Question

In photosynthesis, for the fixation of one mole of glucose, the number of ATP and NADPH${}_2$ required is

• A. 12 and 18.
• B. 18 and 12.
• C. 6 and 12.
• D. 18 and 18.

Answer 1

Answer: (B)

18 and 12.

Formation of one molecule of glucose (6C) requires 6 turns of Calvin cycle as 3 turns of carbon fixation obtains net one molecule of glyceraldehyde 3 phosphates (G3P; 3C) and two molecules of G3P are required to make one molecule of 6 carbon glucose. One turn of Calvin cycle fixes one molecule of carbon dioxide and requires 2 molecules of $NADP{H}_2$ for reduction of 1,3 bisphosphoglycerate into glyceraldehyde 3 phosphate. So, 6 turns of Calvin cycle require 6 x 2= 12$NADP{H}_2$. One turn of Calvin cycle requires total 3ATPs; 2 ATP molecules for phosphorylation of two molecules of 3PGA into two molecules of 1,3-bisphosphoglycerate and 1 ATP during the regeneration phase. One turn of Calvin cycle requires total 3 ATP molecules, 6 turns of Calvin cycle will consume 6x3= 18 ATPs. Thus, the correct answer is B.