Question

If $4$ molecules of glucose are synthesized during photosynthesis, how many ATP molecules are released and how many molecules of ATP are utilized respectively?

• A. 2 ATP, 18 ATP
• B. 8 ATP, 72 ATP
• C. 8 ATP, 36 ATP
• D. 4 ATP, 36 ATP

Answer 1

Answer: (B)

For the synthesis of one molecule of glucose by photosynthesis, 2 ATP molecules are released and 18 ATP molecules are utilized. 18 ATP molecules are utilized in the sense that in most of the plants the mode of CO${}_2$ fixation is C${}_3$ cycle or Calvin cycle. In C${}_3$ cycle for each molecule of CO${}_2$ to be fixed, 3 ATP are required. Thus, for the synthesis of one Glucose molecule, 6 CO${}_2$ molecules are fixed. Thus, 18 ATP molecules in total are utilized for the formation of one Glucose molecule.

Hence, for the synthesis of 4 glucose molecules in photosynthesis,

ATP molecules released = 4 × 2 = 8

ATP molecules utilized = 4 × 18 = 72.

Thus the correct answer is A.