Question

Which of the following does notoccur during the Calvin cycle?

(A) carbon fixation

(B) oxidation of NADPH

(C) release of oxygen

(D) regeneration of the CO₂ acceptor

Short answer

a. The option “carbon fixation” is false.

b. The option “oxidation of NADPH” is false.

c. The option “release of oxygen” is true.

d. The option “regeneration of the CO₂ acceptor” is false.

Step-by-step solution

Calvin cycle

The Calvin cycle is the light-independent reaction of photosynthesis that is anabolic. It means synthesizing molecules like carbohydrates by using small molecules like CO₂ and ATP. It is the process in which CO₂ is fixed by reacting with organic molecules, and NADPH is consumed as reducing power.

Explanation of option (A)

The first step in the Calvin cycle is carbon fixation, in which a molecule of CO2 is attached to RuBP (ribulose bisphosphate) with the help of RuBP carboxylase-oxygenase or rubisco. A six-carbon intermediate forms that immediately splits into two 3-phosphoglycerate molecules.

Thus, carbon fixation occurs in the Calvin cycle.

Therefore, the given option is false.

Explanation of option (B)

Each of the two 3-phosphoglycerate moleculesis phosphorylated by using ATP and 1,3-bisphosphoglycerate is formed that is reduced by NADPH. In this process, NADPH is oxidized by donating a pair of electrons. In this process, glyceraldehyde 3-phosphate is formed, which is a sugar.

Thus, NADPH is oxidized in the Calvin cycle.

Therefore, the given option is false.

Explanation of option (C)

Oxygen is released in the light reaction of the photosynthesis that needs sunlight. This step uses solar energy and converts it into chemical energy in the form of ATP. Water is split to release electrons, protons, and oxygen. Oxygen acts as a byproduct of the light reaction of photosynthesis.

Thus, oxygen is released in a light reaction, not in the Calvin cycle.

Therefore, the given option is true.

Explanation of option (D)

In the last step of the Calvin cycle, five molecules ofglyceraldehyde 3-phosphate are rearranged into three RuBP molecules that accept CO₂ again, and the cycle occurs again.

Thus, the CO₂ acceptor regenerates that isRuBP.

Therefore, the given option is false.