Question

How many molecules of 3-PGA are formed during the synthesis of one molecule of glucose?

• A. 6
• B. 18
• C. 12
• D. 3

Answer 1

The correct option is C 12

All the light independent reactions of photosynthesis occur in the stroma of chloroplast. These reactions lead to the synthesis of glucose. The detailed pathway of synthesis of glucose was studied by Calvin and his co-workers. This biosynthetic pathway is called Calvin cycle.

Carboxylation or the carbon dioxide fixation is the first step of the Calvin cycle. RuBP (ribulose-1,5-bisphosphate) acts as the primary acceptor of carbon dioxide. RuBP combines with CO${}_2$ and forms an unstable compound that splits into two molecules of 3-PGA. So, two molecules of 3-PGA (3-phosphoglycerate) are synthesized for every CO${}_2$ fixed in the Calvin cycle. For the synthesis of one molecule of glucose, 6 carbon dioxide molecules are required and hence six Calvin cycles occur. So, 12 molecules of 3-PGA are formed during the carboxylation stage for synthesising one molecule of glucose.