How many NADPH and ATP are required to fix three CO2 in the Calvin cycle?

How many NADPH and ATP are required to fix three CO2 in the Calvin cycle?

• A. 12 NADPH and 18 ATP
• B. 3 NADPH and 6 ATP
• C. 6 NADPH and 12 ATP
• D. 6 NADPH and 9 ATP

Answer: (D)

The Calvin cycle uses NADPH as the reducing power and ATP as the energy source to convert fixed CO2 into carbohydrate and to rebuild the molecule that accepts CO2, RuBP, so the cycle can continue. When three CO2 enter, they are fixed to form six molecules of 3-phosphoglycerate. Reducing those six molecules to glyceraldehyde-3-phosphate requires one ATP and one NADPH for each 3-PGA, totaling 6 ATP and 6 NADPH. But not all the resulting G3P leaves the cycle; most must be rearranged to regenerate RuBP, and this regeneration step consumes additional ATP. When you add that regeneration energy to the reduction energy, the total comes to 9 ATP and 6 NADPH for fixing three CO2. So the energy and reducing power needed for fixing three CO2 is 9 ATP and 6 NADPH.

The Calvin cycle uses NADPH as the reducing power and ATP as the energy source to convert fixed CO2 into carbohydrate and to rebuild the molecule that accepts CO2, RuBP, so the cycle can continue. When three CO2 enter, they are fixed to form six molecules of 3-phosphoglycerate. Reducing those six molecules to glyceraldehyde-3-phosphate requires one ATP and one NADPH for each 3-PGA, totaling 6 ATP and 6 NADPH. But not all the resulting G3P leaves the cycle; most must be rearranged to regenerate RuBP, and this regeneration step consumes additional ATP. When you add that regeneration energy to the reduction energy, the total comes to 9 ATP and 6 NADPH for fixing three CO2. So the energy and reducing power needed for fixing three CO2 is 9 ATP and 6 NADPH.