron transport chain, generating a proton gradient that drives ATP synthesis via oxidative phosphorylation. NADH does not directly form ATP, act as glycolysis substrate, or bind oxygen.

6. Which vitamin is a precursor of NAD+?

(A) Niacin (Vitamin B3)

(B) Riboflavin (Vitamin B2)

(C) Thiamine (Vitamin B1)

(D) Pantothenic acid (Vitamin B5)

Explanation:

The correct answer is (A) Niacin (Vitamin B3). Niacin is converted into NAD+ and NADP+, essential electron carriers in cellular respiration. Riboflavin forms FAD/FMN, thiamine forms TPP, and pantothenic acid forms CoA, but only niacin is the precursor for NAD+.

7. Assertion-Reason Question:

Assertion (A): NAD+ is essential for glycolysis and Krebs cycle.

Reason (R): NAD+ accepts electrons and is reduced to NADH.

(A) Both A and R true, R explains A

(B) Both A and R true, R does not explain A

(C) A true, R false

(D) A false, R true

Explanation:

Correct answer is (A). NAD+ is required in glycolysis and Krebs cycle to accept electrons, forming NADH. This reduction is essential for continued metabolic flux and ATP generation. Both assertion and reason are true, and the reason correctly explains the assertion.

8. Matching Type Question:

Match the molecule with its function:

(i) NAD+ – (a) Electron carrier

(ii) FAD – (b) Electron carrier

(iii) ADP – (c) Substrate for ATP formation

(iv) Oxygen – (d) Final electron acceptor

(A) i-a, ii-b, iii-c, iv-d

(B) i-b, ii-a, iii-d, iv-c

(C) i-c, ii-d, iii-a, iv-b

(D) i-a, ii-d, iii-b, iv-c

Explanation:

Correct answer is (A). NAD+ and FAD function as electron carriers, ADP serves as substrate for ATP synthesis, and oxygen is the final electron acceptor in the electron transport chain. Matching reinforces understanding of roles of molecules in cellular respiration.

9. Fill in the Blanks:

NAD+ functions as a ______ by accepting electrons and forming ______.

(A) Enzyme, NADH

(B) Electron carrier, NADH

(C) Nucleotide source, ATP

(D) Oxygen carrier, NADH

Explanation:

Correct answer is (B) Electron carrier, NADH. NAD+ accepts electrons and hydrogen ions, forming NADH, which transports electrons to the electron transport chain, facilitating ATP production. It is not an enzyme, nucleotide source, or oxygen carrier.

10. Choose the correct statements:

(A) NAD+ is reduced to NADH during glycolysis and Krebs cycle

(B) NAD+ acts as a coenzyme

(C) NADH donates electrons to electron transport chain

(D) NAD+ directly forms ATP

Options:

(1) A, B, C

(2) A, B, D

(3) B, C, D

(4) A, C, D

Explanation:

Correct answer is (1) A, B, C. NAD+ is reduced to NADH, acts as coenzyme, and NADH donates electrons to electron transport chain for ATP synthesis. NAD+ does not directly produce ATP. Understanding NAD+ role is essential for NEET UG questions on cellular respiration.

Subtopic: Glycolysis and Mitochondrial Function

Keyword Definitions:

• Oxidative phosphorylation: Process in mitochondria generating ATP using electron transport chain and chemiosmosis.

• Mitochondrial matrix: Innermost compartment of mitochondria containing enzymes for TCA cycle.

• Glycolysis: Cytosolic pathway that converts glucose into pyruvate with net ATP production.

• NAD: Nicotinamide adenine dinucleotide, electron carrier involved in redox reactions.

• TCA cycle: Tricarboxylic acid cycle, central pathway of aerobic respiration producing energy intermediates.

• Cytosol: Fluid portion of cytoplasm where glycolysis and other metabolic reactions occur.

Lead Question - 2018

Which of these statements is incorrect :

(A) Oxidative phosphorylation takes place in outer mitochondrial membrane

(B) Enzymes of TCA cycle are present in mitochondrial matrix

(C) Glycolysis operates as long as it is supplied with NAD that can pick up hydrogen atoms

(D) Glycolysis occurs in cytosol

Explanation:

Answer is (A). Oxidative phosphorylation occurs on the inner mitochondrial membrane, not the outer membrane. This membrane contains electron transport chain complexes and ATP synthase, facilitating proton gradient-driven ATP synthesis. TCA enzymes reside in the matrix, and glycolysis functions in the cytosol, dependent on NAD availability for hydrogen acceptance.

Guessed Questions for NEET UG:

1) Single Correct: Site of TCA cycle enzymes:

(A) Cytosol

(B) Mitochondrial matrix

(C) Inner mitochondrial membrane

(D) Outer mitochondrial membrane

Explanation:

Answer is (B). TCA cycle enzymes are located in the mitochondrial matrix, where acetyl-CoA oxidation produces NADH, FADH2, and GTP for energy metabolism.

2) Single Correct: Electron transport chain is embedded in:

(A) Cytosol

(B) Inner mitochondrial membrane

(C) Outer mitochondrial membrane

(D) Mitochondrial matrix

Explanation:

Answer is (B). The inner mitochondrial membrane houses the electron transport chain, creating a proton gradient for ATP synthesis during oxidative phosphorylation.

3) Single Correct: Glycolysis produces ATP in:

(A) Mitochondrial matrix

(B) Cytosol

(C) Golgi apparatus

(D) Endoplasmic reticulum

Explanation:

Answer is (B). Glycolysis occurs in cytosol, breaking glucose into pyruvate and producing a net of 2 ATP and 2 NADH per glucose molecule.

4) Assertion-Reason:

Assertion: NAD is essential for glycolysis.

Reason: It accepts electrons from glucose intermediates to maintain pathway flux.

(A) Both true, Reason correct

(B) Both true, Reason incorrect

(C) Assertion true, Reason false

(D) Both false

Explanation:

Answer is (A). NAD acts as an electron acceptor in glycolysis. Without NAD, glycolytic intermediates cannot be oxidized, halting ATP production.

5) Single Correct: Main purpose of oxidative phosphorylation:

(A) Produce pyruvate

(B) Generate ATP

(C) Reduce NAD

(D) Oxidize glucose in cytosol

Explanation:

Answer is (B). Oxidative phosphorylation generates ATP by utilizing the proton gradient formed by electron transport through inner mitochondrial membrane complexes.

6) Single Correct: NADH donates electrons to:

(A) TCA cycle enzymes

(B) Glycolytic enzymes

(C) Electron transport chain

(D) ATP synthase

Explanation:

Answer is (C). NADH produced in glycolysis and TCA cycle donates electrons to the electron transport chain, driving proton pumping and ATP synthesis.

7) Matching Type:

Column I | Column II

a. Glycolysis | i. Cytosol

b. TCA cycle | ii. Mitochondrial matrix

c. Oxidative phosphorylation | iii. Inner mitochondrial membrane

(A) a-i, b-ii, c-iii

(B) a-ii, b-i, c-iii

(C) a-iii, b-ii, c-i

(D) a-i, b-iii, c-ii

Explanation:

Answer is (A). Glycolysis occurs in cytosol, TCA in matrix, and oxidative phosphorylation on inner membrane of mitochondria.

8) Fill in the Blank:

Glycolysis can continue as long as sufficient ______ is available to accept electrons.

(A) FAD

(B) ATP

(C) NAD

(D) Oxygen

Explanation:

Answer is (C). NAD acts as an electron acceptor in glycolysis, regenerating NAD+ is critical for maintaining continuous ATP production.

9) Choose the correct statements:

(i) Oxidative phosphorylation occurs in inner mitochondrial membrane.

(ii) TCA enzymes are in cytosol.

(iii) Glycolysis is cytosolic and NAD-dependent.

(A) i and iii only

(B) i and ii only

(C) ii and iii only

(D) i, ii, iii

Explanation:

Answer is (A). Oxidative phosphorylation occurs on inner membrane (i), glycolysis is cytosolic and requires NAD (iii). TCA enzymes are in mitochondrial matrix, not cytosol.

10) Clinical-type: In mitochondrial disorders affecting inner membrane, which process is directly impaired?

(A) Glycolysis

(B) TCA cycle

(C) Oxidative phosphorylation

(D) Gluconeogenesis

Explanation:

Answer is (C). Inner membrane defects impair electron transport chain and ATP production, reducing oxidative phosphorylation efficiency, while glycolysis in cytosol remains functional.

Chapter: Cellular Respiration

Topic: Krebs Cycle / Citric Acid Cycle

Subtopic: Steps and Energy Yield of Krebs Cycle

Keyword Definitions:

• Krebs Cycle – Series of chemical reactions generating energy via oxidation of acetyl-CoA.

• Acetyl-CoA – Two-carbon molecule entering Krebs cycle from pyruvate decarboxylation.

• Citric Acid – Six-carbon compound formed in first step of Krebs cycle.

• NAD+ – Nicotinamide adenine dinucleotide, electron carrier reduced to NADH.

• FAD – Flavin adenine dinucleotide, electron carrier reduced to FADH2.

• GTP – Guanosine triphosphate, energy-carrying molecule similar to ATP.

• Succinyl CoA – Four-carbon intermediate in Krebs cycle.

• Succinic Acid – Four-carbon molecule formed from succinyl CoA.

• Oxidative Phosphorylation – ATP synthesis using electron transport chain and chemiosmosis.

• Pyruvic Acid – End product of glycolysis converted into acetyl-CoA.

Lead Question – 2017:

Which statement is wrong for Krebs’ cycle ?

(A) The cycle starts with condensation of acetyl group (acetylCoA) with pyruvic acid to yield citric acid

(B) There are three points in the cycle where NAD+ is reduced to NADH + H+

(C) There is one point in the cycle where FAD+ is reduced to FADH2

(D) During conversion of succinyl CoA to succinic acid, a molecule of GTP is synthesised

Explanation:

Statement (A) is incorrect. The cycle begins with condensation of acetyl-CoA with oxaloacetate to form citric acid, not pyruvic acid. Three NAD+ reductions, one FAD reduction, and GTP formation during succinyl-CoA conversion are correct. This ensures energy capture from the acetyl group oxidation. (Answer: A)

1) Single Correct Answer MCQ:

Which molecule condenses with acetyl-CoA to start Krebs cycle?

(A) Pyruvate

(B) Oxaloacetate

(C) Citrate

(D) Succinyl CoA

Explanation:

Krebs cycle begins with acetyl-CoA condensing with oxaloacetate to form citrate (citric acid). Pyruvate is first converted to acetyl-CoA, succinyl CoA is a later intermediate, and citrate is the product. (Answer: B)

2) Single Correct Answer MCQ:

How many NADH molecules are produced per acetyl-CoA in Krebs cycle?

(A) 1

(B) 2

(C) 3

(D) 4

Explanation:

Each acetyl-CoA oxidation produces three NADH molecules at different steps: isocitrate → α-ketoglutarate, α-ketoglutarate → succinyl-CoA, malate → oxaloacetate. NADH feeds electrons into the electron transport chain for ATP generation. (Answer: C)

3) Single Correct Answer MCQ:

Which intermediate produces GTP in Krebs cycle?

(A) Citrate

(B) α-Ketoglutarate

(C) Succinyl-CoA

(D) Fumarate

Explanation:

Conversion of succinyl-CoA to succinate produces one molecule of GTP via substrate-level phosphorylation. Citrate, α-ketoglutarate, and fumarate do not directly produce GTP. (Answer: C)

4) Single Correct Answer MCQ:

Which electron carrier is reduced to FADH2 in Krebs cycle?

(A) NAD+

(B) FAD

(C) Coenzyme A

(D) ADP

Explanation:

FAD is reduced to FADH2 during succinate → fumarate conversion. NAD+ is reduced to NADH at other steps. Coenzyme A and ADP are not electron carriers. FADH2 contributes electrons to the electron transport chain. (Answer: B)

5) Single Correct Answer MCQ:

During Krebs cycle, how many CO2 molecules are released per acetyl-CoA?

(A) 1

(B) 2

(C) 3

(D) 4

Explanation:

Each acetyl-CoA generates two CO2 molecules during oxidative decarboxylation: isocitrate → α-ketoglutarate and α-ketoglutarate → succinyl-CoA. These CO2 molecules represent complete carbon oxidation of acetyl group. (Answer: B)

6) Single Correct Answer MCQ:

Krebs cycle occurs in:

(A) Cytoplasm

(B) Mitochondrial matrix

(C) Golgi apparatus

(D) Endoplasmic reticulum

Explanation:

Krebs cycle enzymes are located in the mitochondrial matrix, enabling acetyl-CoA oxidation and NADH/FADH2 generation for oxidative phosphorylation. Cytoplasm hosts glycolysis; Golgi and ER are not involved in Krebs cycle. (Answer: B)

7) Assertion-Reason MCQ:

Assertion (A): GTP is synthesized in Krebs cycle.

Reason (R): Conversion of succinyl-CoA to succinate generates GTP via substrate-level phosphorylation.

(A) Both A and R true, R correct explanation

(B) Both A and R true, R not correct explanation

(C) A true, R false

(D) A false, R true

Explanation:

Both statements are correct. During succinyl-CoA → succinate, GTP is produced via substrate-level phosphorylation. This is the only step in Krebs cycle generating GTP directly. (Answer: A)

8) Matching Type MCQ:

Match step with product:

1. Isocitrate → α-Ketoglutarate – (i) CO2 + NADH

2. α-Ketoglutarate → Succinyl-CoA – (ii) CO2 + NADH

3. Succinate → Fumarate – (iii) FADH2

4. Succinyl-CoA → Succinate – (iv) GTP

Options:

(A) 1-i, 2-ii, 3-iii, 4-iv

(B) 1-ii, 2-i, 3-iv, 4-iii

(C) 1-i, 2-iii, 3-ii, 4-iv

(D) 1-iv, 2-iii, 3-i, 4-ii

Explanation:

Correct matching reflects biochemical events: NADH is formed at steps 1 & 2, FADH2 at succinate → fumarate, and GTP at succinyl-CoA → succinate. (Answer: A)

9) Fill in the Blanks MCQ:

During conversion of α-ketoglutarate to succinyl-CoA, ________ is released.

(A) CO2

(B) GTP

(C) FADH2

(D) Citrate

Explanation:

Oxidative decarboxylation of α-ketoglutarate releases CO2 and reduces NAD+ to NADH, forming succinyl-CoA. This is one of two CO2-generating steps in Krebs cycle. (Answer: A)

10) Choose the correct statements MCQ:

1. Three NADH and one FADH2 are produced per acetyl-CoA.

2. Two CO2 molecules are released per acetyl-CoA.

3. GTP is produced from succinyl-CoA.

4. Acetyl-CoA condenses with pyruvate to form citrate.

Options:

(A) 1, 2, 3

(B) 1, 2, 4

(C) 2, 3, 4

(D) 1, 3, 4

Explanation:

Statements 1, 2, 3 are correct. Statement 4 is wrong because acetyl-CoA condenses with oxaloacetate, not pyruvate. This clarifies the sequence and energy yield of Krebs cycle. (Answer: A)

Topic: Stomatal Physiology

Subtopic: Mechanism of Stomatal Opening

Keyword Definitions:

• Stomata – Small pores on leaf surface for gas exchange.

• Guard cells – Specialized cells surrounding stomatal pore, controlling its opening and closing.

• Turgidity – Pressure of water inside cells, causing swelling.

• Cellulose microfibrils – Structural components of cell wall influencing shape and flexibility.

• Aperture – The opening of the stomatal pore.

• Longitudinal orientation – Arrangement of microfibrils along the length of guard cells.

• Radial orientation – Microfibrils arranged perpendicular to guard cell length.

• Osmosis – Movement of water into guard cells affecting turgor pressure.

• Potassium ions – Ions involved in guard cell turgor regulation.

• Clinical relevance – Stomatal regulation affects plant water use, drought resistance.

Lead Question – 2017:

Which of the following facilitates opening of stomatal aperture?

(A) Longitudinal orientation of cellulose microfibrils in the cell wall of guard cells

(B) Contraction of outer wall of guard cells

(C) Decrease in turgidity of guard cells

(D) Radial orientation of cellulose microfibrils in the cell wall of guard cells

Explanation:

Stomatal opening occurs due to radial orientation of cellulose microfibrils in guard cells, allowing the cells to bow out when turgid. Water influx increases turgor pressure, causing radial expansion and opening the pore. Longitudinal microfibrils or decreased turgidity would not facilitate opening. (Answer: D)

1) Single Correct Answer MCQ:

Which ion accumulation triggers stomatal opening?

(A) Sodium

(B) Potassium

(C) Calcium

(D) Magnesium

Explanation:

Accumulation of potassium ions in guard cells lowers water potential, leading to water influx and increased turgor pressure, opening stomata. Sodium or calcium play secondary roles, but potassium is the primary driver for guard cell swelling. (Answer: B)

2) Single Correct Answer MCQ:

Stomatal closure is induced by:

(A) Light

(B) High CO2

(C) Potassium influx

(D) Radial microfibrils

Explanation:

Stomata close in response to high CO2 concentration or water stress. Loss of potassium from guard cells reduces turgor pressure, closing the pore. Light and radial microfibrils facilitate opening, not closure. (Answer: B)

3) Single Correct Answer MCQ:

Which hormone promotes stomatal closure?

(A) Auxin

(B) Abscisic acid

(C) Gibberellin

(D) Cytokinin

Explanation:

Abscisic acid (ABA) signals guard cells during drought, causing potassium efflux, reduced turgor, and stomatal closure. Auxins, gibberellins, and cytokinins generally promote growth, not closure. ABA is key for plant water conservation under stress. (Answer: B)

4) Single Correct Answer MCQ:

The swelling of guard cells is due to:

(A) Osmosis of water

(B) Photosynthesis

(C) Cell division

(D) Evaporation

Explanation:

Osmosis of water into guard cells increases turgor pressure, causing swelling and stomatal opening. Photosynthesis or cell division does not directly cause swelling, and evaporation reduces turgor rather than increasing it. (Answer: A)

5) Single Correct Answer MCQ:

Which cell wall feature allows bowing of guard cells?

(A) Radial microfibrils

(B) Longitudinal microfibrils

(C) Lignin deposits

(D) Pectin accumulation

Explanation:

Radial microfibrils permit guard cells to bow outward when turgid, widening the stomatal pore. Longitudinal microfibrils restrict bowing. Lignin or pectin provide rigidity but do not facilitate the dynamic opening mechanism. (Answer: A)

6) Single Correct Answer MCQ:

Which environmental factor increases stomatal opening?

(A) Darkness

(B) Light

(C) High CO2

(D) Drought

Explanation:

Light stimulates stomatal opening by activating proton pumps and potassium uptake, increasing turgor in guard cells. Darkness or drought induces closure, while high CO2 can signal partial closure. (Answer: B)

7) Assertion-Reason MCQ:

Assertion (A): Guard cells open when turgor increases.

Reason (R): Radial microfibrils in cell walls facilitate expansion.

(A) Both A and R true, R correct explanation

(B) Both A and R true, R not correct explanation

(C) A true, R false

(D) A false, R true

Explanation:

Both A and R are true. Increased turgor pressure causes guard cells to expand, and radial microfibrils direct the expansion outward, opening the stomatal pore. The orientation of microfibrils is the correct mechanism. (Answer: A)

8) Matching Type MCQ:

Match the term with its feature:

(A) Radial microfibrils – (i) Facilitate bowing of guard cells

(B) Longitudinal microfibrils – (ii) Restrict cell expansion

(C) Potassium influx – (iii) Increases turgor

(D) ABA – (iv) Induces closure

Options:

(A) A-i, B-ii, C-iii, D-iv

(B) A-ii, B-i, C-iv, D-iii

(C) A-i, B-iii, C-ii, D-iv

(D) A-iii, B-ii, C-i, D-iv

Explanation:

Correct matching: Radial microfibrils facilitate bowing, longitudinal restrict expansion, potassium influx increases turgor, ABA induces closure. This explains structural and hormonal regulation of stomatal aperture. (Answer: A)

9) Fill in the Blanks MCQ:

Stomatal opening is promoted by ________ turgor in guard cells.

(A) Increased

(B) Decreased

(C) No change

(D) Negative

Explanation:

Stomatal aperture is controlled by guard cell turgor. Increased turgor due to water influx causes guard cells to swell, bow outward, and open the stomatal pore, facilitating gas exchange. Decreased turgor leads to closure. (Answer: A)

10) Choose the correct statements MCQ:

1. Radial microfibrils facilitate stomatal opening.

2. Potassium ions accumulation in guard cells increases turgor.

3. Abscisic acid promotes stomatal opening.

4. Light stimulates stomatal opening.

Options:

(A) 1, 2, 3, 4

(B) 1, 2, 4

(C) 2, 3, 4

(D) 1, 3, 4

Explanation:

Statements 1, 2, and 4 are correct. Radial microfibrils allow bowing, potassium influx increases turgor, and light promotes opening. Abscisic acid induces closure, not opening. Knowledge of structural, ionic, and environmental factors is crucial for understanding stomatal physiology. (Answer: B)

Subtopic: Cellular Respiration

Keyword Definitions:

- Oxidative phosphorylation: ATP production using energy from electron transport chain.

- ATP: Adenosine triphosphate, primary energy currency of the cell.

- Substrate-level phosphorylation: Direct transfer of phosphate group to ADP from a substrate.

- Electron transport chain (ETC): Series of protein complexes in mitochondria transferring electrons to generate proton gradient.

- Mitochondria: Cell organelle where oxidative phosphorylation occurs.

- Proton gradient: Electrochemical gradient of H+ ions used to drive ATP synthesis.

- ADP: Adenosine diphosphate, precursor to ATP.

- ATP synthase: Enzyme that synthesizes ATP using proton motive force.

Lead Question - 2016 (Phase 2)

Oxidative phosphorylation is:

(1) formation of ATP by energy released from electrons removed during substrate oxidation

(2) formation of ATP by transfer of phosphate group from a substrate to ADP

(3) oxidation of phosphate group in ATP

(4) addition of phosphate group to ATP

Answer & Explanation:

Correct answer: formation of ATP by energy released from electrons removed during substrate oxidation. Oxidative phosphorylation occurs in mitochondria, where electrons from NADH and FADH2 pass through the electron transport chain, creating a proton gradient. ATP synthase uses this energy to phosphorylate ADP to ATP efficiently.

1. Where does oxidative phosphorylation occur?

(1) Cytoplasm

(2) Nucleus

(3) Mitochondrial matrix

(4) Mitochondrial inner membrane

Answer & Explanation:

Oxidative phosphorylation occurs in the inner mitochondrial membrane. Electrons pass through the electron transport chain complexes, generating a proton gradient across the membrane. ATP synthase uses this gradient to produce ATP. This process is essential for cellular energy production and efficient metabolism.

2. Assertion (A): Oxidative phosphorylation requires oxygen.

Reason (R): Oxygen acts as the final electron acceptor.

(1) Both A and R are true, and R is correct explanation of A

(2) Both A and R are true, but R is not correct explanation of A

(3) A is true, R is false

(4) A is false, R is true

Answer & Explanation:

Both A and R are true, and R is correct explanation of A. Oxygen accepts electrons at the end of the electron transport chain, forming water. Without oxygen, the electron transport chain would halt, preventing ATP production via oxidative phosphorylation.

3. Match the following:

A. Substrate-level phosphorylation - (i) Direct phosphate transfer

B. Oxidative phosphorylation - (ii) ATP via electron transport

C. Photophosphorylation - (iii) ATP via light energy

(1) A-i, B-ii, C-iii

(2) A-ii, B-i, C-iii

(3) A-iii, B-ii, C-i

(4) A-i, B-iii, C-ii

Answer & Explanation:

Correct answer: A-i, B-ii, C-iii. Substrate-level phosphorylation transfers phosphate directly to ADP. Oxidative phosphorylation generates ATP using energy from electrons in the ETC. Photophosphorylation occurs in chloroplasts during photosynthesis, using light energy to produce ATP.

4. Fill in the blank:

The enzyme that synthesizes ATP during oxidative phosphorylation is ________.

(1) Hexokinase

(2) Phosphofructokinase

(3) ATP synthase

(4) Pyruvate kinase

Answer & Explanation:

ATP synthase synthesizes ATP by utilizing the proton gradient generated across the mitochondrial inner membrane. Protons flow through ATP synthase, driving conformational changes that phosphorylate ADP to ATP, which is the primary energy currency for cellular functions.

5. Clinical-type Question:

Deficiency in oxidative phosphorylation may result in:

(1) Hypoglycemia

(2) Mitochondrial diseases

(3) Excess ATP

(4) Reduced glycolysis

Answer & Explanation:

Deficiency in oxidative phosphorylation impairs ATP production, leading to mitochondrial disorders, muscle weakness, neurological defects, and lactic acidosis. Cells rely on less efficient glycolysis, which cannot meet energy demands, highlighting the clinical significance of proper mitochondrial function.

6. Which molecules donate electrons for oxidative phosphorylation?

(1) Glucose

(2) NADH and FADH2

(3) Oxygen

(4) ADP

Answer & Explanation:

NADH and FADH2 donate electrons to the electron transport chain during oxidative phosphorylation. These electrons drive the formation of a proton gradient, which ATP synthase uses to produce ATP. Efficient electron donation is critical for cellular energy supply.

7. Which process is coupled to ATP synthesis in mitochondria?

(1) Glycolysis

(2) Citric acid cycle

(3) Electron transport chain

(4) Gluconeogenesis

Answer & Explanation:

The electron transport chain is coupled to ATP synthesis. Electron flow through the chain generates a proton gradient across the inner mitochondrial membrane, which ATP synthase uses to phosphorylate ADP, forming ATP efficiently in oxidative phosphorylation.

8. Choose the correct statements:

(a) Oxidative phosphorylation requires oxygen

(b) It occurs in cytoplasm

(c) It produces most ATP in cellular respiration

(d) Substrate-level phosphorylation produces more ATP

Answer & Explanation:

Correct statements are a and c. Oxidative phosphorylation occurs in mitochondria, requires oxygen, and produces the majority of ATP during cellular respiration. Substrate-level phosphorylation contributes less ATP and occurs in glycolysis and citric acid cycle.