A leaf fixes 12 molecules of CO2 in the Calvin cycle. How many ATP and NADPH molecules are used?
- Each CO2 fixed needs 3 ATP and 2 NADPH.
- ATP = 12 × 3 = 36.
- NADPH = 12 × 2 = 24.
Answer: 36 ATP and 24 NADPH (twice the amount needed for one glucose).
Photosynthesis: light-independent reactions and photorespiration
Biology · Bioenergetics. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
The idea
The Calvin cycle takes place in the stroma. It does not use light directly, but it needs the ATP and NADPH made by the light reactions. It has three phases: carbon fixation, reduction and regeneration of RuBP.
The enzyme RuBisCO joins CO2 to ribulose bisphosphate (RuBP, 5 carbons). The 6-carbon product splits at once into two molecules of phosphoglyceric acid (PGA, 3 carbons). PGA is reduced to glyceraldehyde 3-phosphate (G3P). Most G3P is used to remake RuBP; a small part leaves to form glucose.
RuBisCO can also bind oxygen. This starts photorespiration, which releases CO2 already fixed and makes no ATP or sugar. It is fastest on hot, dry days when stomata close.
Rules to remember
- Fixation: 3CO2 + 3RuBP → 6PGA (first stable product, 3 carbons, so these are C3 plants).
- Reduction: 6PGA → 6G3P, using 6 ATP and 6 NADPH.
- Regeneration: 5 of the 6 G3P remake 3RuBP, using 3 ATP; 1 G3P is the net product.
- Per CO2 fixed: 3 ATP and 2 NADPH.
- One glucose needs 6CO2: 18 ATP and 12 NADPH.
- Photorespiration: RuBP + O2 gives one PGA and one 2-carbon glycolate; CO2 is later released. Organelles involved: chloroplast, peroxisome, mitochondrion.
Common mistake
Students call the Calvin cycle the dark reaction and conclude it runs at night. It stops soon after dark because its supply of ATP and NADPH from the light reactions stops.
Worked example
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