Only reduction spends NADPH in the Calvin cycle
- The idea
- Predict
- See it
- Why
- Check
The idea
“Fixation is the big step, so it must be the one that burns the ATP and NADPH.”
Many people call the whole cycle carbon fixation, so that step sounds expensive. ATP and NADPH arrive together, so it's easy to picture every phase using both.
The short version
The mistake: Fixation is the big step, so it must be the one that burns the ATP and NADPH.
What's actually true: Fixation spends no ATP or NADPH. Reduction uses both to make G3P, and regeneration uses only ATP to rebuild RuBP.
- In fixation, the enzyme rubisco attaches CO2 to RuBP, a five-carbon sugar. The result splits into two 3-PGA. Nothing from the light reactions is spent.
- Reduction means gaining electrons. ATP and NADPH power the change from 3-PGA to G3P, and NADPH supplies the electrons. This is where sugar gets made.
- Most G3P stays in the cycle. Regeneration spends ATP to rebuild RuBP, so fixation can run again.
On the exam: NADPH goes where electrons are added, at reduction. Fixation spends nothing. Regeneration spends only ATP.