Question

The end product of the Calvin cycle is
a. Ribulose bisphosphate
b. Phosphoglyceraldehyde (PGAL)
c. PEP carboxylase (PEP case)
d. Carbon dioxide

Answer 1

Hint: Plant’s produces and stores sucrose as starch. Calvin cycle works in three major steps that are carboxylation, reduction and regeneration to produce sucrose. During each step various products are made. The Calvin cycle consists of various chain reactions to produce sucrose.

Complete answer:
• Calvin cycle is carried out in all photosynthetic plants. Let’s go through all three stages of Calvin cycle.
• The first stage is carboxylation of RuBP (Rubisco bisphosphate). In which the atmospheric $CO_2$ gets fixed with RuBP to produce stable organic intermediate. This reaction is carried out by RuBP carboxylase enzyme and produces two molecules of 3-phosphoglycerate.
• The second stage is reduction. In this process, 2 molecules of ATP used for phosphorylation and two NADPH for reduction per $CO_2$. 3-phosphoglycerate gets converted into glyceraldehyde-3-phosphate or phosphoglyceraldehyde.
• Third stage is regeneration. After the six rounds of Calvin cycle, 2 molecules of PGAL goes for production of one molecule of glucose while another 15 PGAL goes for regeneration of 5-C RuBP that can be used to accept new carbon molecules. This process requires ATP.

Hence, the correct answer is option (B).

Note: Here the regenerated RuBP cannot be the end product because it undergoes regeneration. So the last product should be PGAL. The last product will sucrose instead of glucose because glucose cannot be transported directly to storage by plants. Plant stem can transport sucrose as it is non-reducing sugar and it does not react with oxygen while being transported in the stem for storage. The stored sucrose can be used during dormancy.