Photosynthesis

Topic: Essential Elements; Subtopic: Micronutrients in Photosynthesis

Keyword Definitions:

• Micronutrients: Essential elements required by plants in very small quantities for normal growth and metabolism.

• Photosynthesis: The process by which green plants use sunlight to synthesize food from carbon dioxide and water.

• Light Reaction: The phase of photosynthesis occurring in thylakoid membranes where light energy is converted into chemical energy (ATP and NADPH).

• Oxygen Evolution: The release of oxygen during the photolysis of water in photosystem II.

• Manganese and Chlorine: Essential micronutrients involved in the splitting of water molecules during photosynthesis.

Lead Question - 2022 (Abroad)
Which of the following pair of micronutrients would help in the light phase of photosynthesis to help in the reaction leading to oxygen evolution?
1. Zinc and Chlorine
2. Manganese and Molybdenum
3. Molybdenum and Iron
4. Manganese and Chlorine
Explanation: The light phase of photosynthesis involves the photolysis of water, which leads to oxygen evolution. The oxygen-evolving complex (OEC) of photosystem II requires manganese (Mn) and chlorine (Cl) ions as cofactors for splitting water molecules into protons, electrons, and oxygen. Hence, the correct answer is option 4 — Manganese and Chlorine.

1. Manganese plays an important role in:
1. Activation of nitrogenase enzyme
2. Splitting of water to release oxygen
3. Formation of chlorophyll pigment
4. Reduction of nitrates to ammonia
Explanation: Manganese (Mn) acts as a cofactor in the oxygen-evolving complex (OEC) of photosystem II. It facilitates the splitting of water molecules into protons, electrons, and oxygen during the light reaction of photosynthesis. Hence, the correct answer is option 2 — Splitting of water to release oxygen.

2. Chlorine deficiency in plants results in:
1. Reduced oxygen evolution and leaf wilting
2. Increased nitrate accumulation
3. Enhanced chlorophyll synthesis
4. Excessive transpiration
Explanation: Chlorine (Cl⁻) helps in maintaining osmotic balance and acts as an essential ion in the oxygen-evolving complex of photosystem II. Its deficiency reduces oxygen evolution, leading to wilting and chlorosis. Therefore, the correct answer is option 1 — Reduced oxygen evolution and leaf wilting.

3. The oxygen-evolving complex (OEC) is associated with:
1. Photosystem I
2. Photosystem II
3. Calvin cycle
4. Glycolysis
Explanation: The oxygen-evolving complex (OEC) or water-splitting complex is part of photosystem II, located on the inner surface of the thylakoid membrane. It catalyzes the photolysis of water to release oxygen, protons, and electrons. Hence, the correct answer is option 2 — Photosystem II.

4. Which of the following micronutrients is involved in nitrogen metabolism rather than oxygen evolution?
1. Manganese
2. Molybdenum
3. Chlorine
4. Iron
Explanation: Molybdenum (Mo) acts as a cofactor for nitrate reductase and nitrogenase enzymes, helping in nitrogen reduction and fixation. It is not directly involved in the oxygen-evolving complex of photosynthesis. Hence, the correct answer is option 2 — Molybdenum.

5. The splitting of water molecules during photosynthesis provides:
1. ATP
2. NADPH
3. Protons, electrons, and oxygen
4. Glucose and CO₂
Explanation: During the light reaction of photosynthesis, water is split by the oxygen-evolving complex in photosystem II into protons (H⁺), electrons (e⁻), and oxygen (O₂). This process provides electrons for photosynthetic electron transport and contributes to proton gradient formation. Hence, the correct answer is option 3.

6. Which pair of elements are directly required for photolysis of water in chloroplasts?
1. Mn and Cl
2. Fe and Mo
3. Zn and Cu
4. Mg and K
Explanation: The photolysis of water requires manganese (Mn) and chloride (Cl) ions as essential cofactors in the oxygen-evolving complex (OEC) of photosystem II. These elements help in the oxidation of water molecules, leading to oxygen evolution. Hence, the correct answer is option 1 — Mn and Cl.

7. Assertion (A): Manganese and Chlorine are required for oxygen evolution in plants.
Reason (R): They form part of the oxygen-evolving complex in photosystem II.
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: Manganese and Chlorine ions are essential for the oxygen-evolving complex (OEC) of photosystem II, which catalyzes the photolysis of water, releasing oxygen. Both statements are true, and the reason correctly explains the assertion. Hence, the correct answer is option 1.

8. Match the following:
A. Manganese — (i) Oxygen-evolving complex
B. Molybdenum — (ii) Nitrate reductase
C. Iron — (iii) Electron transport chain
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)
Explanation: Manganese participates in oxygen evolution in PS II, Molybdenum in nitrate reduction, and Iron in electron transport during photosynthesis. Hence, the correct matching is A-(i), B-(ii), C-(iii), corresponding to option 1.

9. Fill in the blank:
The oxygen-evolving complex of photosystem II contains __________ ions essential for splitting of water.
1. Iron
2. Molybdenum
3. Manganese and Chlorine
4. Zinc and Copper
Explanation: The oxygen-evolving complex (OEC) consists of manganese and chloride ions that are critical for the photolysis of water molecules in photosystem II. These ions help stabilize the complex and enable the release of oxygen. Hence, the correct answer is option 3.

10. Choose the correct statements:
1. Manganese and Chlorine are involved in water splitting.
2. Iron is part of chlorophyll structure.
3. Molybdenum helps in nitrogen fixation.
4. Zinc acts as a cofactor for carbonic anhydrase.
Explanation: All four statements are scientifically correct. Manganese and Chlorine assist in oxygen evolution, Iron aids in electron transport, Molybdenum assists in nitrogen metabolism, and Zinc activates carbonic anhydrase for CO₂ fixation. Therefore, the correct answer is option 1, 2, 3, and 4 — all are correct.

Subtopic: CO2 Enrichment in Agriculture

• CO2 Enrichment: Increasing carbon dioxide concentration in the atmosphere to enhance photosynthesis and crop yield.

• Sugar Beet: A root crop rich in sucrose, cultivated for sugar production.

• Cabbage: Leafy vegetable belonging to Brassicaceae family, responds well to CO2 enrichment.

• Photosynthesis: Process by which plants convert CO2 and water into glucose and oxygen using sunlight.

• Crop Yield: Quantity of crop harvested per unit area.

• Tomatoes: Fruiting vegetable crop sensitive to CO2 enrichment for improved growth and yield.

• Wheat: Cereal crop; growth can be enhanced under higher CO2 conditions.

• Bell Pepper: Fruiting vegetable crop; yield responds to CO2-enriched environment.

• CO2 Fertigation: Technique of supplying CO2 to greenhouse crops for enhanced photosynthesis.

• Brassica Crops: Plants of mustard family, includes cabbage, respond to CO2 enrichment.

• Controlled Environment Agriculture: Using greenhouses or growth chambers to manipulate CO2, temperature, and light for crop productivity.

Lead Question - 2022 (Abroad)

Following crops have been extensively cultivated in CO2 rich atmosphere for higher yield:

1. Sugar beet and Cabbage

2. Carrots and Tomatoes

3. Wheat and Sugar beet

4. Tomatoes and Bell pepper

Explanation: Sugar beet and cabbage are among the crops that show significant increase in growth and yield under CO2-enriched atmospheres due to enhanced photosynthetic rates. These crops are commonly cultivated in controlled environment agriculture or greenhouses to utilize CO2 enrichment. Answer: Sugar beet and Cabbage. Answer: 1

Q1: Which photosynthetic pathway is more responsive to CO2 enrichment for increased yield?

1. C3 pathway

2. C4 pathway

3. CAM pathway

4. Both C4 and CAM

Explanation: C3 plants like sugar beet and cabbage show a higher increase in photosynthesis and yield under elevated CO2 conditions. C4 and CAM plants have mechanisms that already concentrate CO2, making them less responsive. Answer: C3 pathway. Answer: 1

Q2: Which greenhouse technique enhances crop yield using higher CO2?

1. Hydroponics

2. CO2 Fertigation

3. Mulching

4. Intercropping

Explanation: CO2 fertigation supplies additional carbon dioxide to greenhouse crops, boosting photosynthesis and yield. Hydroponics relates to soil-less cultivation, mulching reduces evaporation, and intercropping involves planting multiple crops together. Answer: CO2 Fertigation. Answer: 2

Q3: Which leafy vegetable responds well to CO2 enrichment?

1. Spinach

2. Cabbage

3. Lettuce

4. Beetroot

Explanation: Cabbage, a Brassica crop, shows enhanced growth and biomass accumulation under CO2-enriched conditions due to higher photosynthesis rates. Spinach and lettuce respond moderately, while beetroot is a root crop. Answer: Cabbage. Answer: 2

Q4: Elevated CO2 mainly enhances which process in plants?

1. Respiration

2. Transpiration

3. Photosynthesis

4. Germination

Explanation: Elevated CO2 increases the rate of photosynthesis in C3 plants by providing more substrate for carbon fixation, leading to higher biomass and yield. Respiration and transpiration are indirectly affected, while germination is not directly influenced. Answer: Photosynthesis. Answer: 3

Q5: Which crop is both a C3 plant and highly responsive to CO2 enrichment?

1. Maize

2. Sugar beet

3. Sorghum

4. Millet

Explanation: Sugar beet is a C3 plant that exhibits increased growth and yield under elevated CO2 due to enhanced photosynthetic efficiency. Maize, sorghum, and millet are C4 crops and show less response. Answer: Sugar beet. Answer: 2

Q6: CO2 enrichment is most commonly applied in which environment?

1. Open fields

2. Greenhouses

3. Hydroponics

4. Polyculture systems

Explanation: CO2 enrichment is typically applied in greenhouses where CO2 concentration can be controlled to boost photosynthesis and yield. Open fields cannot maintain elevated CO2 levels consistently. Hydroponics and polyculture are cultivation methods but not specifically CO2 enrichment environments. Answer: Greenhouses. Answer: 2

Q7: Assertion (A): CO2 enrichment increases yield of C3 crops.
Reason (R): Higher CO2 improves water use efficiency and photosynthetic rate in C3 plants.

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: Elevated CO2 increases photosynthesis in C3 crops, improving biomass accumulation and yield. It also reduces stomatal opening, enhancing water use efficiency. Therefore, the reason explains the assertion correctly. Answer: Both A and R are correct and R explains A. Answer: 3

Q8: Match the crop with response to CO2 enrichment:

1. Sugar beet    A. High response
2. Wheat    B. Moderate response
3. Maize    C. Low response

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

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

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

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

Explanation: Sugar beet, a C3 crop, responds highly to CO2 enrichment. Wheat shows moderate increase, while maize, a C4 plant, responds minimally because its carbon concentrating mechanism reduces CO2 limitation. Correct match is 1-A, 2-B, 3-C. Answer: 1

Q9: Elevated CO2 reduces stomatal opening, thereby improving ______ in plants.

1. Photosynthetic rate

2. Water use efficiency

3. Respiration rate

4. Seed germination

Explanation: Elevated CO2 causes partial stomatal closure, reducing water loss and improving water use efficiency while simultaneously enhancing photosynthesis in C3 crops. Respiration and germination are not significantly affected. Answer: Water use efficiency. Answer: 2

Q10: Select the correct statements regarding CO2 enrichment:

1. Enhances photosynthesis in C3 plants

2. Improves water use efficiency

3. Used in greenhouses for higher yield

4. C4 plants show minimal response

Explanation: CO2 enrichment enhances photosynthesis and biomass accumulation in C3 crops, improves water use efficiency, and is applied in greenhouses for higher yield. C4 crops are less responsive due to their carbon-concentrating mechanism. All statements correctly describe CO2 enrichment effects. Answer: 1,2,3,4

Subtopic: Carbon Fixation in C3 and C4 Plants

• C3 Plants: Plants that fix CO2 through the Calvin cycle forming a 3-carbon compound (3-phosphoglycerate).

• C4 Plants: Plants that fix CO2 into a 4-carbon compound (oxaloacetate) before entering Calvin cycle, reducing photorespiration.

• Carbon Fixation: Conversion of inorganic CO2 into organic compounds during photosynthesis.

• Calvin Cycle: Light-independent reactions in chloroplasts that synthesize sugars using CO2.

• Photorespiration: Process where oxygen is consumed and CO2 released, reducing photosynthetic efficiency.

• Mesophyll Cells: Cells in leaves where initial CO2 fixation occurs in C4 plants.

• Bundle Sheath Cells: Specialized cells in C4 plants where Calvin cycle occurs.

• Efficiency: Rate of CO2 fixation per unit time under same conditions.

• PEP Carboxylase: Enzyme in C4 plants that fixes CO2 into oxaloacetate.

• Rubisco: Enzyme in Calvin cycle that fixes CO2 in both C3 and C4 plants.

• Photosynthetic Pathway: Biochemical steps by which plants convert CO2 into organic compounds.

Lead Question - 2022 (Abroad)

The ratio of carbon dioxide fixation between C4 plants and C3 plants is:

1. 2:1

2. 2:3

3. 1:1

4. 1:2

Explanation: C4 plants fix CO2 more efficiently than C3 plants due to their specialized mechanism that minimizes photorespiration. Under similar light and CO2 conditions, the rate of CO2 fixation in C4 plants is roughly twice that of C3 plants. Hence, the ratio of carbon dioxide fixation between C4 and C3 plants is 2:1. Answer: 1

Q1: Which enzyme initially fixes CO2 in C4 plants?

1. Rubisco

2. PEP Carboxylase

3. ATP Synthase

4. Hexokinase

Explanation: In C4 plants, CO2 is first fixed into oxaloacetate by PEP carboxylase in mesophyll cells, which reduces photorespiration. Rubisco later fixes CO2 in bundle sheath cells. ATP synthase and hexokinase are unrelated to initial CO2 fixation. Answer: PEP Carboxylase. Answer: 2

Q2: Where does the Calvin cycle occur in C4 plants?

1. Mesophyll cells

2. Bundle sheath cells

3. Stomata

4. Root hairs

Explanation: In C4 plants, CO2 is transported from mesophyll cells to bundle sheath cells where Rubisco fixes CO2 in the Calvin cycle. Mesophyll cells handle initial fixation, stomata are openings for gas exchange, and root hairs absorb water and minerals. Answer: Bundle sheath cells. Answer: 2

Q3: Main advantage of C4 pathway over C3 pathway is:

1. Higher water use efficiency

2. Reduced photorespiration

3. Better CO2 fixation under high temperature

4. All of the above

Explanation: C4 pathway reduces photorespiration, enhances water use efficiency, and allows better CO2 fixation under high temperature. These combined advantages make C4 plants more productive in tropical and subtropical climates. Answer: All of the above. Answer: 4

Q4: C3 plants predominantly use which photosynthetic pathway?

1. Hatch-Slack pathway

2. Calvin cycle

3. Cyclic photophosphorylation

4. Photorespiration pathway

Explanation: C3 plants fix CO2 directly through the Calvin cycle forming 3-phosphoglycerate. Hatch-Slack is specific to C4 plants, cyclic photophosphorylation generates ATP, and photorespiration is a wasteful process. Hence, Calvin cycle is the primary pathway. Answer: Calvin cycle. Answer: 2

Q5: In which cell type of C4 plants does PEP carboxylase operate?

1. Bundle sheath cells

2. Mesophyll cells

3. Guard cells

4. Parenchyma cells

Explanation: PEP carboxylase fixes CO2 into oxaloacetate in mesophyll cells of C4 plants, before transferring to bundle sheath cells for the Calvin cycle. Guard cells regulate stomata, and parenchyma cells have general metabolic roles. Answer: Mesophyll cells. Answer: 2

Q6: Which condition favors C4 photosynthesis over C3?

1. Low light intensity

2. High CO2 concentration

3. High temperature and drought

4. Low temperature

Explanation: C4 plants are more efficient than C3 under high temperature and drought because their mechanism minimizes photorespiration. Low light or low temperature favors C3 efficiency, and high CO2 reduces photorespiration in C3 plants. Answer: High temperature and drought. Answer: 3

Q7: Assertion (A): C4 plants have higher photosynthetic efficiency than C3 plants.
Reason (R): C4 plants concentrate CO2 around Rubisco, reducing photorespiration.

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: C4 plants concentrate CO2 in bundle sheath cells around Rubisco, minimizing photorespiration and enhancing photosynthetic efficiency. Both assertion and reason are true, and the reason correctly explains the assertion. Answer: Both A and R are correct and R explains A. Answer: 3

Q8: Match the plant type with characteristic:

1. C3 Plant    A. Calvin cycle only
2. C4 Plant    B. CO2 first fixed into oxaloacetate
3. CAM Plant    C. Fixes CO2 at night, stores as malate

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

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

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

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

Explanation: C3 plants use Calvin cycle directly. C4 plants first fix CO2 into oxaloacetate via PEP carboxylase. CAM plants fix CO2 at night storing it as malate to minimize water loss. Correct matching is 1-A, 2-B, 3-C. Answer: 1

Q9: The enzyme responsible for fixing CO2 in Calvin cycle is ______.

1. PEP Carboxylase

2. Rubisco

3. ATP Synthase

4. NADP Reductase

Explanation: Rubisco catalyzes the fixation of CO2 into 3-phosphoglycerate during the Calvin cycle in both C3 and C4 plants. PEP carboxylase operates in C4 initial fixation, ATP synthase generates ATP, and NADP reductase participates in electron transport. Answer: Rubisco. Answer: 2

Q10: Select correct statements regarding C4 photosynthesis:

1. C4 plants reduce photorespiration

2. C4 plants have higher CO2 fixation under high temperature

3. PEP carboxylase fixes CO2 initially

4. CO2 is concentrated in bundle sheath cells for Calvin cycle

Explanation: C4 plants utilize PEP carboxylase for initial CO2 fixation, concentrate CO2 in bundle sheath cells, reduce photorespiration, and maintain high fixation rates under high temperature. All four statements accurately describe C4 photosynthesis. Answer: All of the above. Answer: 1,2,3,4

Topic: Light Reaction of Photosynthesis
Subtopic: Products of Light Reaction

Keyword Definitions:
- Light Reaction: The photochemical phase of photosynthesis where light energy is converted into chemical energy.
- ATP: Adenosine Triphosphate, an energy currency molecule formed during photophosphorylation.
- NADPH: Nicotinamide Adenine Dinucleotide Phosphate (reduced form), a reducing agent used in the Calvin cycle.
- Photolysis: The splitting of water by light energy, releasing O2, electrons, and protons.
- Photosystems: Complexes of pigments (PS I and PS II) that absorb light energy in chloroplasts.
- Electron Transport Chain: A series of carriers transferring electrons, producing ATP and NADPH.
- Photophosphorylation: Formation of ATP from ADP and inorganic phosphate using light energy.
- Oxygen Evolution: The release of O2 as a by-product of water splitting in photosynthesis.
- Chloroplast: Organelle where photosynthesis occurs in plant cells.
- Calvin Cycle: The dark phase using ATP and NADPH to fix CO2 into carbohydrates.

Lead Question - 2022 (Abroad):
The products of light reaction in photosynthesis are:
(1) ATP, NADPH and O2
(2) ATP, NADPH and O2 + H2O
(3) ATP, NADPH and H2O
(4) ATP, NADPH and CO2
Explanation: The correct answer is (1). The light reaction forms ATP and NADPH as energy molecules and releases O2 through photolysis of water. These products fuel the Calvin cycle to synthesize glucose during the dark reaction phase of photosynthesis.

1. Which pigment directly participates in the light reaction?
(a) Chlorophyll a
(b) Chlorophyll b
(c) Carotenoids
(d) Xanthophyll
Explanation: The correct answer is (a). Chlorophyll a acts as the primary pigment capturing light energy in the reaction center. It converts absorbed light energy into chemical energy that drives photochemical reactions in photosystems I and II.

2. The splitting of water molecule during light reaction releases:
(a) Hydrogen and Oxygen only
(b) Electrons, protons, and oxygen
(c) ATP and NADPH
(d) Glucose and oxygen
Explanation: The correct answer is (b). Water photolysis in photosystem II yields electrons, protons, and oxygen. Electrons replace those lost by chlorophyll, protons aid ATP synthesis, and oxygen is released as a by-product into the atmosphere.

3. Which photosystem is responsible for oxygen evolution?
(a) Photosystem I
(b) Photosystem II
(c) Both PSI and PSII
(d) Cytochrome complex
Explanation: The correct answer is (b). Photosystem II splits water molecules in the thylakoid lumen, releasing oxygen, protons, and electrons. This process is known as photolysis and is unique to PSII during the light-dependent reactions.

4. In non-cyclic photophosphorylation, electrons lost by PSII ultimately reduce:
(a) NADP+
(b) ATP
(c) O2
(d) H2O
Explanation: The correct answer is (a). In non-cyclic photophosphorylation, electrons from PSII are transferred to NADP+ via the electron transport chain, forming NADPH. This reducing power, along with ATP, supports CO2 fixation in the Calvin cycle.

5. Which of the following occurs in the stroma of chloroplast?
(a) Photolysis of water
(b) Photophosphorylation
(c) Calvin cycle
(d) Light absorption
Explanation: The correct answer is (c). The Calvin cycle, also called the dark reaction, takes place in the chloroplast stroma. It utilizes ATP and NADPH from the light reaction to convert carbon dioxide into glucose.

6. The oxygen released during photosynthesis comes from:
(a) CO2
(b) H2O
(c) Glucose
(d) NADPH
Explanation: The correct answer is (b). Oxygen released during photosynthesis originates from the photolysis of water molecules in Photosystem II. This process splits H2O into electrons, protons, and molecular oxygen, which diffuses out of the chloroplast.

7. Assertion-Reason:
Assertion (A): ATP and NADPH are produced during the light reaction.
Reason (R): These molecules are required in the Calvin cycle for carbon fixation.
(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, but R is false
(d) A is false, but R is true
Explanation: Option (a) is correct. ATP and NADPH generated in the light reaction provide energy and reducing power for carbon fixation in the Calvin cycle, linking both phases of photosynthesis efficiently.

8. Matching Type:
Match the following:
(a) Photosystem I (i) O2 evolution
(b) Photosystem II (ii) NADPH formation
(c) Photolysis (iii) Water splitting
(d) ATP synthase (iv) ATP formation
Options:
(1) a-ii, b-i, c-iii, d-iv
(2) a-iv, b-ii, c-i, d-iii
(3) a-i, b-iii, c-ii, d-iv
(4) a-ii, b-iv, c-i, d-iii
Explanation: The correct option is (1). Photosystem I forms NADPH, Photosystem II initiates O2 evolution, photolysis splits water, and ATP synthase produces ATP. These coordinated reactions sustain photosynthetic efficiency.

9. Fill in the Blank:
During photolysis, ______ is released as a by-product.
(a) Carbon dioxide
(b) Glucose
(c) Oxygen
(d) NADPH
Explanation: The correct answer is (c). Oxygen is released during photolysis in Photosystem II as water splits under light energy, providing electrons and protons for photosynthetic reactions.

10. Choose the Correct Statements:
1. Light reaction occurs in thylakoids.
2. O2 is released from photolysis of water.
3. ATP is produced in stroma.
4. NADPH is formed in Photosystem II.
(a) 1 and 2
(b) 1, 2, and 3
(c) 2, 3, and 4
(d) 1, 3, and 4
Explanation: Option (a) is correct. The light reaction occurs in thylakoids, releasing O2 from photolysis. ATP forms via ATP synthase, while NADPH forms in Photosystem I, not II.

Topic: C4 Pathway (Hatch and Slack Pathway)
Subtopic: Role of Bundle Sheath Cells and Kranz Anatomy

Keyword Definitions:

• Bundle Sheath Cells: Large cells surrounding vascular bundles in C4 plants, containing numerous chloroplasts for the Calvin cycle.

• Kranz Anatomy: Leaf anatomy in C4 plants with concentric arrangement of mesophyll and bundle sheath cells.

• Photorespiration: Wasteful process where oxygen is used instead of carbon dioxide, reducing photosynthetic efficiency.

• C4 Pathway: A photosynthetic adaptation minimizing photorespiration and increasing efficiency in high light and temperature.

• Calvin Cycle: The light-independent phase of photosynthesis occurring in the stroma or bundle sheath chloroplasts.

Lead Question (2022):
What is the role of large bundle sheath cells found around the vascular bundles in C4 plants:
(1) To increase the number of chloroplast for the operation of Calvin cycle
(2) To enable the plant to tolerate high temperature
(3) To protect the vascular tissue from high light intensity
(4) To provide the site for photorespiratory pathway

Explanation: In C4 plants, bundle sheath cells contain numerous chloroplasts that perform the Calvin cycle isolated from oxygen, reducing photorespiration. These cells concentrate CO₂ via the malate transport from mesophyll cells, enhancing photosynthetic efficiency. Hence, the correct answer is (1) To increase the number of chloroplast for the operation of Calvin cycle.

1. Guessed Question:
Which of the following is a characteristic feature of C4 plants?
(1) Presence of Kranz anatomy
(2) Absence of bundle sheath cells
(3) High rate of photorespiration
(4) Lack of PEP carboxylase enzyme

Explanation: C4 plants exhibit Kranz anatomy with mesophyll and bundle sheath cells functioning in coordination to reduce photorespiration. PEP carboxylase fixes CO₂ efficiently in mesophyll cells. Hence, the correct answer is (1) Presence of Kranz anatomy.

2. Guessed Question:
In C4 plants, the first stable compound formed during CO₂ fixation is:
(1) 3-phosphoglyceric acid
(2) Oxaloacetic acid
(3) Malic acid
(4) Pyruvic acid

Explanation: C4 plants initially fix CO₂ in mesophyll cells through PEP carboxylase forming oxaloacetic acid (OAA), a four-carbon compound. This compound is then reduced to malate or aspartate before entering bundle sheath cells. Hence, the correct answer is (2) Oxaloacetic acid.

3. Guessed Question:
PEP carboxylase is active in which part of C4 plant leaf?
(1) Bundle sheath cells
(2) Mesophyll cells
(3) Epidermal cells
(4) Guard cells

Explanation: PEP carboxylase is an enzyme located in mesophyll cells of C4 plants. It fixes atmospheric CO₂ into oxaloacetic acid before transport to bundle sheath cells, where the Calvin cycle occurs. Hence, the correct answer is (2) Mesophyll cells.

4. Guessed Question:
Which enzyme is responsible for the fixation of CO₂ in bundle sheath cells of C4 plants?
(1) RuBisCO
(2) PEP carboxylase
(3) Malate dehydrogenase
(4) Pyruvate kinase

Explanation: RuBisCO enzyme present in the chloroplasts of bundle sheath cells catalyzes CO₂ fixation during the Calvin cycle. This compartmentalization helps prevent photorespiration and enhances photosynthetic efficiency. Hence, the correct answer is (1) RuBisCO.

5. Guessed Question:
Which one of the following statements about C4 plants is correct?
(1) They show high photorespiration
(2) They perform Calvin cycle only in mesophyll cells
(3) They possess Kranz anatomy
(4) They grow best under low temperature and shade

Explanation: C4 plants possess Kranz anatomy that minimizes photorespiration and increases CO₂ fixation efficiency, especially under high temperature and light intensity. Hence, the correct answer is (3) They possess Kranz anatomy.

6. Guessed Question:
Which of the following pairs is correctly matched?
(1) PEP carboxylase – Bundle sheath cell
(2) RuBisCO – Mesophyll cell
(3) PEP carboxylase – Mesophyll cell
(4) RuBisCO – Epidermal cell

Explanation: In C4 plants, PEP carboxylase is located in mesophyll cells, and RuBisCO is found in bundle sheath cells. This separation of enzymes prevents photorespiration. Hence, the correct answer is (3) PEP carboxylase – Mesophyll cell.

7. Assertion-Reason Question:
Assertion (A): C4 plants have higher photosynthetic efficiency than C3 plants.
Reason (R): They lack photorespiration due to spatial separation of initial CO₂ fixation and Calvin cycle.
(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, R is false.
(4) A is false, R is true.

Explanation: C4 plants exhibit spatial separation between mesophyll and bundle sheath cells, reducing photorespiration and enhancing carbon fixation efficiency. Thus, both A and R are true, and R correctly explains A. Hence, the correct answer is (1).

8. Matching Type Question:
Match the following enzymes with their functions:
A. PEP carboxylase — (i) Fixes CO₂ in bundle sheath cells
B. RuBisCO — (ii) Fixes CO₂ in mesophyll cells
C. Malate dehydrogenase — (iii) Converts OAA to malate
Options:
(1) A–ii, B–i, C–iii
(2) A–iii, B–ii, C–i
(3) A–i, B–ii, C–iii
(4) A–ii, B–iii, C–i

Explanation: PEP carboxylase fixes CO₂ in mesophyll cells, RuBisCO fixes CO₂ in bundle sheath cells, and malate dehydrogenase converts oxaloacetate to malate. Hence, the correct answer is (1) A–ii, B–i, C–iii.

9. Fill in the Blanks:
In C4 plants, CO₂ is initially fixed into a ______ carbon compound called ______.
(1) Three, PGA
(2) Four, OAA
(3) Two, Acetate
(4) One, CO

Explanation: In C4 plants, CO₂ fixation in mesophyll cells produces a four-carbon compound called oxaloacetic acid (OAA), catalyzed by PEP carboxylase. Hence, the correct answer is (2) Four, OAA.

10. Choose the Correct Statements:
(a) C4 plants show Kranz anatomy.
(b) Photorespiration is negligible in C4 plants.
(c) PEP carboxylase is active in bundle sheath cells.
(d) Calvin cycle occurs in bundle sheath cells.
(1) (a), (b), and (d) only
(2) (a), (c), and (d) only
(3) (a) and (b) only
(4) (b), (c), and (d) only

Explanation: C4 plants exhibit Kranz anatomy, minimal photorespiration, and Calvin cycle activity in bundle sheath cells, while PEP carboxylase acts in mesophyll cells. Hence, the correct answer is (1) (a), (b), and (d) only.

Subtopic: Chemiosmosis and Energy Release

Keyword Definitions:

• ATP Synthesis: Formation of adenosine triphosphate from ADP and inorganic phosphate during cellular respiration or photosynthesis.

• Chemiosmosis: Movement of protons across a membrane, generating ATP through ATP synthase.

• Proton gradient: Difference in proton concentration across a membrane, driving ATP synthesis.

• NADP/NADPH: Nicotinamide adenine dinucleotide phosphate; NADP is reduced to NADPH during photosynthesis to carry electrons.

• Stroma: Fluid-filled space inside chloroplasts surrounding thylakoids.

• Electron gradient: Flow of electrons through an electron transport chain creating proton gradient.

• Membrane: Thylakoid membrane in chloroplasts where chemiosmosis occurs.

• Energy release: Conversion of chemical energy in proton gradients to ATP.

• Proton-motive force: Force generated by proton gradient that drives ATP synthase.

• ATP synthase: Enzyme that synthesizes ATP using energy from proton flow.

Lead Question (2022)
Which one of the following is not true regarding the release of energy during ATP synthesis through chemiosmosis ? It involves:
(1) Breakdown of electron gradient
(2) Movement of protons across the membrane to the stroma
(3) Reduction of NADP to NADPH2 on the stroma side of the membrane
(4) Breakdown of proton gradient

Explanation:
ATP synthesis via chemiosmosis depends on the proton gradient across the thylakoid membrane. Protons move to the stroma, releasing energy for ATP formation. Electron flow and proton gradient breakdown drive this process. Reduction of NADP to NADPH occurs separately in photosynthesis, not directly part of ATP energy release. Correct answer is (3).

1. Single Correct Answer MCQ:
Which membrane enzyme directly synthesizes ATP during chemiosmosis?
(1) NADP reductase
(2) ATP synthase
(3) Cytochrome b6f
(4) Rubisco

Explanation:
ATP synthase is the enzyme that produces ATP using energy from proton flow across the thylakoid membrane. NADP reductase reduces NADP to NADPH, cytochrome b6f transfers electrons, and Rubisco fixes CO2. Correct answer is (2).

2. Single Correct Answer MCQ:
The proton gradient for ATP synthesis is established by:
(1) Proton pumps in mitochondrial matrix
(2) Electron transport chain in thylakoid membrane
(3) Cytoplasmic diffusion
(4) Stroma enzymes

Explanation:
The electron transport chain in the thylakoid membrane pumps protons into the lumen, creating a gradient. This gradient drives ATP synthesis. Cytoplasmic diffusion or stroma enzymes do not establish the proton gradient. Correct answer is (2).

3. Single Correct Answer MCQ:
Energy for ATP synthesis is released by:
(1) Electron absorption
(2) Proton flow down gradient
(3) Water splitting
(4) CO2 fixation

Explanation:
Protons flow down their gradient through ATP synthase, releasing energy used to synthesize ATP. Electron absorption initiates proton pumping, water splitting provides electrons, and CO2 fixation is part of Calvin cycle, not ATP energy release. Correct answer is (2).

4. Single Correct Answer MCQ:
The side of the membrane where protons accumulate is:
(1) Stroma
(2) Thylakoid lumen
(3) Cytoplasm
(4) Mitochondrial matrix

Explanation:
Protons accumulate inside the thylakoid lumen, creating a high concentration. ATP synthase allows them to move to the stroma. Cytoplasm and mitochondrial matrix are not relevant to chloroplast chemiosmosis. Correct answer is (2).

5. Single Correct Answer MCQ:
Which molecule carries electrons in the light reactions?
(1) NADPH
(2) NADP+
(3) FAD
(4) Cytochrome b6f

Explanation:
NADP+ accepts electrons and is reduced to NADPH in light reactions. Cytochrome b6f transfers electrons but does not carry them long-term. FAD is involved in respiration, not photosynthesis. Correct answer is (2).

6. Single Correct Answer MCQ:
Chemiosmosis in chloroplasts primarily occurs in:
(1) Outer membrane
(2) Thylakoid membrane
(3) Stroma
(4) Cytoplasm

Explanation:
Chemiosmosis occurs across the thylakoid membrane where protons flow from lumen to stroma through ATP synthase, releasing energy. Outer membrane, stroma, or cytoplasm are not sites of chemiosmotic ATP synthesis. Correct answer is (2).

7. Assertion-Reason MCQ:
Assertion (A): ATP synthesis requires proton movement.
Reason (R): Electron transport generates proton gradient across thylakoid membrane.
Options:
(1) Both A and R correct, R explains A
(2) A correct, R incorrect
(3) A incorrect, R correct
(4) Both A and R incorrect

Explanation:
ATP synthesis depends on protons moving through ATP synthase, which is driven by the gradient created by electron transport. Both statements are correct and R explains A. Correct answer is (1).

8. Matching Type MCQ:
Match molecule with its role:
A. NADP+ — 1. Electron carrier
B. ATP synthase — 2. ATP production
C. Proton gradient — 3. Energy source
D. Water — 4. Electron donor
Options:
(1) A–1, B–2, C–3, D–4
(2) A–2, B–1, C–4, D–3
(3) A–1, B–3, C–2, D–4
(4) A–3, B–2, C–1, D–4

Explanation:
NADP+ is an electron carrier (1), ATP synthase produces ATP (2), proton gradient provides energy (3), and water donates electrons in photolysis (4). Correct answer is (1).

9. Fill in the Blanks MCQ:
Protons move from thylakoid lumen to ________ during ATP synthesis.
(1) Cytoplasm
(2) Stroma
(3) Matrix
(4) Outer membrane

Explanation:
Protons flow from the thylakoid lumen to the stroma through ATP synthase, releasing energy to produce ATP. Cytoplasm, matrix, or outer membrane are incorrect. Correct answer is (2).

10. Choose the correct statements MCQ:
(a) ATP is synthesized using proton-motive force
(b) NADPH production is part of ATP chemiosmosis
(c) Electron transport generates proton gradient
(d) Protons move from lumen to stroma through ATP synthase
Options:
(1) a,

Topic: C4 Pathway

Subtopic: Carbon Dioxide Fixation and Enzymatic Distribution

Keyword Definitions:

• Phosphoenolpyruvate (PEP): A 3-carbon compound that acts as the primary CO₂ acceptor in C4 plants.

• RuBisCo: Ribulose-1,5-bisphosphate carboxylase/oxygenase, an enzyme responsible for CO₂ fixation in the Calvin cycle.

• Mesophyll Cells: Photosynthetic cells present in C4 plants where the initial fixation of CO₂ occurs.

• Bundle Sheath Cells: Cells surrounding vascular bundles, where the Calvin cycle operates in C4 plants.

Lead Question (2022):

Given below are two statements:

Statement I: The primary CO₂ acceptor in C4 plants is phosphoenolpyruvate and is found in the mesophyll cells.
Statement II: Mesophyll cells of C4 plants lack RuBisCo enzyme.
In the light of the above statements, choose the correct answer from the options given below:

(1) Both Statement I and Statement II are incorrect
(2) Statement I is correct but Statement II is incorrect
(3) Statement I is incorrect but Statement II is correct
(4) Both, Statement I and Statement II are correct

Explanation (Answer: 4)
In C4 plants, phosphoenolpyruvate (PEP) acts as the first CO₂ acceptor within mesophyll cells, forming oxaloacetate. RuBisCo enzyme is restricted to bundle sheath cells and absent in mesophyll cells, reducing photorespiration. Hence, both statements are correct, describing spatial separation of CO₂ fixation and the Calvin cycle.

1. Which enzyme fixes CO₂ in C4 plants?

(1) RuBisCo
(2) PEP carboxylase
(3) Hexokinase
(4) Pyruvate kinase

Explanation (Answer: 2)
PEP carboxylase catalyzes the first CO₂ fixation step in C4 plants, converting phosphoenolpyruvate into oxaloacetate in mesophyll cells. This spatial separation reduces photorespiration, making photosynthesis more efficient under high temperature and light conditions.

2. In which part of C4 plants does the Calvin cycle occur?

(1) Mesophyll cells
(2) Epidermal cells
(3) Bundle sheath cells
(4) Guard cells

Explanation (Answer: 3)
The Calvin cycle occurs exclusively in the bundle sheath cells of C4 plants. These cells contain RuBisCo enzyme, which fixes CO₂ released from malate. This compartmentalization minimizes photorespiration and enhances photosynthetic efficiency.

3. The first stable product of C4 cycle is:

(1) Malate
(2) Oxaloacetate
(3) Pyruvate
(4) PGA

Explanation (Answer: 2)
The first stable product of the C4 pathway is oxaloacetate, a four-carbon compound formed by carboxylation of phosphoenolpyruvate (PEP) by PEP carboxylase in mesophyll cells. It is later converted to malate or aspartate for CO₂ transfer to bundle sheath cells.

4. C4 plants are more efficient because:

(1) They require less ATP
(2) They minimize photorespiration
(3) They fix CO₂ directly with RuBisCo
(4) They lack bundle sheath cells

Explanation (Answer: 2)
C4 plants minimize photorespiration through spatial separation of initial CO₂ fixation and the Calvin cycle. CO₂ is first fixed by PEP carboxylase in mesophyll cells, then concentrated in bundle sheath cells where RuBisCo operates efficiently.

5. Which of the following is a C4 plant?

(1) Wheat
(2) Rice
(3) Maize
(4) Potato

Explanation (Answer: 3)
Maize (corn) is a C4 plant exhibiting Kranz anatomy. Its mesophyll and bundle sheath cells coordinate CO₂ fixation through PEP carboxylase and RuBisCo, respectively, reducing photorespiration and improving photosynthetic efficiency in tropical climates.

6. What is Kranz anatomy?

(1) Arrangement of guard cells
(2) Specialized leaf anatomy in C4 plants
(3) Root modification
(4) Chloroplast distribution in epidermis

Explanation (Answer: 2)
Kranz anatomy refers to the specialized leaf anatomy in C4 plants where mesophyll cells surround bundle sheath cells. This structure facilitates efficient carbon fixation and minimizes photorespiration by spatial separation of PEP carboxylase and RuBisCo enzymes.

7. Assertion-Reason Type:

Assertion (A): C4 plants are more efficient in hot climates.
Reason (R): They possess a mechanism to concentrate CO₂ around RuBisCo.
(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 (Answer: 1)
C4 plants concentrate CO₂ around RuBisCo, preventing oxygenase activity and reducing photorespiration. This adaptation makes them highly efficient in hot, dry climates with high light intensity.

8. Matching Type:

Match the following:
A. PEP carboxylase — (i) Bundle sheath cell
B. RuBisCo — (ii) Mesophyll cell
C. Calvin cycle — (iii) C4 plants
(1) A–ii, B–i, C–iii
(2) A–i, B–ii, C–iii
(3) A–iii, B–i, C–ii
(4) A–ii, B–iii, C–i

Explanation (Answer: 1)
PEP carboxylase acts in mesophyll cells; RuBisCo functions in bundle sheath cells where the Calvin cycle occurs. This coordination defines the spatial organization in C4 plants.

9. Fill in the Blanks:

In C4 photosynthesis, the Calvin cycle occurs in ________ cells.

(1) Mesophyll
(2) Bundle sheath
(3) Epidermal
(4) Guard

Explanation (Answer: 2)
The Calvin cycle in C4 plants occurs in bundle sheath cells containing RuBisCo, ensuring efficient CO₂ fixation and low photorespiration rates.

10. Choose the Correct Statements:

(1) RuBisCo is present in both mesophyll and bundle sheath cells of C4 plants.
(2) PEP carboxylase acts only in mesophyll cells.
(3) C4 plants perform photorespiration at high rates.
(4) Calvin cycle occurs in mesophyll cells of C4 plants.

Explanation (Answer: 2)
PEP carboxylase acts only in mesophyll cells for initial CO₂ fixation in C4 plants. RuBisCo operates in bundle sheath cells, leading to reduced photorespiration and improved photosynthetic efficiency.

Topic: Electron Transport Chain

Subtopic: Oxidative Phosphorylation

• Electron Transport Chain (ETC): Series of protein complexes transferring electrons to produce ATP.

• NADH: Nicotinamide adenine dinucleotide, electron donor in respiration.

• FADH2: Flavin adenine dinucleotide, electron carrier in ETC.

• ATP Synthase (Complex V): Enzyme synthesizing ATP using proton gradient.

• Proton Gradient: Difference in proton concentration across inner mitochondrial membrane.

• Oxidation-Reduction Reactions: Reactions transferring electrons, generating energy.

• Aerobic Respiration: Energy-producing process using oxygen as terminal electron acceptor.

• Terminal Stage: Final step in ETC where oxygen accepts electrons forming water.

• Oxidative Phosphorylation: ATP formation coupled to electron transfer through ETC.

• ATP Yield: Number of ATP molecules generated per NADH or FADH2.

• Mitochondrial Inner Membrane: Location of ETC and ATP synthase.

Lead Question - 2021

Which of the following statements is incorrect?

1. In ETC, one molecule of NADH + H+ gives rise to 2 ATP molecules, and one FADH2 gives rise to 3 ATP molecules.
Options:
A. Correct statement
B. Incorrect statement
C. Partially correct
D. Cannot be determined

Explanation: This statement is incorrect because in oxidative phosphorylation, one molecule of NADH produces approximately 3 ATP molecules, while FADH2 yields about 2 ATP molecules. The reverse given in the statement is a common misconception. Answer: Incorrect statement.

2. ATP is synthesized through complex V.
Options:
A. True
B. False
C. Partially true
D. Cannot say

Explanation: ATP synthesis occurs through ATP synthase (complex V) in the inner mitochondrial membrane, utilizing the proton gradient generated by ETC. It couples proton flow with phosphorylation of ADP to ATP, forming the primary energy currency for cellular processes. Answer: True.

3. Oxidation-reduction reactions produce proton gradient in respiration.
Options:
A. True
B. False
C. Only in anaerobic respiration
D. Only in cytoplasm

Explanation: During respiration, oxidation-reduction reactions in ETC transfer electrons, pumping protons across the inner mitochondrial membrane, generating a proton gradient. This gradient is essential for ATP synthesis via chemiosmosis. Answer: True.

4. During aerobic respiration, role of oxygen is limited to the terminal stage.
Options:
A. True
B. False
C. Oxygen acts throughout
D. Only in glycolysis

Explanation: Oxygen acts as the terminal electron acceptor at the end of the ETC, forming water. Its role is crucial only in this terminal stage of aerobic respiration, enabling continued electron flow and maintenance of the proton gradient for ATP synthesis. Answer: True.

5. The main site of ETC and oxidative phosphorylation is:
Options:
A. Cytoplasm
B. Mitochondrial inner membrane
C. Nucleus
D. Lysosome

Explanation: The mitochondrial inner membrane houses ETC complexes I–IV and ATP synthase (complex V). The membrane’s impermeability to protons allows proton gradient formation, driving oxidative phosphorylation to generate ATP. Answer: Mitochondrial inner membrane.

6. NADH and FADH2 differ in ATP yield because:
Options:
A. NADH enters at complex I, FADH2 at complex II
B. Both enter at same complex
C. Only FADH2 produces ATP
D. NADH is non-functional

Explanation: NADH contributes electrons to complex I, pumping more protons and producing ~3 ATP. FADH2 donates electrons at complex II, bypassing one proton pump, yielding ~2 ATP. This explains the difference in energy yield. Answer: NADH enters at complex I, FADH2 at complex II.

7. Assertion-Reason:
Assertion (A): ETC generates a proton gradient.
Reason (R): ATP synthase uses this gradient to produce ATP.
Options:
A. Both A and R are true, R is correct explanation
B. Both A and R are true, R is not correct explanation
C. A is true, R is false
D. A is false, R is true

Explanation: The ETC pumps protons into the intermembrane space, creating a proton gradient. ATP synthase utilizes this gradient to drive ADP phosphorylation, producing ATP. Both the assertion and reason are correct, with the reason explaining the assertion. Answer: Both A and R are true, R is correct explanation.

8. Match the following:
Column I: 1. Complex I 2. Complex II 3. Complex III 4. Complex IV
Column II: A. Cytochrome c oxidase B. Succinate dehydrogenase C. NADH dehydrogenase D. Cytochrome bc1 complex
Options:
A. 1-C, 2-B, 3-D, 4-A
B. 1-B, 2-C, 3-A, 4-D
C. 1-A, 2-D, 3-B, 4-C
D. 1-D, 2-A, 3-C, 4-B

Explanation: Correct matching: Complex I – NADH dehydrogenase (C), Complex II – Succinate dehydrogenase (B), Complex III – Cytochrome bc1 complex (D), Complex IV – Cytochrome c oxidase (A). These complexes transfer electrons and contribute to the proton gradient. Answer: 1-C, 2-B, 3-D, 4-A.

9. Fill in the blank: Oxidative phosphorylation occurs in the __________.
Options:
A. Cytoplasm
B. Mitochondrial inner membrane
C. Nucleus
D. Endoplasmic reticulum

Explanation: Oxidative phosphorylation takes place in the mitochondrial inner membrane, where ETC generates a proton gradient. ATP synthase utilizes this gradient to synthesize ATP efficiently. This is the final stage of aerobic respiration, producing the majority of cellular ATP. Answer: Mitochondrial inner membrane.

10. Choose the correct statements:
1. NADH yields ~3 ATP per molecule.
2. FADH2 yields ~2 ATP per molecule.
3. Oxygen acts as terminal electron acceptor.
Options:
A. 1 and 2 only
B. 2 and 3 only
C. 1 and 3 only
D. 1, 2 and 3

Explanation: All statements are correct. NADH generates ~3 ATP, FADH2 produces ~2 ATP, and oxygen is the terminal electron acceptor forming water, allowing continuous electron flow through ETC. Answer: 1, 2 and 3.

Topic: Light Reaction
Subtopic: Photophosphorylation

Keyword Definitions:

• Photophosphorylation: Formation of ATP using light energy during photosynthesis.

• Photosystem I (PS I): Protein-pigment complex absorbing light at 700 nm, responsible for NADPH formation.

• Photosystem II (PS II): Protein-pigment complex absorbing light at 680 nm, responsible for splitting water.

• Cyclic Photophosphorylation: ATP synthesis involving only PS I, no NADPH formation.

• Non-Cyclic Photophosphorylation: Involves both PS I and PS II, producing ATP and NADPH.

Lead Question - 2021

Which of the following statements is incorrect?

(1) Stroma lamellae have PS I only and lack NADP reductase
(2) Grana lamellae have both PS I and PS II
(3) Cyclic photophosphorylation involves both PS I and PS II
(4) Both ATP and NADPH + H+ are synthesized during non-cyclic photophosphorylation

Explanation: The correct answer is (3) Cyclic photophosphorylation involves both PS I and PS II. Cyclic photophosphorylation uses only PS I, cycling electrons back to the same photosystem. Non-cyclic photophosphorylation requires both PS I and PS II, producing ATP and NADPH. Thus, statement (3) is incorrect, while the others are correct.

Guessed Questions:

1. Which photosystem is involved in water splitting during light reaction?
(1) Photosystem I
(2) Photosystem II
(3) Both PS I and PS II
(4) None of these
Explanation: The correct answer is (2) Photosystem II. PS II contains the oxygen-evolving complex, which splits water molecules into protons, electrons, and oxygen. Electrons enter the electron transport chain, while PS I functions later in NADPH production. Water splitting is unique to PS II, making it essential for oxygen evolution.

2. Which of the following occurs during cyclic photophosphorylation?
(1) ATP synthesis only
(2) Both ATP and NADPH synthesis
(3) Oxygen evolution
(4) Splitting of water
Explanation: The correct answer is (1) ATP synthesis only. In cyclic photophosphorylation, electrons from PS I cycle back to the same system, generating ATP but not NADPH. Water is not split, and oxygen is not released. This process supplements ATP demand when Calvin cycle requires extra energy.

3. Non-cyclic photophosphorylation produces:
(1) ATP only
(2) NADPH only
(3) Both ATP and NADPH
(4) Oxygen only
Explanation: The correct answer is (3) Both ATP and NADPH. Non-cyclic photophosphorylation involves electron flow from water through PS II and PS I to NADP+, producing ATP, NADPH, and oxygen. These products are essential for the Calvin cycle, providing both reducing power and energy required for carbon fixation.

4. Where are PS I and PS II located in the chloroplast?
(1) PS I in stroma lamellae, PS II in grana
(2) Both PS I and PS II in stroma lamellae
(3) Both PS I and PS II in grana
(4) PS I in grana, PS II in stroma lamellae
Explanation: The correct answer is (1) PS I in stroma lamellae, PS II in grana. PS I is mainly present in stroma thylakoids, while PS II is localized in grana thylakoids. This distribution allows efficient electron transport. Grana contain both systems together, but stroma lamellae have only PS I.

5. Which of the following is a product of photolysis of water?
(1) ATP
(2) Oxygen
(3) NADPH
(4) Glucose
Explanation: The correct answer is (2) Oxygen. Photolysis of water occurs in PS II, producing electrons for electron transport, protons for ATP synthesis, and oxygen as a by-product. ATP and NADPH are products of photophosphorylation, while glucose is formed later during the Calvin cycle, not directly by photolysis.

6. Light-harvesting complexes (LHC) are associated with:
(1) Photosystems
(2) Calvin cycle
(3) Photorespiration
(4) Glycolysis
Explanation: The correct answer is (1) Photosystems. Light-harvesting complexes contain pigments like chlorophyll and carotenoids, which absorb light and transfer energy to the reaction center. They are essential for efficient light capture in both PS I and PS II. LHCs ensure energy funneling for photochemical reactions during photosynthesis.

7. Assertion (A): Cyclic photophosphorylation produces ATP but not NADPH.
Reason (R): Electrons return to the same photosystem instead of reducing NADP+.
(1) Both A and R true, R correct explanation of A
(2) Both A and R true, R not correct explanation of A
(3) A true, R false
(4) A false, R true
Explanation: The correct answer is (1) Both A and R true, R correct explanation of A. In cyclic photophosphorylation, electrons from PS I are cycled back into its electron transport chain, producing only ATP. NADPH is not synthesized because NADP+ is not reduced in this pathway.

8. Match the following:
A. PS I    1. Absorbs at 700 nm
B. PS II    2. Absorbs at 680 nm
C. Photolysis of water    3. Occurs in PS II
(1) A-1, B-2, C-3
(2) A-2, B-1, C-3
(3) A-3, B-2, C-1
(4) A-1, B-3, C-2
Explanation: The correct answer is (1) A-1, B-2, C-3. Photosystem I absorbs at 700 nm, Photosystem II absorbs at 680 nm, and photolysis of water occurs in PS II. These properties are crucial for distinguishing the two photosystems and their roles in light reactions of photosynthesis.

9. Fill in the blank: The Z-scheme of photosynthesis represents ______.
(1) Calvin cycle
(2) Pathway of electron flow
(3) Photolysis of water
(4) Carbon fixation
Explanation: The correct answer is (2) Pathway of electron flow. The Z-scheme illustrates the movement of electrons from water through PS II and PS I to NADP+, forming NADPH. It highlights the energy changes in electrons during non-cyclic photophosphorylation, showing both ATP and NADPH production for carbon assimilation.

10. Choose the correct statements:
(a) PS I absorbs light at 700 nm
(b) PS II is involved in water splitting
(c) Cyclic photophosphorylation produces ATP and NADPH
(d) Non-cyclic photophosphorylation produces ATP, NADPH, and O2
Options:
(1) a, b, c
(2) a, b, d
(3) b, c, d
(4) a, c, d
Explanation: The correct answer is (2) a, b, d. PS I absorbs at 700 nm, PS II splits water, and non-cyclic photophosphorylation yields ATP, NADPH, and oxygen. Cyclic photophosphorylation produces only ATP, not NADPH, so statement (c) is incorrect. Thus, statements a, b, and d are correct.

Subtopic: CO2 Fixation in C4 Plants

Keyword Definitions:

• CO2 Fixation: The process of converting atmospheric carbon dioxide into organic compounds during photosynthesis.

• Sorghum: A C4 plant in which CO2 fixation occurs initially via a four-carbon intermediate.

• Oxaloacetic Acid: A four-carbon compound and the first stable product of CO2 fixation in C4 plants.

• Phosphoglyceric Acid (PGA): A three-carbon compound formed as the first stable product in C3 photosynthesis.

• Succinic Acid: A four-carbon compound in the Krebs cycle, not directly involved in initial CO2 fixation.

• Pyruvic Acid: A three-carbon intermediate in glycolysis and metabolism, not the first product in CO2 fixation in sorghum.

• C4 Pathway: A photosynthetic pathway in certain plants that initially forms a four-carbon compound to minimize photorespiration.

• Stable Product: A molecule that remains in the pathway long enough to be further metabolized into sugars.

Lead Question - 2021

The first stable product of CO2 fixation in sorghum is:
(1) Oxaloacetic acid
(2) Succinic acid
(3) Phosphoglyceric acid
(4) Pyruvic acid

Explanation: The correct answer is (1) Oxaloacetic acid. In C4 plants like sorghum, CO2 is initially fixed by PEP carboxylase to form oxaloacetic acid, a stable four-carbon compound. This adaptation reduces photorespiration and enhances efficiency in hot, dry environments, unlike C3 plants where phosphoglyceric acid is the first product.

Guessed Questions:

1) Which enzyme catalyzes the formation of oxaloacetic acid in C4 plants?
(1) Rubisco
(2) PEP carboxylase
(3) NADP reductase
(4) ATP synthase

Explanation: The correct answer is (2) PEP carboxylase. This enzyme fixes CO2 in mesophyll cells of C4 plants, forming oxaloacetic acid. It has a high affinity for CO2 and prevents photorespiration, making it essential for efficient photosynthesis in sorghum and other C4 species.

2) Single Correct Answer: The C4 pathway initially produces which type of carbon compound?
(1) Three-carbon
(2) Four-carbon
(3) Five-carbon
(4) Six-carbon

Explanation: The correct answer is (2) Four-carbon. C4 plants first form a four-carbon compound, oxaloacetic acid, during CO2 fixation. This intermediate is later converted to malate or aspartate and transported to bundle-sheath cells for decarboxylation and entry into the Calvin cycle.

3) Assertion (A): C4 plants reduce photorespiration.
Reason (R): Oxaloacetic acid is formed as the first stable product in mesophyll cells.
(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: The correct answer is (1). C4 plants reduce photorespiration because CO2 is initially fixed as oxaloacetic acid by PEP carboxylase in mesophyll cells, concentrating CO2 around Rubisco in bundle-sheath cells, enhancing carbon fixation efficiency in hot, dry environments.

4) Fill in the blank: The first stable product of CO2 fixation in maize and sorghum is _______.
(1) Phosphoglyceric acid
(2) Oxaloacetic acid
(3) Succinic acid
(4) Pyruvic acid

Explanation: The correct answer is (2) Oxaloacetic acid. In C4 plants like maize and sorghum, CO2 is fixed into a four-carbon compound, oxaloacetic acid, in mesophyll cells, which is then transported to bundle-sheath cells for efficient carbon assimilation.

5) Which compound is directly converted to malate after CO2 fixation in C4 plants?
(1) Oxaloacetic acid
(2) Pyruvate
(3) Phosphoglyceric acid
(4) Succinic acid

Explanation: The correct answer is (1) Oxaloacetic acid. Oxaloacetic acid is rapidly converted to malate or aspartate in mesophyll cells, which then transports CO2 to bundle-sheath cells for decarboxylation and Calvin cycle entry.

6) Single Correct Answer: C4 photosynthesis primarily occurs in which type of cells?
(1) Bundle-sheath cells
(2) Epidermal cells
(3) Mesophyll cells
(4) Guard cells

Explanation: The correct answer is (3) Mesophyll cells. CO2 is initially fixed into oxaloacetic acid by PEP carboxylase in mesophyll cells of C4 plants, then shuttled to bundle-sheath cells for the Calvin cycle, minimizing photorespiration.

7) Matching Type: Match the compound with its role in C4 photosynthesis.
List-I    List-II
(a) Oxaloacetic acid    (i) First stable product
(b) Malate    (ii) CO2 carrier to bundle-sheath
(c) Pyruvate    (iii) Regenerated for PEP formation
(d) Rubisco    (iv) Catalyzes CO2 fixation in Calvin cycle
Select correct answer:
(a) (b) (c) (d)
(1) i, ii, iii, iv
(2) ii, i, iii, iv
(3) i, iii, ii, iv
(4) i, ii, iv, iii

Explanation: The correct answer is (1). Oxaloacetic acid is the first stable product (i), malate carries CO2 to bundle-sheath cells (ii), pyruvate regenerates PEP (iii), and Rubisco catalyzes CO2 fixation in the Calvin cycle (iv), illustrating the C4 photosynthetic pathway.

8) Single Correct Answer: Which plant is a typical C4 plant?
(1) Wheat
(2) Sorghum
(3) Rice

Subtopic: Calvin Cycle and RuBisCO Function

Keyword Definitions:

• RuBisCO: Ribulose-1,5-bisphosphate carboxylase/oxygenase, enzyme catalyzing CO2 fixation in Calvin cycle.

• Bifunctional enzyme: Enzyme that catalyzes two reactions, here carboxylase and oxygenase activities.

• C4 plants: Plants with specialized Kranz anatomy, separating initial CO2 fixation and RuBisCO activity.

• Mesophyll cells: Photosynthetic cells where initial CO2 fixation by PEP carboxylase occurs in C4 plants.

• Substrate molecule: 5-carbon compound ribulose-1,5-bisphosphate (RuBP) used by RuBisCO for carboxylation.

• ATP and NADPH: Energy and reducing power from light reactions used in Calvin cycle, not directly by RuBisCO.

• Calvin cycle: Light-independent reactions converting CO2 to carbohydrates using RuBisCO and other enzymes.

• Carboxylase activity: RuBisCO catalyzes CO2 fixation with RuBP to form 3-phosphoglycerate.

• Oxygenase activity: RuBisCO catalyzes reaction with O2, leading to photorespiration.

• Kranz anatomy: Structural adaptation in C4 leaves separating mesophyll and bundle sheath cells.

• Photorespiration: Process where RuBisCO uses O2 as substrate, reducing photosynthetic efficiency.

Lead Question - 2020 (COVID Reexam)

Which of the following statements is incorrect?

1. RuBisCO is a bifunctional enzyme
2. In C4 plants, the site of RuBisCO activity is mesophyll cell
3. The substrate molecule for RuBisCO activity is a 5-carbon compound
4. RuBisCO action requires ATP and NADPH

Explanation: In C4 plants, RuBisCO activity occurs in bundle sheath cells, not mesophyll, to minimize photorespiration. RuBisCO is bifunctional and acts on RuBP (5-carbon). ATP and NADPH are used in Calvin cycle, not directly by RuBisCO. Correct answer is option 2: In C4 plants, site of RuBisCO is mesophyll cell. (50 words)

Guessed Question 1. Single Correct Answer MCQ: RuBisCO catalyzes reaction with which substrate?

1. 3-phosphoglycerate
2. Ribulose-1,5-bisphosphate
3. Phosphoenolpyruvate
4. Glucose

Explanation: RuBisCO catalyzes carboxylation of ribulose-1,5-bisphosphate (RuBP), a 5-carbon compound, to form two molecules of 3-phosphoglycerate. Correct answer is option 2: Ribulose-1,5-bisphosphate. RuBP is the primary substrate for CO2 fixation in Calvin cycle. (50 words)

Guessed Question 2. Single Correct Answer MCQ: RuBisCO is bifunctional because it has:

1. Carboxylase activity only
2. Oxygenase activity only
3. Both carboxylase and oxygenase activities
4. Kinase activity

Explanation: RuBisCO catalyzes carboxylation of RuBP and oxygenation leading to photorespiration. This dual functionality defines it as bifunctional. Correct answer is option 3: Both carboxylase and oxygenase activities. Its efficiency affects carbon fixation and photorespiration rates in plants. (50 words)

Guessed Question 3. Single Correct Answer MCQ: In C4 plants, RuBisCO is localized in:

1. Mesophyll cells
2. Bundle sheath cells
3. Epidermal cells
4. Xylem

Explanation: C4 plants separate initial CO2 fixation and RuBisCO activity. RuBisCO is localized in bundle sheath cells to minimize oxygenation and photorespiration. Correct answer is option 2: Bundle sheath cells. Mesophyll cells perform initial CO2 fixation via PEP carboxylase. (50 words)

Guessed Question 4. Single Correct Answer MCQ: Calvin cycle requires energy in the form of:

1. ATP and NADPH
2. ADP only
3. Glucose
4. Oxygen

Explanation: Calvin cycle uses ATP and NADPH produced during light reactions for CO2 fixation and reduction of 3-phosphoglycerate. Correct answer is option 1: ATP and NADPH. RuBisCO catalyzes only carboxylation and oxygenation; it does not directly use ATP or NADPH. (50 words)

Guessed Question 5. Assertion-Reason MCQ:

Assertion (A): RuBisCO is found in bundle sheath cells of C4 plants.
Reason (R): It reduces oxygenation and photorespiration in C4 pathway.
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: RuBisCO is localized in bundle sheath cells in C4 plants to avoid oxygenation and minimize photorespiration. Both assertion and reason are true, with R correctly explaining A. Correct answer is option 1. This spatial separation enhances carbon fixation efficiency. (50 words)

Guessed Question 6. Matching Type MCQ:

Column I - Component
(a) RuBP (i) Substrate for RuBisCO
(b) RuBisCO (ii) Enzyme
(c) Bundle sheath (iii) Site of RuBisCO in C4
(d) PEP carboxylase (iv) Initial CO2 fixation in C4
Options:
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)-(i), (c)-(ii), (d)-(iv)

Explanation: RuBP is substrate, RuBisCO is enzyme, bundle sheath is RuBisCO site in C4, PEP carboxylase fixes CO2 initially. Correct answer is option 1. This mapping clarifies Calvin cycle reactions and C4 photosynthetic adaptations. (50 words)

Guessed Question 7. Fill in the blank:

RuBisCO catalyzes fixation of CO2 with _______ as substrate.

1. 3-phosphoglycerate
2. Ribulose-1,5-bisphosphate
3. Phosphoenolpyruvate
4. ATP

Explanation: RuBisCO catalyzes CO2 fixation with ribulose-1,5-bisphosphate (RuBP), a 5-carbon compound, forming 3-phosphoglycerate. Correct answer is option 2: Ribulose-1,5-bisph

Subtopic: Non-cyclic Photophosphorylation

Keyword Definitions:

• Non-cyclic photophosphorylation: Light-dependent process producing ATP and NADPH with electrons moving from PS II to PS I.

• PS II (Photosystem II): Reaction center that absorbs light, loses electrons to electron transport chain.

• Reaction center: Chlorophyll-protein complex where primary photochemical reactions occur.

• Water (H2O): Electron donor in PS II, split to release electrons, protons, and oxygen.

• Oxygen (O2): By-product of water splitting in photolysis during photosynthesis.

• Carbon dioxide (CO2): Substrate for Calvin cycle, not an electron donor in light reactions.

• Light: Energy source that excites electrons in chlorophyll molecules.

• Electron transport chain: Series of carriers transferring electrons from PS II to PS I and ultimately to NADP+.

• Photolysis: Splitting of water molecules into O2, H+, and electrons by light energy.

• ATP and NADPH: Energy carriers generated during non-cyclic photophosphorylation for carbon fixation.

• Chlorophyll: Pigment that absorbs light and initiates electron transfer in photosystems.

Lead Question - 2020 (COVID Reexam)

During non-cyclic photophosphorylation, when electrons are lost from the reaction center at PS II, what is the source which replaces these electrons?

1. Oxygen
2. Water
3. Carbon dioxide
4. Light

Explanation: In non-cyclic photophosphorylation, water undergoes photolysis at PS II to provide electrons lost from the reaction center. This process releases protons and oxygen as a by-product. Correct answer is option 2: Water. Water splitting is essential to maintain continuous electron flow and ATP/NADPH production. (50 words)

Guessed Question 1. Single Correct Answer MCQ: Photolysis of water occurs at which photosystem?

1. PS I
2. PS II
3. Both PS I and PS II
4. Neither PS I nor PS II

Explanation: Photolysis occurs at PS II, where light energy splits water to provide electrons, protons, and oxygen. This replenishes electrons lost from the reaction center and maintains the electron transport chain. Correct answer is option 2: PS II. It ensures continuous ATP and NADPH generation. (50 words)

Guessed Question 2. Single Correct Answer MCQ: Which by-product is formed during water photolysis?

1. NADPH
2. Carbon dioxide
3. Oxygen
4. Glucose

Explanation: Photolysis of water at PS II releases oxygen as a by-product, while electrons and protons are used in the electron transport chain. Correct answer is option 3: Oxygen. This process contributes to atmospheric O2 and sustains light-dependent reactions in photosynthesis. (50 words)

Guessed Question 3. Single Correct Answer MCQ: Source of electrons for non-cyclic photophosphorylation is:

1. Carbon dioxide
2. Water
3. Oxygen
4. NADP+

Explanation: In non-cyclic photophosphorylation, electrons lost from PS II are replenished by splitting water molecules, which releases oxygen and protons. Correct answer is option 2: Water. This ensures continuous electron flow to PS I and generation of ATP and NADPH. (50 words)

Guessed Question 4. Single Correct Answer MCQ: Non-cyclic electron flow produces:

1. ATP only
2. NADPH only
3. ATP and NADPH
4. Oxygen only

Explanation: Non-cyclic photophosphorylation produces both ATP and NADPH. Electrons from PS II move through the electron transport chain to PS I, reducing NADP+ to NADPH, while ATP forms via chemiosmosis. Water provides electrons lost at PS II. Correct answer is option 3: ATP and NADPH. (50 words)

Guessed Question 5. Assertion-Reason MCQ:

Assertion (A): Water is the electron donor in non-cyclic photophosphorylation.
Reason (R): Photolysis splits water to supply electrons to PS II and release O2.
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: Photolysis of water provides electrons to PS II and releases oxygen. Both assertion and reason are true, and R correctly explains A. Correct answer is option 1. This is critical for sustaining non-cyclic electron flow and ATP/NADPH synthesis. (50 words)

Guessed Question 6. Matching Type MCQ:

Column I - Event
(a) PS II (i) Photolysis of water
(b) PS I (ii) NADP+ reduction
(c) Electron transport chain (iii) ATP synthesis
(d) Chemiosmosis (iv) Proton gradient formation
Options:
1. (a)-(i), (b)-(ii), (c)-(iv), (d)-(iii)
2. (a)-(ii), (b)-(i), (c)-(iii), (d)-(iv)
3. (a)-(i), (b)-(ii), (c)-(iii), (d)-(iv)
4. (a)-(iii), (b)-(iv), (c)-(i), (d)-(ii)

Explanation: PS II performs water photolysis, PS I reduces NADP+ to NADPH, the electron transport chain pumps protons to form a gradient, and chemiosmosis uses this gradient to synthesize ATP. Correct answer is option 1. This coordination ensures efficient light-driven energy conversion. (50 words)

Guessed Question 7. Fill in the blank:

Electrons lost from PS II reaction center are replaced by _______.

1. Carbon dioxide
2. Water
3. Oxygen
4. Light

Explanation: Water molecules are split during photolysis at PS II, replacing electrons lost from the reaction center. This process also generates protons and oxygen. Correct answer is option 2: Water. Essential for continuous electron flow and production of ATP and NADPH. (50 words)

Guessed Question 8. Single Correct Answer MCQ: Oxygen released in photosynthesis originates from:

1. CO2
2. Water
3. Glucose
4. Chlorophyll

Explanation: Oxygen is released from the splitting of water during non-cyclic photophosphorylation at PS II. Electrons from water replace those lost in the reaction center. Correct answer is option 2: Water. This is a vital source of atmospheric oxygen. (50 words)

Guessed Question 9. Single Correct Answer MCQ: Electrons from PS II eventually reduce:

1. NADP+
2. ADP
3. Oxygen
4. Carbon dioxide

Explanation: Electrons from

Topic: Photorespiration

Subtopic: RuBisCo Oxygenation Activity

Keyword Definitions:

• RuBisCo – Ribulose-1,5-bisphosphate carboxylase/oxygenase, enzyme that catalyzes CO2 fixation and oxygenation in Calvin cycle.

• Photorespiration – Process where RuBisCo oxygenates RuBP leading to the formation of glycolate and CO2 release, reducing photosynthetic efficiency.

• RuBP – Ribulose-1,5-bisphosphate, a 5-carbon sugar that is the substrate for RuBisCo.

• 3-C Compound – 3-phosphoglycerate (3-PGA), produced during RuBisCo carboxylation.

• 2-C Compound – Phosphoglycolate, formed during RuBisCo oxygenation in photorespiration.

• 4-C Compound – Glycolate metabolite formed during photorespiration after oxygenation of RuBP.

Lead Question - 2020

The oxygenation activity of RuBisCo enzyme during photorespiration leads to the formation of:
(1) 1 molecule of 6-C compound
(2) 1 molecule of 4-C compound and 1 molecule of 2-C compound
(3) 2 molecules of 3-C compound
(4) 1 molecule of 3-C compound

Explanation: During photorespiration, RuBisCo oxygenates RuBP, a 5-carbon compound, producing one molecule of 3-phosphoglycerate (3-C) and one molecule of 2-phosphoglycolate (2-C). The 2-C compound is then metabolized to a 4-C intermediate. Correct answer is (2) 1 molecule of 4-C compound and 1 molecule of 2-C compound.

1. Single Correct Answer: The main substrate for RuBisCo oxygenase activity is:
(1) ATP
(2) RuBP
(3) NADPH
(4) G3P

Explanation: Ribulose-1,5-bisphosphate (RuBP) is the primary substrate for RuBisCo. During oxygenation, RuBP reacts with O2, initiating photorespiration and producing 3-C and 2-C compounds. Correct answer is (2) RuBP.

2. Single Correct Answer: Photorespiration reduces photosynthetic efficiency because:
(1) It fixes extra CO2
(2) It releases CO2 and consumes ATP
(3) It generates more sugar
(4) It synthesizes NADPH

Explanation: Photorespiration consumes energy (ATP) and releases CO2 without producing sugars, reducing net photosynthetic efficiency. Correct answer is (2).

3. Single Correct Answer: The 2-C compound formed during photorespiration is:
(1) 2-phosphoglycolate
(2) Oxaloacetate
(3) Glyoxylate
(4) Pyruvate

Explanation: Phosphoglycolate (2-phosphoglycolate) is formed when RuBP is oxygenated by RuBisCo. It is a 2-carbon molecule that enters the photorespiratory pathway. Correct answer is (1).

4. Assertion-Reason:
Assertion (A): Photorespiration generates both 3-C and 2-C compounds.
Reason (R): RuBisCo has dual activity as carboxylase and oxygenase.
(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: RuBisCo oxygenase activity leads to one 3-C and one 2-C compound, while its carboxylase activity generates two 3-C compounds. Both statements are true and the dual activity of RuBisCo explains the products formed. Correct answer is (1).

5. Single Correct Answer: The 4-C compound in photorespiration is derived from:
(1) 3-PGA
(2) Phosphoglycolate
(3) RuBP
(4) G3P

Explanation: Phosphoglycolate (2-C) formed during RuBisCo oxygenation is converted into glyoxylate and then to 4-C metabolites as part of the photorespiratory pathway. Correct answer is (2) Phosphoglycolate.

6. Single Correct Answer: Which enzyme catalyzes oxygenation of RuBP?
(1) RuBisCo
(2) PEP carboxylase
(3) G3P dehydrogenase
(4) Rubisco activase

Explanation: RuBisCo has both carboxylase and oxygenase activity. Oxygenation of RuBP initiates photorespiration, forming 3-C and 2-C compounds. Correct answer is (1) RuBisCo.

7. Matching Type: Match Column I with Column II:
a. 3-C compound – i. 3-phosphoglycerate
b. 2-C compound – ii. Phosphoglycolate
c. Enzyme – iii. RuBisCo
d. Pathway – iv. Photorespiration
(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-i, b-iii, c-ii, d-iv

Explanation: 3-C compound is 3-PGA (a-i), 2-C compound is phosphoglycolate (b-ii), enzyme is RuBisCo (c-iii), pathway is photorespiration (d-iv). Correct answer is (1).

8. Fill in the blank: Photorespiration mainly occurs when _______ competes with CO2 for RuBisCo active site.
(1) O2
(2) H2O
(3) NADPH
(4) ATP

Explanation: Oxygen competes with CO2 for the RuBisCo active site, leading to the oxygenation reaction that initiates photorespiration. Correct answer is (1) O2.

9. Single Correct Answer: Photorespiration results in net loss of:
(1) CO2 only
(2) Carbon and energy
(3) Water
(4) Oxygen

Explanation: Photorespiration consumes ATP and NADPH, releases CO2, and does not produce sugar, leading to a net loss of carbon and energy. Correct answer is (2).

10. Choose the correct statements:
(a) RuBisCo oxygenation produces 3-C and 2-C compounds
(b) Photorespiration reduces photosynthetic efficiency
(c) Photorespiration occurs only in C4 plants
(d) Oxygenation is catalyzed by RuBisCo
(1) a, b, d
(2) a,

Topic: Light Reaction

Subtopic: Electron Transport Chain

Keyword Definitions:

• Light Reaction – The first stage of photosynthesis where light energy is converted into chemical energy in the form of ATP and NADPH.

• Plastoquinone – Lipid-soluble electron carrier in the thylakoid membrane, transferring electrons from PS-II to the Cyt b6f complex.

• Photosystem II (PS-II) – Protein-pigment complex that absorbs light and initiates electron transport in photosynthesis.

• Cyt b6f Complex – Protein complex that mediates electron transfer between PS-II and PS-I and pumps protons for ATP synthesis.

• PS-I – Photosystem I, captures light energy to transfer electrons ultimately to NADP+ forming NADPH.

• ATP Synthase – Enzyme complex that synthesizes ATP using the proton gradient created during electron transport.

Lead Question - 2020

In light reaction, plastoquinone facilitates the transfer of electrons from:
(1) PS-I to NADP+
(2) PS-I to ATP synthase
(3) PS-II to Cyt b6f complex
(4) Cyt b6f complex to PS-I

Explanation: Plastoquinone is a mobile electron carrier that shuttles electrons from Photosystem II (PS-II) to the Cytochrome b6f complex during the light reaction of photosynthesis. This transfer also helps in proton pumping across the thylakoid membrane, contributing to the proton gradient for ATP synthesis. Correct answer is (3).

1. Single Correct Answer: Which molecule carries electrons from PS-I to NADP+?
(1) Plastoquinone
(2) Ferredoxin
(3) Cytochrome b6f
(4) Plastocyanin

Explanation: Ferredoxin is the final electron carrier that transfers electrons from PS-I to NADP+, forming NADPH. Plastoquinone transfers electrons earlier from PS-II. Correct answer is (2) Ferredoxin.

2. Single Correct Answer: The primary function of Cytochrome b6f complex is:
(1) Absorb light energy
(2) Pump protons and transfer electrons
(3) Fix carbon dioxide
(4) Synthesize NADPH

Explanation: Cytochrome b6f complex receives electrons from plastoquinone, transfers them to plastocyanin, and pumps protons into the thylakoid lumen, generating a proton gradient for ATP synthesis. Correct answer is (2).

3. Single Correct Answer: Plastoquinone is a:
(1) Water-soluble protein
(2) Lipid-soluble electron carrier
(3) Pigment in PS-I
(4) Enzyme of Calvin cycle

Explanation: Plastoquinone is a lipid-soluble quinone molecule that shuttles electrons from PS-II to Cyt b6f complex. It is embedded in the thylakoid membrane. Correct answer is (2).

4. Assertion-Reason:
Assertion (A): Plastoquinone transfers electrons from PS-II to Cyt b6f complex.
Reason (R): Electron transport creates a proton gradient used for ATP synthesis.
(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: Plastoquinone transfers electrons from PS-II to Cyt b6f, which helps pump protons into thylakoid lumen. This proton gradient drives ATP synthase activity. Both statements are correct and R explains A. Correct answer is (1).

5. Single Correct Answer: Plastocyanin carries electrons from:
(1) PS-II to Cyt b6f
(2) Cyt b6f to PS-I
(3) PS-I to NADP+
(4) Water to PS-II

Explanation: Plastocyanin is a copper-containing protein that transfers electrons from Cyt b6f complex to PS-I during the light reaction. Correct answer is (2).

6. Single Correct Answer: Which of the following contributes directly to the proton gradient in thylakoids?
(1) Plastoquinone
(2) NADP+
(3) Ferredoxin
(4) Ribulose bisphosphate

Explanation: Plastoquinone transfers electrons and also facilitates proton pumping across the thylakoid membrane via Cyt b6f complex, creating a proton gradient used for ATP synthesis. Correct answer is (1).

7. Matching Type: Match column I with column II:
a. PS-II – i. Reduces NADP+
b. Plastoquinone – ii. Electron carrier from PS-II
c. Cyt b6f – iii. Transfers electrons to PS-I
d. PS-I – iv. Captures light to energize electrons
(1) a-iv, b-ii, c-iii, d-i
(2) a-ii, b-iii, c-iv, d-i
(3) a-i, b-ii, c-iv, d-iii
(4) a-iii, b-iv, c-i, d-ii

Explanation: PS-II captures light energy (a-iv), plastoquinone carries electrons to Cyt b6f (b-ii), Cyt b6f transfers electrons to PS-I (c-iii), and PS-I reduces NADP+ (d-i). Correct answer is (1).

8. Fill in the blank: The mobile electron carrier between PS-II and Cyt b6f complex is _______.
(1) Ferredoxin
(2) Plastoquinone
(3) Plastocyanin
(4) NADP+

Explanation: Plastoquinone shuttles electrons from PS-II to Cyt b6f complex during the light reaction of photosynthesis. Correct answer is (2).

9. Single Correct Answer: Which component is directly involved in ATP synthesis in thylakoid membrane?
(1) PS-II
(2) Plastoquinone
(3) ATP synthase
(4) PS-I

Explanation: ATP synthase utilizes the proton gradient created by electron transport (including plastoquinone activity) to synthesize ATP in the thylakoid membrane. Correct answer is (3).

10. Choose the correct statements:
(a) Plastoquinone transfers electrons from PS-II to Cyt b6f
(b) Ferredoxin transfers electrons from PS-I to NADP+
(c) Plastocyanin transfers electrons from Cyt b6f to PS-I
(d) NADP+ receives electrons from PS-II directly
(1) a, b, c
(2) a, b, d
(3) a, c, d
(4) b, c, d

Explanation: Plastoquinone transfers electrons from PS-II to Cyt b6f (a), Ferredoxin transfers electrons from PS-I to NADP+ (b), Plastocyanin carries electrons from Cyt b6f to PS-I (c). NADP+ does not receive electrons directly from PS-II. Correct answer is (1) a, b, c.

Topic: Light Reactions

Subtopic: Products of Light Reaction

Keyword Definitions:

• Light reaction: First stage of photosynthesis occurring in thylakoid membranes, converting light energy into chemical energy.

• ATP: Energy currency of the cell, produced during light reactions.

• NADPH: Electron carrier molecule generated in light reactions, used in Calvin cycle.

• Oxygen: Byproduct of water splitting during light reaction.

• NADH: Electron carrier in cellular respiration, not produced in photosynthesis.

• Thylakoid: Membrane-bound compartment inside chloroplasts where light reactions occur.

Lead Question (2018):

Which of the following is not a product of light reaction of photosynthesis?

(A) Oxygen

(B) ATP

(C) NADPH

(D) NADH

Explanation:

The correct answer is (D) NADH. Light reactions of photosynthesis produce ATP, NADPH, and oxygen as a byproduct of water splitting. NADH is an electron carrier involved in cellular respiration, not photosynthesis. This distinction is important for understanding energy conversion in plant cells.

1. Light reaction of photosynthesis occurs in:

(A) Stroma

(B) Thylakoid membrane

(C) Cytoplasm

(D) Mitochondria

Explanation:

The correct answer is (B) Thylakoid membrane. Light reactions take place in chloroplast thylakoid membranes, where pigments absorb light to produce ATP and NADPH. The stroma is the site for the Calvin cycle, not the light reaction.

2. Oxygen produced in light reactions comes from:

(A) CO2

(B) Water

(C) NADPH

(D) Glucose

Explanation:

The correct answer is (B) Water. Photolysis of water in photosystem II releases electrons, protons, and molecular oxygen. CO2 is reduced in the Calvin cycle, not during light reactions.

3. NADPH is required for:

(A) Calvin cycle

(B) Glycolysis

(C) Krebs cycle

(D) Electron transport chain

Explanation:

The correct answer is (A) Calvin cycle. NADPH generated in the light reaction provides high-energy electrons for carbon fixation in the Calvin cycle. Glycolysis and Krebs cycle occur in cellular respiration, not photosynthesis.

4. ATP generated in light reactions is used for:

(A) Light absorption

(B) Carbon fixation

(C) Oxygen evolution

(D) Water splitting

Explanation:

The correct answer is (B) Carbon fixation. ATP from light reactions provides energy for the Calvin cycle to convert CO2 into glucose. It is not directly used for light absorption or water splitting.

5. Photosystem II absorbs light at wavelength around:

(A) 700 nm

(B) 680 nm

(C) 600 nm

(D) 500 nm

Explanation:

The correct answer is (B) 680 nm. Photosystem II absorbs red light near 680 nm, initiating photolysis of water and electron transport. Photosystem I absorbs at 700 nm.

6. Primary electron donor in light reaction is:

(A) NADPH

(B) Water

(C) ATP

(D) CO2

Explanation:

The correct answer is (B) Water. Water molecules donate electrons during photolysis in photosystem II. NADPH carries electrons, ATP stores energy, and CO2 is a carbon source, not an electron donor.

7. Assertion-Reason Question:

Assertion (A): Light reactions produce ATP, NADPH, and oxygen.

Reason (R): NADH is the main electron carrier in photosynthesis.

(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 (C). Light reactions produce ATP, NADPH, and oxygen. NADH is not produced in photosynthesis; it is used in cellular respiration. Hence, the assertion is true, and the reason is false.

8. Matching Type Question:

Match products with their origin:

(i) ATP – (a) Light reaction

(ii) NADPH – (b) Light reaction

(iii) Oxygen – (c) Light reaction

(iv) Glucose – (d) Calvin cycle

(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). ATP, NADPH, and oxygen are products of light reactions, while glucose is synthesized in the Calvin cycle. This distinction helps separate energy capture from carbon fixation stages in photosynthesis.

9. Fill in the Blanks:

The light reaction of photosynthesis produces ______, ______, and ______.

(A) Oxygen, NADH, ATP

(B) NADPH, ATP, Oxygen

(C) Glucose, ATP, NADPH

(D) CO2, NADPH, Oxygen

Explanation:

Correct answer is (B) NADPH, ATP, Oxygen. Light reactions produce chemical energy (ATP), reducing power (NADPH), and oxygen as a byproduct. NADH and glucose are not products of light reactions.

10. Choose the correct statements:

(A) Light reactions produce ATP

(B) Oxygen is released

(C) NADPH is formed

(D) NADH is formed

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. Light reactions produce ATP, NADPH, and oxygen. NADH is not produced in photosynthesis but in cellular respiration. Understanding the distinction between NADPH and NADH is crucial for NEET UG questions.

Topic: Oxygen Evolution

Subtopic: Oxygenic vs Anoxygenic Photosynthesis

Keyword Definitions:

• Oxygenic photosynthesis: Type of photosynthesis where oxygen is released, as in plants, algae, and cyanobacteria.

• Anoxygenic photosynthesis: Photosynthesis that does not release oxygen, using molecules like H2S instead of water.

• Chara: Freshwater green algae performing oxygenic photosynthesis.

• Green sulphur bacteria: Photosynthetic bacteria that perform anoxygenic photosynthesis, do not produce O2.

• Cycas: Gymnosperm plant performing oxygenic photosynthesis.

• Nostoc: Cyanobacterium performing oxygenic photosynthesis.

Lead Question (2018):

Oxygen is not produced during photosynthesis by:

(A) Chara

(B) Green sulphur bacteria

(C) Cycas

(D) Nostoc

Explanation:

The correct answer is (B) Green sulphur bacteria. Green sulphur bacteria perform anoxygenic photosynthesis using H2S or other donors, producing no oxygen. Chara, Cycas, and Nostoc are oxygenic photosynthesizers, releasing O2 by splitting water molecules during light-dependent reactions.

1. Which organism releases oxygen during photosynthesis?

(A) Green sulphur bacteria

(B) Chara

(C) Purple bacteria

(D) Heliobacteria

Explanation:

The correct answer is (B) Chara. Chara is a green alga performing oxygenic photosynthesis, splitting water to release oxygen. Green sulphur bacteria, purple bacteria, and heliobacteria perform anoxygenic photosynthesis, producing no O2. Oxygen evolution is characteristic of plants, algae, and cyanobacteria.

2. In anoxygenic photosynthesis, the electron donor is usually:

(A) Water

(B) Hydrogen sulfide

(C) Carbon dioxide

(D) Oxygen

Explanation:

The correct answer is (B) Hydrogen sulfide. Anoxygenic photosynthetic bacteria like green sulphur bacteria use H2S or other compounds as electron donors instead of water. This process does not release oxygen, unlike oxygenic photosynthesis where water is split.

3. Which plant type is an oxygenic photosynthesizer?

(A) Gymnosperms

(B) Green sulphur bacteria

(C) Purple bacteria

(D) Heliobacteria

Explanation:

The correct answer is (A) Gymnosperms. Gymnosperms like Cycas perform oxygenic photosynthesis, releasing O2. Photosynthetic bacteria like green sulphur, purple bacteria, and heliobacteria are anoxygenic and do not produce oxygen. Oxygenic photosynthesis involves water splitting in light reactions.

4. Cyanobacteria such as Nostoc contribute to:

(A) Oxygen production

(B) Hydrogen sulfide production

(C) Methane generation

(D) Nitrogen fixation only

Explanation:

The correct answer is (A) Oxygen production. Nostoc, a cyanobacterium, performs oxygenic photosynthesis producing oxygen and can also fix nitrogen. Cyanobacteria were responsible for the initial oxygenation of Earth's atmosphere.

5. Which pigment is present in green sulphur bacteria?

(A) Chlorophyll a

(B) Bacteriochlorophyll

(C) Phycobilin

(D) Carotenoids

Explanation:

The correct answer is (B) Bacteriochlorophyll. Green sulphur bacteria contain bacteriochlorophyll which absorbs light for anoxygenic photosynthesis. Chlorophyll a is in oxygenic photosynthesizers like plants and cyanobacteria. Phycobilins are accessory pigments in cyanobacteria.

6. Oxygen evolution occurs in which photosystem?

(A) Photosystem I

(B) Photosystem II

(C) Photosystem III

(D) Cytochrome system

Explanation:

The correct answer is (B) Photosystem II. Photosystem II splits water molecules in oxygenic photosynthesis to release oxygen. Anoxygenic bacteria lack PSII, using other electron donors. Photosystem I mainly participates in NADPH formation.

7. Assertion-Reason Question:

Assertion (A): Green sulphur bacteria do not produce oxygen.

Reason (R): They perform anoxygenic photosynthesis using H2S as electron donor.

(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). Green sulphur bacteria are anoxygenic; they use H2S instead of water as electron donor, producing no oxygen. The reason correctly explains the assertion, distinguishing oxygenic and anoxygenic photosynthetic organisms.

8. Matching Type Question:

Match organisms with type of photosynthesis:

(i) Chara – (a) Oxygenic

(ii) Cycas – (b) Oxygenic

(iii) Nostoc – (c) Oxygenic

(iv) Green sulphur bacteria – (d) Anoxygenic

(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-d, ii-c, iii-b, iv-a

Explanation:

Correct answer is (A). Chara, Cycas, and Nostoc perform oxygenic photosynthesis, producing O2. Green sulphur bacteria are anoxygenic, using H2S and producing no oxygen. Matching reinforces oxygen evolution understanding in various organisms.

9. Fill in the Blanks:

______ performs anoxygenic photosynthesis and does not release ______.

(A) Chara, oxygen

(B) Green sulphur bacteria, oxygen

(C) Cycas, carbon dioxide

(D) Nostoc, water

Explanation:

Correct answer is (B) Green sulphur bacteria, oxygen. Green sulphur bacteria use H2S as electron donor and perform anoxygenic photosynthesis, producing no oxygen. Chara, Cycas, and Nostoc are oxygenic and release O2.

10. Choose the correct statements:

(A) Chara produces oxygen

(B) Green sulphur bacteria do not produce oxygen

(C) Cycas produces oxygen

(D) Nostoc produces oxygen

Options:

(1) A, B, C

(2) A, B, D

(3) B, C, D

(4) A, B, C, D

Explanation:

Correct answer is (4) A, B, C, D. Chara, Cycas, and Nostoc are oxygenic and release oxygen. Green sulphur bacteria are anoxygenic and do not release oxygen. Recognizing oxygenic versus anoxygenic photosynthesis is essential for NEET UG questions on photosynthesis.

Subtopic: Carbon Fixation Pathways

Keyword Definitions:

• Phosphoenolpyruvate (PEP) – A three-carbon compound acting as CO2 acceptor in C4 photosynthesis.

• C3 plants – Plants using Calvin cycle as primary carbon fixation, first product is 3-phosphoglycerate.

• C4 plants – Plants with Kranz anatomy, PEP carboxylase fixes CO2 initially.

• Calvin cycle – Series of reactions in chloroplast stroma for carbon fixation.

• C2 plants – Plants exhibiting photorespiration; intermediate between C3 and C4.

• PEP carboxylase – Enzyme that fixes CO2 to PEP in C4 pathway.

• Carbon fixation – Incorporation of inorganic CO2 into organic molecules.

• Bundle sheath cells – Cells in C4 plants where Calvin cycle occurs.

• Mesophyll cells – Photosynthetic cells in leaf tissue; site of initial CO2 fixation in C4 plants.

• Photorespiration – Process consuming O2 and releasing CO2 in C3 plants.

Lead Question – 2017:

Phosphoenol pyruvate (PEP) is the primary CO2 acceptor in:

(A) C3 and C4 plants

(B) C3 plant

(C) C4 plant

(D) C2 plant

Explanation:

PEP acts as the primary CO2 acceptor in C4 plants, where CO2 is initially fixed by PEP carboxylase in mesophyll cells, forming oxaloacetate. In C3 plants, CO2 is fixed by RuBP via Calvin cycle. (Answer: C)

1) Single Correct Answer MCQ:

Which enzyme fixes CO2 to PEP in C4 plants?

(A) Rubisco

(B) PEP carboxylase

(C) Malate dehydrogenase

(D) Glyceraldehyde-3-phosphate dehydrogenase

Explanation:

< b>PEP carboxylase

catalyzes CO2 fixation to PEP forming oxaloacetate in mesophyll cells of C4 plants. Rubisco functions in Calvin cycle. This step reduces photorespiration, allowing efficient carbon fixation. (Answer: B)

2) Single Correct Answer MCQ:

In which cells does initial CO2 fixation occur in C4 plants?

(A) Bundle sheath cells

(B) Mesophyll cells

(C) Guard cells

(D) Epidermal cells

Explanation:

Initial CO2 fixation in C4 plants occurs in mesophyll cells where PEP carboxylase converts CO2 and PEP into oxaloacetate. Bundle sheath cells later perform the Calvin cycle. (Answer: B)

3) Single Correct Answer MCQ:

First product of CO2 fixation in C4 plants is:

(A) 3-phosphoglycerate

(B) Oxaloacetate

(C) Pyruvate

(D) Malate

Explanation:

In C4 plants, CO2 fixed by PEP carboxylase forms oxaloacetate (a four-carbon compound), which is later converted to malate or aspartate for transport to bundle sheath cells. (Answer: B)

4) Single Correct Answer MCQ:

Which of the following reduces photorespiration in hot climates?

(A) C3 pathway

(B) C4 pathway

(C) C2 pathway

(D) CAM pathway

Explanation:

The C4 pathway reduces photorespiration because PEP carboxylase has no oxygenase activity, concentrating CO2 in bundle sheath cells for Calvin cycle, making photosynthesis more efficient in hot environments. (Answer: B)

5) Single Correct Answer MCQ:

Which compound regenerates PEP in C4 photosynthesis?

(A) Malate

(B) Pyruvate

(C) Oxaloacetate

(D) Glyceraldehyde-3-phosphate

Explanation:

< b>Pyruvate is transported back to mesophyll cells after CO2 release in bundle sheath cells and converted into PEP, completing the C4 cycle for continuous CO2 fixation. (Answer: B)

6) Single Correct Answer MCQ:

C3 plants use which CO2 acceptor?

(A) PEP

(B) RuBP

(C) Oxaloacetate

(D) Malate

Explanation:

C3 plants fix CO2 using RuBP (ribulose-1,5-bisphosphate) via Calvin cycle. PEP acts only in C4 and CAM plants. (Answer: B)

7) Assertion-Reason MCQ:

Assertion (A): PEP is the primary CO2 acceptor in C4 plants.

Reason (R): PEP carboxylase is more efficient and oxygen-insensitive than Rubisco.

(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:

Both assertion and reason are true. PEP carboxylase fixes CO2 efficiently in mesophyll cells without oxygen interference, explaining why PEP is the primary CO2 acceptor in C4 plants. (Answer: A)

8) Matching Type MCQ:

Match the plant with its primary CO2 acceptor:

1. Maize – (i) RuBP

2. Wheat – (ii) PEP

3. Sugarcane – (iii) PEP

4. Rice – (iv) RuBP

Options:

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

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

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

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

Explanation:

C4 plants (Maize, Sugarcane) use PEP, while C3 plants (Wheat, Rice) use RuBP as primary CO2 acceptor. (Answer: A)

9) Fill in the Blanks MCQ:

In C4 photosynthesis, CO2 is initially fixed to ________ forming oxaloacetate.

(A) RuBP

(B) PEP

(C) Malate

(D) Pyruvate

Explanation:

CO2 is fixed to PEP in C4 plants by PEP carboxylase forming a four-carbon compound oxaloacetate, which is then transported to bundle sheath cells. (Answer: B)

10) Choose the correct statements MCQ:

1. PEP is primary CO2 acceptor in C4 plants.

2. RuBP is primary CO2 acceptor in C3 plants.

3. PEP carboxylase is oxygen-insensitive.

4. C4 pathway is common in cold climates.

Options:

(A) 1, 2, 3

(B) 2, 3, 4

(C) 1, 3, 4

(D) 1, 2, 4

Explanation:

Statements 1, 2, 3 are correct. Statement 4 is incorrect; C4 pathway is common in hot, arid climates to reduce photorespiration. (Answer: A)

Topic: Factors affecting Photosynthesis
Subtopic: CO2, Light and Temperature influence

Keyword Definitions:

• Photosynthesis: Process by which green plants prepare food using sunlight, CO2 and water.

• C3 plants: Plants fixing CO2 initially into a 3-carbon compound (PGA) in Calvin cycle.

• C4 plants: Plants fixing CO2 initially into a 4-carbon compound (oxaloacetate) using PEP carboxylase.

• Light saturation: Level of light intensity beyond which photosynthesis cannot increase.

• CO2 concentration: Level of carbon dioxide in the atmosphere influencing photosynthesis rate.

Lead Question - 2017

With reference to factors affecting the rate of photosynthesis, which of the following statements is not correct ?
(A) Tomato is a greenhouse crop which can be grown in CO2 enriched atmosphere for higher yield
(B) Light saturation for CO2 fixation occurs at 10% of full sunlight
(C) Increasing atmospheric CO2 concentration up to 0.05% can enhance CO2 fixation rate
(D) C3 plants respond to higher temperatures with enhanced photosynthesis while C4 plants

Explanation: The incorrect statement is (D). C3 plants show photorespiration at higher temperatures leading to reduced photosynthesis, while C4 plants thrive better under high temperature. Options (A), (B), and (C) are correct. Thus, statement D is not correct regarding photosynthesis. Hence, the correct answer is (D).

1) In maize, which is a C4 plant, the primary CO2 acceptor is:
(A) RuBP
(B) PEP
(C) PGA
(D) OAA

Explanation: In C4 plants like maize, the primary CO2 acceptor is phosphoenolpyruvate (PEP), which forms oxaloacetate. This adaptation minimizes photorespiration and enhances efficiency under high light and temperature. Therefore, the correct answer is (B) PEP.

2) Photorespiration occurs in which of the following cellular organelles?
(A) Mitochondria, Cytoplasm, Chloroplast
(B) Chloroplast, Peroxisome, Mitochondria
(C) Peroxisome, Lysosome, Cytoplasm
(D) Chloroplast, Ribosome, Nucleus

Explanation: Photorespiration occurs through coordinated functioning of chloroplast, peroxisome, and mitochondria. This process consumes O2, releases CO2, and reduces photosynthetic efficiency in C3 plants. Thus, the correct answer is (B) Chloroplast, Peroxisome, and Mitochondria.

3) Clinical type: A patient with genetic deficiency of RuBisCO enzyme would show:
(A) Impaired oxygen transport
(B) Impaired CO2 fixation in Calvin cycle
(C) No mitochondrial activity
(D) Defective glycolysis

Explanation: RuBisCO catalyzes CO2 fixation in the Calvin cycle. Its deficiency leads to impaired CO2 fixation, affecting carbohydrate synthesis and overall photosynthetic efficiency. Oxygen transport, glycolysis, and mitochondria remain unaffected. Hence, the correct answer is (B) Impaired CO2 fixation in Calvin cycle.

4) Which one is the major limiting factor for photosynthesis in tropical plants?
(A) Temperature
(B) CO2 concentration
(C) Light intensity
(D) Water availability

Explanation: In tropical plants, the major limiting factor is CO2 concentration. Despite high sunlight and suitable temperatures, limited CO2 restricts photosynthesis rates. Thus, the correct answer is (B) CO2 concentration.

5) Assertion-Reason Type:
Assertion (A): C4 plants show better photosynthetic efficiency at high temperatures.
Reason (R): They have a CO2 concentrating mechanism minimizing photorespiration.
(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

Explanation: Both assertion and reason are true. C4 plants possess a special CO2 concentrating mechanism which minimizes photorespiration, hence they are efficient in high temperature environments. Therefore, the correct answer is (A).

6) Match the following regarding photosynthesis pathways:
Column I
(a) Calvin Cycle
(b) Hatch and Slack Pathway
(c) Photorespiration
(d) Kranz Anatomy
Column II
1. C3 cycle
2. C4 pathway
3. C2 cycle
4. Specialized leaf anatomy
Options:
(A) a-1, b-2, c-3, d-4
(B) a-2, b-1, c-4, d-3
(C) a-3, b-4, c-2, d-1
(D) a-4, b-3, c-1, d-2

Explanation: Correct matching is a-1 (Calvin cycle = C3), b-2 (Hatch and Slack = C4), c-3 (Photorespiration = C2), d-4 (Kranz Anatomy = Specialized anatomy). Hence, the correct answer is (A).

7) Fill in the blank: The enzyme PEP carboxylase is absent in ______ plants.
(A) C3
(B) C4
(C) CAM
(D) None

Explanation: PEP carboxylase is absent in C3 plants. It is present in C4 and CAM plants to fix CO2, thereby reducing photorespiration. Hence, the correct answer is (A) C3 plants.

8) In greenhouse cultivation of tomato, yield can be increased by:
(A) Providing high O2 concentration
(B) Enriching CO2 concentration
(C) Decreasing humidity
(D) Lowering CO2 concentration

Explanation: Enriching CO2 concentration in greenhouse cultivation increases the rate of photosynthesis, thereby improving yield in crops like tomato. Thus, the correct answer is (B) Enriching CO2 concentration.

9) Choose the correct statements:
(i) C4 plants are more efficient in hot climates
(ii) C3 plants perform better in cool climates
(iii) CAM plants open stomata during day
(iv) Photorespiration reduces efficiency in C3 plants
Options:
(A) i, ii, iii
(B) ii, iii, iv
(C) i, ii, iv
(D) i, iii, iv

Explanation: Statements (i), (ii), and (iv) are correct. C4 plants excel in hot climates, C3 plants are better in cooler climates, and photorespiration reduces C3 efficiency. CAM plants open stomata at night, not day. Hence, the correct answer is (C) i, ii, iv.

10) Clinical type: A farmer notices stunted growth and poor yield in wheat crops grown under high temperature. The probable reason is:
(A) Increased glycolysis
(B) Enhanced photorespiration in C3 pathway
(C) Lack of water absorption
(D) Nitrogen deficiency

Explanation: Wheat is a C3 plant. Under high temperature, photorespiration increases, reducing photosynthetic output and crop yield. Thus, the correct answer is (B) Enhanced photorespiration in C3 pathway.

Subtopic: C3 and C4 Pathways

Keyword Definitions:

• Photorespiration: Process in which RuBisCO fixes O₂ instead of CO₂, reducing photosynthetic efficiency in C3 plants.

• Calvin cycle: Light-independent reaction of photosynthesis producing glucose from CO₂ using ATP and NADPH.

• Glycolysis: Breakdown of glucose to pyruvate, occurs in cytoplasm, not part of photosynthesis.

• Respiration: Process of energy release from glucose through glycolysis, Krebs cycle, and electron transport.

Lead Question - 2016 (Phase 2):

The process which makes major difference between C3 and C4 plants is

(1) Respiration

(2) Glycolysis

(3) Calvin cycle

(4) Photorespiration

Explanation: Photorespiration significantly affects C3 plants, reducing photosynthetic efficiency because RuBisCO fixes O₂ instead of CO₂. C4 plants have a CO₂-concentrating mechanism that minimizes photorespiration. Correct answer: (4) Photorespiration. Understanding this difference is crucial for NEET UG plant physiology and crop productivity studies.

1. Single Correct Answer MCQ:

C4 plants minimize photorespiration by

(1) Using PEP carboxylase

(2) Using RuBisCO exclusively

(3) Performing glycolysis faster

(4) Increasing mitochondrial respiration

Explanation: C4 plants utilize PEP carboxylase to initially fix CO₂ into a four-carbon compound, delivering CO₂ to RuBisCO in bundle sheath cells, minimizing photorespiration. Correct answer: (1) Using PEP carboxylase, essential for NEET UG photosynthesis understanding.

2. Single Correct Answer MCQ:

The first product of carbon fixation in C3 plants is

(1) 3-Phosphoglycerate

(2) Oxaloacetate

(3) Malate

(4) Pyruvate

Explanation: In C3 plants, RuBisCO fixes CO₂ to RuBP producing 3-phosphoglycerate as the first stable product. Correct answer: (1) 3-Phosphoglycerate, fundamental for NEET UG photosynthesis questions.

3. Single Correct Answer MCQ:

In C4 plants, the Calvin cycle occurs in

(1) Mesophyll cells

(2) Bundle sheath cells

(3) Guard cells

(4) Epidermal cells

Explanation: In C4 plants, CO₂ is concentrated in bundle sheath cells where RuBisCO operates efficiently in the Calvin cycle, reducing photorespiration. Correct answer: (2) Bundle sheath cells, key concept for NEET UG plant physiology.

4. Single Correct Answer MCQ:

Photorespiration in C3 plants leads to

(1) Increased glucose production

(2) Loss of fixed carbon and energy

(3) Faster growth under high light

(4) Formation of starch in chloroplast

Explanation: Photorespiration reduces photosynthetic efficiency by consuming O₂ and releasing CO₂, resulting in energy and carbon loss. Correct answer: (2) Loss of fixed carbon and energy, crucial for NEET UG photosynthesis concepts.

5. Single Correct Answer MCQ (Clinical-type):

Under high temperature and low CO₂, C3 plants exhibit

(1) Reduced photorespiration

(2) Increased photorespiration

(3) Faster Calvin cycle

(4) Inhibited glycolysis

Explanation: High temperatures and low CO₂ increase oxygenation activity of RuBisCO, enhancing photorespiration in C3 plants, lowering efficiency. Correct answer: (2) Increased photorespiration, essential for NEET UG environmental plant physiology.

6. Single Correct Answer MCQ:

PEP carboxylase differs from RuBisCO because

(1) It fixes O₂

(2) It fixes CO₂ without oxygenase activity

(3) It is in mitochondria

(4) It produces 3-phosphoglycerate

Explanation: PEP carboxylase fixes CO₂ efficiently without binding O₂, avoiding photorespiration in C4 plants. RuBisCO has both carboxylase and oxygenase activity. Correct answer: (2) It fixes CO₂ without oxygenase activity, vital for NEET UG photosynthesis comparison.

7. Assertion-Reason MCQ:

Assertion (A): C4 plants show higher photosynthetic efficiency than C3 in hot climates.

Reason (R): C4 pathway minimizes photorespiration by CO₂ concentration.

(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 but R is false

(4) A is false but R is true

Explanation: Both assertion and reason are true. C4 plants minimize photorespiration via CO₂ concentration in bundle sheath cells, maintaining higher efficiency under heat and drought. Correct answer: (1) Both A and R are true and R is correct explanation of A.

8. Matching Type MCQ:

Match the plant type with its feature:

A. C3

B. C4

1. Photorespiration high

2. Photorespiration minimized

Options:

(1) A-1, B-2

(2) A-2, B-1

(3) A-1, B-1

(4) A-2, B-2

Explanation: C3 plants experience high photorespiration; C4 plants concentrate CO₂ in bundle sheath cells minimizing photorespiration. Correct answer: (1) A-1, B-2, essential for NEET UG photosynthesis differences.

9. Fill in the Blanks MCQ:

The major difference between C3 and C4 plants is ______.

(1) Glycolysis

(2) Calvin cycle

(3) Photorespiration

(4) Respiration

Explanation: Photorespiration occurs extensively in C3 plants but is minimized in C4 plants due to CO₂ concentration mechanism. Correct answer: (3) Photorespiration, fundamental for NEET UG photosynthesis topics.

10. Choose the Correct Statements MCQ:

Select correct statements:

(1) C4 plants fix CO₂ in mesophyll then bundle sheath cells

(2) C3 plants have high photorespiration

(3) PEP carboxylase prevents photorespiration in C4 plants

(4) C3 and C4 plants perform glycolysis differently

Options:

(1) 1, 2, 3 only

(2) 2 and 4 only

(3) 1, 3, 4 only

(4) All statements are correct

Explanation: Statements 1, 2, and 3 are correct. C4 plants fix CO₂ initially via PEP carboxylase in mesophyll; C3 plants show high photorespiration; PEP carboxylase reduces photorespiration. Glycolysis occurs similarly in both. Correct answer: (1) 1, 2, 3 only.

Topic: Photosynthesis and Plant Adaptations

Subtopic: Photosynthetic Pathways and Efficiency

• C3 Plants: Plants using Calvin cycle for carbon fixation, common in cool, wet climates.

• C4 Plants: Plants with specialized anatomy to reduce photorespiration, adapted to high light and temperature.

• CAM Plants: Plants that open stomata at night to minimize water loss.

• Nitrogen Fixer: Plants that convert atmospheric nitrogen into usable forms via symbiotic relationships.

Lead Question - 2016 (Phase 1)

A plant in your garden avoids photorespiratory losses, has improved water use efficiency, shows high rates of photosynthesis at high temperatures, and has improved efficiency of nitrogen utilization. In which of the following physiological groups would you assign this plant:

• (1) C3

• (2) C4

• (3) CAM

• (4) Nitrogen fixer

Answer & Explanation: (2) C4. C4 plants are adapted to high light and temperature conditions, reducing photorespiration by spatial separation of initial CO₂ fixation and the Calvin cycle. They show better water use efficiency and nitrogen utilization, making them ideal in hot climates compared to C3 and CAM plants.

MCQ 1 (Single Correct Answer)

Which of the following is a key feature of C4 photosynthesis?

• (A) Temporal separation of carbon fixation and Calvin cycle

• (B) Spatial separation of carbon fixation and Calvin cycle

• (C) Carbon fixation occurs at night only

• (D) Nitrogen fixation within chloroplasts

Answer & Explanation: (B) C4 plants spatially separate initial CO₂ fixation (in mesophyll cells) and the Calvin cycle (in bundle sheath cells). This mechanism significantly reduces photorespiration, improving photosynthetic efficiency, especially under high temperature and light intensity conditions, unlike C3 or CAM pathways.

MCQ 2 (Single Correct Answer)

Which enzyme is primarily responsible for carbon fixation in C4 plants?

• (A) Ribulose-1,5-bisphosphate carboxylase (RuBisCO)

• (B) Phosphoenolpyruvate carboxylase (PEP Carboxylase)

• (C) ATP synthase

• (D) NADPH oxidase

Answer & Explanation: (B) Phosphoenolpyruvate carboxylase (PEP Carboxylase) fixes CO₂ in C4 plants. It has a higher affinity for CO₂ than RuBisCO, enabling efficient carbon fixation even at low CO₂ levels, thus reducing photorespiration and enhancing productivity in high temperature and light conditions.

MCQ 3 (Single Correct Answer)

CAM plants primarily open their stomata during:

• (A) Daytime

• (B) Nighttime

• (C) Both Day and Night equally

• (D) Only when water is abundant

Answer & Explanation: (B) CAM plants open stomata at night to fix CO₂, minimizing water loss in arid environments. This temporal separation enables survival under water scarcity, differing from C3 and C4 plants which open stomata during the day.

MCQ 4 (Single Correct Answer)

Why is photorespiration considered inefficient in C3 plants?

• (A) It produces more ATP

• (B) Oxygen competes with CO₂ at RuBisCO, reducing sugar production

• (C) It enhances nitrogen fixation

• (D) It increases chlorophyll production

Answer & Explanation: (B) In C3 plants, photorespiration occurs when RuBisCO fixes O₂ instead of CO₂, wasting energy and reducing photosynthetic efficiency. This leads to lower sugar production and is exacerbated under high temperature and light, unlike C4 plants which bypass this by efficient carbon fixation pathways.

MCQ 5 (Single Correct Answer)

Which of the following is an advantage of C4 plants over C3 plants?

• (A) Less ATP requirement for carbon fixation

• (B) Higher photorespiration rates

• (C) Greater water use efficiency

• (D) Limited to cool climates

Answer & Explanation: (C) C4 plants exhibit greater water use efficiency. The spatial separation of carbon fixation steps enables them to concentrate CO₂ at the site of the Calvin cycle, lowering stomatal opening time, thus conserving water in high temperature and light environments.

MCQ 6 (Single Correct Answer)

Which plant is an example of a CAM plant?

• (A) Sugarcane

• (B) Maize

• (C) Aloe Vera

• (D) Wheat

Answer & Explanation: (C) Aloe Vera is a CAM plant. It opens stomata at night for CO₂ uptake and stores it as malic acid for use in photosynthesis during the day. This strategy minimizes water loss and allows survival in arid climates, unlike C3 or C4 plants.

MCQ 7 (Assertion-Reason)

Assertion (A): C4 plants perform better under high light intensity and temperature compared to C3 plants.

Reason (R): C4 plants possess specialized anatomy and biochemical pathways reducing photorespiration.

• (A) Both A and R are true and R is the correct explanation of A.

• (B) Both A and R are true, but R is not the correct explanation of A.

• (C) A is true, but R is false.

• (D) A is false, but R is true.

Answer & Explanation: (A) Both A and R are true. C4 plants have Kranz anatomy and PEP carboxylase for initial CO₂ fixation, reducing photorespiration, especially in high temperatures and light intensity, thereby outperforming C3 plants under such conditions.

MCQ 8 (Matching Type)

Match the plant type with its characteristic:

• 1. C3 Plant

• 2. C4 Plant

• 3. CAM Plant

• 4. Nitrogen Fixer

1. Performs carbon fixation during the day, susceptible to photorespiration

2. Spatial separation of steps for efficient carbon fixation

3. Temporal separation of carbon fixation to reduce water loss

4. Converts atmospheric nitrogen to ammonia

• (A) 1-A, 2-B, 3-C, 4-D

• (B) 1-B, 2-A, 3-D, 4-C

• (C) 1-D, 2-C, 3-B, 4-A

• (D) 1-C, 2-D, 3-A, 4-B

Answer & Explanation: (A) 1-A, 2-B, 3-C, 4-D. C3 plants perform carbon fixation during the day and are prone to photorespiration; C4 plants separate fixation spatially for efficiency; CAM plants fix carbon at night to minimize water loss; nitrogen fixers convert atmospheric N₂ into ammonia.

MCQ 9 (Fill in the Blanks)

In ______ plants, carbon fixation occurs in mesophyll cells and the Calvin cycle occurs in ______ cells.

• (A) C3, Guard

• (B) C4, Bundle sheath

• (C) CAM, Epidermal

• (D) Nitrogen-fixing, Parenchymal

Answer & Explanation: (B) C4, Bundle sheath. In C4 plants, CO₂ is first fixed in mesophyll cells by PEP carboxylase and transported to bundle sheath cells where the Calvin cycle occurs. This arrangement minimizes photorespiration and enhances efficiency under high temperature conditions.

MCQ 10 (Choose the Correct Statements)

Choose correct statements about C4 and CAM plants:

• 1. Both have adaptations to minimize photorespiration.

• 2. C4 plants separate steps spatially, CAM plants temporally.

• 3. CAM plants are mainly found in aquatic environments.

• 4. C4 plants are more efficient in hot, dry environments than C3 plants.

• (A) 1, 2, and 4 only

• (B) 2 and 3 only

• (C) 1 and 3 only

• (D) All statements are correct

Answer & Explanation: (A) Statements 1, 2, and 4 are correct. Both C4 and CAM plants minimize photorespiration. C4 plants spatially separate carbon fixation steps; CAM plants do so temporally. CAM plants are typically found in arid environments, not aquatic ones, while C4 plants thrive in hot, dry areas.

Keywords:

• Anthocyanins: Water-soluble pigments present in vacuoles responsible for red, purple, and blue colors in flowers and fruits.

• Xanthophylls: Yellow pigments found in chloroplasts, involved in photosynthesis and photoprotection.

• Chlorophylls: Green pigments located in chloroplasts, essential for photosynthesis.

• Carotenoids: Orange to yellow pigments in chloroplasts, protect against photooxidative damage.

• Vacuole: Membrane-bound organelle in plant cells storing water, pigments, and waste.

• Water-soluble pigments: Pigments that dissolve in water, often found in vacuoles, affecting flower and fruit coloration.

• Plastids: Organelles like chloroplasts, chromoplasts, containing pigments like chlorophyll and carotenoids.

• pH-sensitive pigments: Anthocyanin color varies with vacuolar pH.

• Clinical relevance: Anthocyanins have antioxidant properties beneficial for human health.

• Photosynthetic pigments: Chlorophyll, carotenoids, and xanthophylls participate in light absorption.

• Flower pigmentation: Determines attraction of pollinators.

Chapter: Plant Physiology

Topic: Pigments

Subtopic: Water Soluble Pigments

Lead Question - 2016 (Phase 1): Water soluble pigments found in plant cell vacuoles are:

(1) Xanthophylls
(2) Chlorophylls
(3) Carotenoids
(4) Anthocyanins

Answer: 4

Explanation: Anthocyanins are water-soluble pigments located in vacuoles, responsible for red, purple, and blue coloration in plants. Other pigments like chlorophylls, carotenoids, and xanthophylls are lipid-soluble and found in plastids. Their location and solubility differentiate them from vacuolar pigments.

1. Single Correct Answer MCQ: Which pigment is responsible for red coloration in petals?

(A) Chlorophyll
(B) Xanthophyll
(C) Anthocyanin
(D) Carotenoid

Answer: C

Explanation: Anthocyanins provide red, purple, or blue colors in flowers and fruits, attracting pollinators and seed dispersers.

2. Single Correct Answer MCQ: Which pigment is lipid-soluble and located in chloroplasts?

(A) Anthocyanin
(B) Carotenoid
(C) Betalain
(D) Flavonoid

Answer: B

Explanation: Carotenoids are lipid-soluble pigments in chloroplasts, involved in light harvesting and photoprotection.

3. Single Correct Answer MCQ: Water-soluble pigments in vacuoles include:

(A) Chlorophylls
(B) Carotenoids
(C) Anthocyanins
(D) Xanthophylls

Answer: C

Explanation: Anthocyanins dissolve in vacuolar sap, unlike chlorophylls or carotenoids that are in plastid membranes.

4. Single Correct Answer MCQ: Which pigment is pH-sensitive?

(A) Carotenoids
(B) Anthocyanins
(C) Chlorophylls
(D) Xanthophylls

Answer: B

Explanation: Anthocyanins change color depending on vacuolar pH, showing red in acidic and blue in alkaline conditions.

5. Single Correct Answer MCQ: Which pigment is responsible for yellow coloration in leaves?

(A) Anthocyanin
(B) Chlorophyll
(C) Xanthophyll
(D) Phycobilin

Answer: C

Explanation: Xanthophylls are yellow pigments located in chloroplasts, assisting in light absorption for photosynthesis.

6. Single Correct Answer MCQ: Vacuolar pigments mainly function in:

(A) Photosynthesis
(B) Attracting pollinators
(C) Respiration
(D) Electron transport

Answer: B

Explanation: Anthocyanins in vacuoles impart bright colors to flowers and fruits, attracting pollinators and aiding reproduction.

7. Assertion-Reason MCQ:
Assertion (A): Anthocyanins are water-soluble pigments.
Reason (R): They are located in plastids and help in photosynthesis.

(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

Answer: C

Explanation: Anthocyanins are water-soluble pigments in vacuoles, not plastids, and do not participate in photosynthesis.

8. Matching Type MCQ: Match pigment with its solubility/location:

1. Chlorophyll   A. Water-soluble
2. Anthocyanin   B. Lipid-soluble
3. Carotenoid   C. Lipid-soluble
4. Xanthophyll   D. Lipid-soluble

(A) 1-B, 2-A, 3-C, 4-D
(B) 1-A, 2-B, 3-C, 4-D
(C) 1-C, 2-A, 3-B, 4-D
(D) 1-B, 2-D, 3-A, 4-C

Answer: A

Explanation: Chlorophyll, carotenoids, and xanthophylls are lipid-soluble in plastids. Anthocyanins are water-soluble in vacuoles.

9. Fill in the Blanks: ______ pigments are water-soluble and stored in vacuoles, while ______ pigments are lipid-soluble and found in chloroplasts.

(A) Chlorophyll; Anthocyanins
(B) Carotenoids; Anthocyanins
(C) Anthocyanins; Carotenoids
(D) Xanthophylls; Anthocyanins

Answer: C

Explanation: Anthocyanins are water-soluble pigments in vacuoles; carotenoids are lipid-soluble pigments in chloroplasts.

10. Choose the Correct Statements:
(A) Anthocyanins are water-soluble pigments in vacuoles.
(B) Chlorophylls are water-soluble and found in vacuoles.
(C) Carotenoids are lipid-soluble pigments in plastids.
(D) Xanthophylls are lipid-soluble pigments in chloroplasts.

(1) A, B, C
(2) A, C, D
(3) B, C, D
(4) All are correct

Answer: 2

Explanation: A, C, and D are correct. Chlorophylls are lipid-soluble and located in chloroplasts, not vacuoles.

Chapter: Photosynthesis in Higher Plants
Topic: Light Reactions
Subtopic: Chemiosmotic Hypothesis

Keywords:

• Chloroplast – Double-membrane cell organelle responsible for photosynthesis.

• Thylakoid – Flattened sacs in chloroplasts where light reactions occur.

• Stroma – Fluid-filled region outside thylakoids, site of Calvin cycle.

• Lumen – Internal space of thylakoids where proton accumulation occurs.

• ATP synthase – Enzyme that utilizes proton gradient to form ATP.

• Proton gradient – Difference in proton concentration across membranes, driving ATP synthesis.

• Photosystem – Protein-pigment complex capturing light energy for electron transport.

• Light reactions – Stage of photosynthesis converting light into chemical energy.

• NADPH – Reduced coenzyme produced during light reactions.

• Electron transport chain (ETC) – Series of carriers transporting electrons and pumping protons.

Lead Question – 2016 (Phase 1)

In a chloroplast the highest number of protons are found in :

(1) Stroma
(2) Lumen of thylakoids
(3) Inter membrane space
(4) Antennae complex

Explanation: During light reactions, water splitting and proton pumping occur inside thylakoid membranes, leading to maximum proton accumulation in the thylakoid lumen. This creates a gradient used by ATP synthase. Answer: (2) Lumen of thylakoids.

Question 2. Protons generated during photolysis of water are released into:

(1) Stroma
(2) Lumen of thylakoid
(3) Intermembrane space
(4) Nucleus

Explanation: Water photolysis at Photosystem II releases protons into the thylakoid lumen, strengthening the proton gradient essential for ATP generation. Answer: (2) Lumen of thylakoid.

Question 3. ATP synthesis in chloroplasts is driven by:

(1) Proton gradient
(2) Oxygen gradient
(3) Electron gradient
(4) NADPH gradient

Explanation: According to the chemiosmotic hypothesis, ATP synthesis in chloroplasts occurs when protons flow down their gradient from the lumen into the stroma via ATP synthase. Answer: (1) Proton gradient.

Question 4. Assertion (A): Protons accumulate inside the thylakoid lumen during light reactions.
Reason (R): Proton pumping occurs across the thylakoid membrane through cytochrome b6-f complex.

(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 the correct explanation
(3) A is true but R is false
(4) A is false but R is true

Explanation: Protons accumulate in the thylakoid lumen because cytochrome b6-f complex actively pumps protons, and water photolysis adds more protons. R explains A correctly. Answer: (1)

Question 5. Match the following:

A. Photosystem II → (i) NADPH formation
B. Photosystem I → (ii) Water splitting
C. Cytochrome b6-f → (iii) Proton pumping
D. ATP synthase → (iv) ATP synthesis

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

Explanation: PSII splits water, PSI forms NADPH, cytochrome b6-f pumps protons, and ATP synthase generates ATP. Answer: (1)

Question 6. During cyclic photophosphorylation, which product is formed?

(1) ATP only
(2) NADPH only
(3) Oxygen only
(4) ATP and NADPH both

Explanation: Cyclic photophosphorylation involves PSI only and does not produce NADPH or oxygen. It generates ATP exclusively for balancing energy requirements. Answer: (1) ATP only.

Question 7. Fill in the blank: The enzyme responsible for ATP synthesis in chloroplasts is ________.

(1) ATPase
(2) ATP synthase
(3) Kinase
(4) Phosphatase

Explanation: ATP synthase, embedded in the thylakoid membrane, utilizes proton flow to catalyze the phosphorylation of ADP to ATP. Answer: (2) ATP synthase.

Question 8. Which clinical condition may result from defective chloroplast electron transport in plants?

(1) Chlorosis
(2) Anaemia
(3) Osteoporosis
(4) Rickets

Explanation: Impaired chloroplast function affects chlorophyll production and electron transport, leading to leaf yellowing, termed chlorosis, often due to deficiencies in essential nutrients like Mg or Fe. Answer: (1) Chlorosis.

Question 9. Passage-based MCQ:
"Light reactions of photosynthesis generate ATP and NADPH. Protons accumulate in the lumen, and their diffusion back into the stroma drives ATP synthesis. Oxygen is evolved as a by-product of water splitting."

Which of the following is correct?

(1) ATP forms in lumen
(2) NADPH forms in stroma
(3) Oxygen forms in stroma
(4) Protons accumulate in stroma

Explanation: ATP synthase releases ATP into the stroma, NADPH is also produced in stroma, oxygen is released from water splitting into atmosphere, and protons accumulate in lumen. Answer: (2) NADPH forms in stroma.

Question 10. The splitting of water in photosynthesis is catalyzed by:

(1) Oxygen-evolving complex
(2) Cytochrome oxidase
(3) Ferredoxin
(4) Plastocyanin

Explanation: The oxygen-evolving complex, associated with Photosystem II, splits water molecules into protons, electrons, and oxygen, contributing to the proton gradient. Answer: (1) Oxygen-evolving complex.

Chapter: Photosynthesis in Higher Plants
Topic: Light Reactions
Subtopic: Discovery of Photosystems

Keywords:

• Emerson’s Red Drop – Fall in photosynthetic efficiency beyond 680 nm light.

• Emerson’s Enhancement Effect – Increased photosynthetic yield when red and far-red light used together.

• Photosystem I (PSI) – Absorbs light at 700 nm, produces NADPH.

• Photosystem II (PSII) – Absorbs light at 680 nm, splits water and releases oxygen.

• Photophosphorylation – ATP formation using light-driven proton gradient.

• Cyclic photophosphorylation – ATP formation without NADPH or O₂ release.

• Non-cyclic photophosphorylation – Produces ATP, NADPH, and oxygen.

• Light reactions – First stage of photosynthesis converting light into chemical energy.

• Chlorosis – Yellowing of leaves due to defective chloroplasts or pigments.

• Electron transport chain – Series of carriers transferring electrons in thylakoids.

Lead Question – 2016 (Phase 1)

Emerson's enhancement effect and Red drop have been instrumental in the discovery of:

(1) Photophosphorylation and non-cyclic electron transport
(2) Two photosystems operating simultaneously
(3) Photophosphorylation and cyclic electron transport
(4) Oxidative phosphorylation

Explanation: Emerson observed decreased efficiency under >680 nm (red drop) but increased efficiency when red and far-red light combined (enhancement effect). This proved the existence of two photosystems working together in light reactions. Answer: (2) Two photosystems operating simultaneously.

Question 2. Which pigment molecule is mainly responsible for absorbing far-red light in photosynthesis?

(1) P680
(2) P700
(3) Carotenoids
(4) Chlorophyll b

Explanation: Far-red light is primarily absorbed by Photosystem I, where the special chlorophyll a molecule is called P700. It works in cooperation with PSII during non-cyclic photophosphorylation. Answer: (2) P700.

Question 3. The red drop effect is best explained as:

(1) Loss of pigment molecules
(2) Sharp decline in photosynthetic efficiency beyond 680 nm
(3) Increase in photolysis rate
(4) Inhibition of cyclic phosphorylation

Explanation: The red drop effect refers to reduced photosynthetic efficiency in light of wavelength longer than 680 nm. This occurs because PSII is not efficiently excited by far-red light alone. Answer: (2) Sharp decline in photosynthetic efficiency beyond 680 nm.

Question 4. Assertion (A): Emerson’s enhancement effect shows maximum yield when red and far-red light are used together.
Reason (R): Two photosystems, PSII and PSI, operate in series to enhance photosynthesis.

(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
(3) A is true but R is false
(4) A is false but R is true

Explanation: The enhancement effect arises because PSII absorbs red light while PSI absorbs far-red, and both work sequentially in the Z-scheme. Hence, A and R are true and R correctly explains A. Answer: (1).

Question 5. Match the following:

A. Emerson’s Effect → (i) Two photosystems
B. Red Drop → (ii) Decline in efficiency
C. PSII → (iii) Splits water
D. PSI → (iv) NADPH formation

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

Explanation: Emerson’s effect proved two photosystems, Red Drop indicated efficiency decline, PSII splits water, and PSI produces NADPH. Answer: (1) A-i, B-ii, C-iii, D-iv.

Question 6. In non-cyclic photophosphorylation, electrons lost from PSI are replaced by:

(1) Water
(2) PSII
(3) NADPH
(4) Cytochrome b6f

Explanation: Electrons lost from PSI are replenished by PSII through the Z-scheme. Water splitting at PSII provides electrons that are passed via the electron transport chain to PSI. Answer: (2) PSII.

Question 7. Fill in the blank: The first stable product of the Calvin cycle is ________.

(1) PGA
(2) Glucose
(3) RuBP
(4) ATP

Explanation: The first stable product of the Calvin cycle (dark reaction) is 3-phosphoglycerate (PGA), formed by carboxylation of RuBP catalyzed by RuBisCO. Answer: (1) PGA.

Question 8. A farmer observed yellowing of leaves in young plants despite sufficient sunlight. The most probable reason is:

(1) Iron deficiency affecting chlorophyll synthesis
(2) Excess oxygen release
(3) Overproduction of ATP
(4) Increased photolysis

Explanation: Iron deficiency prevents proper chlorophyll formation, impairing photosystems and electron transport. This leads to chlorosis (yellowing of leaves) even under adequate light. Answer: (1) Iron deficiency affecting chlorophyll synthesis.

Question 9. Passage-based MCQ:
"Two photosystems operate together in plants. PSII absorbs light at 680 nm and splits water, while PSI absorbs at 700 nm and reduces NADP⁺. The cooperation of both enhances photosynthetic yield."

Which of the following is correct?

(1) PSI splits water
(2) PSII produces NADPH
(3) Both PSI and PSII are needed for maximum efficiency
(4) Enhancement effect disproves two photosystems

Explanation: The passage explains the cooperative function of PSI and PSII in photosynthesis. Both are essential for maximum efficiency as proved by Emerson’s enhancement effect. Answer: (3) Both PSI and PSII are needed for maximum efficiency.

Question 10. Which scientist first reported the enhancement effect in photosynthesis?

(1) Calvin
(2) Emerson
(3) Blackman
(4) Priestley

Explanation: Robert Emerson studied light wavelengths in photosynthesis and described both the red drop and enhancement effects, leading to the discovery of two photosystems. Answer: (2) Emerson.