Question

Which is the first stable product formed during the Calvin cycle?

• A. Glyceraldehyde-3-phosphate
• B. 3-phosphoglycerate
• C. Triose phosphate
• D. RuBP

Answer 1

The correct option is B 3-phosphoglycerate

All the light independent reactions of photosynthesis during which sugars are synthesised was discovered by Calvin. Hence, it is also called the Calvin cycle. It includes three stages - carboxylation, reduction and regeneration.

During carboxylation or carbon dioxide fixation, CO${}_2$ that enters the chloroplast through stomata combines with RuBP. Hence, RuBP (ribulose-1,5-bisphosphate) is the primary acceptor of carbon dioxide. RuBP combines with CO${}_2$ and forms an unstable 6 carbon compound. This reaction is catalysed by the enzyme RuBisCO. The 6 carbon compound splits into two molecules of 3-PGA (3-phosphoglycerate or 3-phosphoglyceric acid). 3-PGA is a 3 carbon compound and the first stable product formed during the Calvin cycle.

3-PGA undergoes reduction to form glyceraldehyde-3-phosphate which is also known as triose phosphate. This is called the reduction stage of the calvin cycle. It requires ATP and NADPH.

The third stage is called regeneration where RuBP is regenerated from triose phosphate with the utilization of ATP.