The Calvin Cycle and Carbon Fixation
Rubisco joins carbon dioxide to RuBP, and the ATP and reduced NADP from the light reactions turn the product into triose phosphate. Most of that triose phosphate is spent regenerating RuBP.
What a learner can do afterwards
- Traces one carbon atom from carbon dioxide into a triose phosphate.
- Explains what the ATP and the reduced NADP are used for inside the cycle.
- Says why the cycle stalls quickly once the light-dependent stage stops.
1 · Read
The Calvin cycle fixes carbon in the stroma, starting the sugar that feeds the plant and the animals that eat plants. Its enzyme Rubisco grabs carbon dioxide and bolts it onto RuBP, a five-carbon acceptor. The product splits into two molecules of glycerate 3-phosphate, called GP.
ATP and reduced NADP from the light reactions then reduce GP to triose phosphate. Each turn fixes one carbon, and a triose phosphate holds three, so three turns are needed for one net triose. Those three turns make six triose phosphates: five are recycled to rebuild three RuBP, spending more ATP but no reduced NADP, and one leaves the cycle. Kill the light and the shipped stocks drain within moments, so the cycle stalls.
Follow one carbon atom. It enters as carbon dioxide gas, joins RuBP, becomes part of GP, and is reduced into triose phosphate. From there it either cycles back into RuBP or leaves inside exported sugar. No photon is trapped here; the cycle only spends carriers shipped from the thylakoids.
Light independent does not mean light irrelevant. Kill the light and the shipped stocks of ATP and reduced NADP drain within moments, so reduction halts and RuBP cannot be rebuilt.
Fix carbon onto RuBP, reduce it with ATP and reduced NADP, and rebuild RuBP with most of the product.
Rewritten readings were written for this dataset. Readings summarise a Khan Academy article on the Calvin cycle. Videos are linked from Khan Academy. License terms for these sources are not recorded.
2 · Watch
Take it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.