Question

Consider the following statements and select correct set for photosynthesis in $C_3$-plants.
(a) Bifunctional nature of carboxylating enzyme
(b) It takes only one turns of the Calvin cycle to fix one molecule of glucose
(c) Regeneration of each RUBP requires 6 ATP
(d) First stable product of Calvin cycle undergoes phosphorylation in reduction step
(e) Only ATP is required

• A. (a), (c) and (d)
• B. (b), (d) and (e)
• C. (a) and (d)
• D. (c) and (d)

Answer 1

The correct option is C (a) and (d)

It takes six turns of the Calvin cycle to fix six carbon atoms from $C{O}_2$. These six turns require energy input from 12 ATP molecules and 12 NADPH molecules in the reduction step and 6 ATP molecules in the regeneration step. But for each RUBP only 1 ATP required.

First stable product of Calvin cycle is 3PGA which undergoes phosphorylation in reduction step.

The carboxylating enzyme in C${}_3$ cycle is RuBisCO. It is a bifunctional enzyme that is used in the Calvin cycle to catalyze the major step of carbon fixation, a process by which the atoms of atmospheric carbon dioxide $\left(C{O}_2\right)$ are made available to organisms in the form of energy-rich molecules. Besides its carboxylase activity, RuBisCO also acts as an oxygenase in a reaction involving competition between $O_2$ and $C{O}_2$.