Question 1

Which enzyme catalyzes the carboxylation of ribulose-1,5-bisphosphate (RuBP) in the Calvin–Benson cycle, and what is the main product of this reaction?

Accepted Answer

Rubisco; 3-phosphoglycerate (3-PGA)

Question 2

What is the primary function of the C4 cycle in plants?

Accepted Answer

To conserve CO2 and minimize photorespiration

Question 3

Which of the following best describes the fate of glyceraldehyde-3-phosphate (GAP) produced in the Calvin–Benson cycle?

Accepted Answer

It is exported to the cytosol for sucrose synthesis and also used to regenerate RuBP.

Question 4

Which enzyme is responsible for the reduction of 1,3-bisphosphoglycerate to glyceraldehyde-3-phosphate in the Calvin–Benson cycle?

Accepted Answer

GAPDH

Question 5

What is the role of Rubisco activase in photosynthesis?

Accepted Answer

It releases inhibitory sugar phosphates from Rubisco, activating it.

Question 6

Which of the following statements about photorespiration is correct?

Accepted Answer

It results in the consumption of O2 and release of CO2.

Question 7

Which enzyme catalyzes the phosphorylation of ribulose-5-phosphate to ribulose-1,5-bisphosphate in the Calvin–Benson cycle?

Accepted Answer

PRK

Question 8

What is the effect of high pH and increased Mg2+ concentration in the stroma on Rubisco activity?

Accepted Answer

It stimulates Rubisco activity.

Question 9

Which process is responsible for the generation of ATP without the production of NADPH or O2 in chloroplasts?

Accepted Answer

Cyclic photophosphorylation

Question 10

Which of the following herbicides inhibits photosystem II and blocks electron flow in the light reactions?

Accepted Answer

Diuron

Question 11

What is the main regulatory mechanism for activating chloroplast ATP synthase?

Accepted Answer

Reduction of a disulfide bond in the γ subunit by thioredoxin

Question 12

Which enzyme catalyzes the irreversible hydrolysis of fructose 1,6-bisphosphate to fructose 6-phosphate in the Calvin–Benson cycle?

Accepted Answer

FBPase

Question 13

What is the significance of nonphotochemical quenching (NPQ) in plants?

Accepted Answer

It protects plants from reactive oxygen species by dissipating excess energy as heat.

Question 14

Which of the following best describes the organization of photosynthetic components in the thylakoid membrane?

Accepted Answer

Photosystem II is in stacked regions, while photosystem I and ATP synthase are in unstacked regions.

Question 15

Which enzyme is responsible for the conversion of fructose-6-phosphate to glucose-6-phosphate in the starch biosynthesis pathway?

Accepted Answer

PGI (phosphoglucose isomerase)

Question 16

A mutant plant lacks carotenoids. What is the most likely consequence when exposed to bright sunlight?

Accepted Answer

It will be quickly killed due to the formation of reactive oxygen species.

Question 17

Which of the following is a direct product of the decarboxylation of malate in the C4 cycle?

Accepted Answer

CO2

Question 18

Which enzyme is activated by the reduction of disulfides to sulfhydryls via the ferredoxin/thioredoxin pathway during the light reactions?

Accepted Answer

Rubisco activase

Question 19

What is the main difference between the ATP synthase complexes of chloroplasts and mitochondria?

Accepted Answer

Chloroplast ATP synthase is called the CF1–CF0 complex and has reversed membrane orientation compared to mitochondrial F1–F0.

Question 20

Which of the following best explains why C4 plants are more efficient than C3 plants under high light and temperature conditions?

Accepted Answer

C4 plants minimize photorespiration by concentrating CO2 in bundle sheath cells.

Photosynthesis: The Calvin–Benson Cycle, Photorespiration, and Regulation

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