Lesson 4 of 5 / Photosynthesis and energy capture
Fix, reduce and regenerate carbon
Why do most triose phosphates stay in the Calvin cycle?
I can explain the three phases of the C3 Calvin cycle and ATP/NADPH use.
I can explain carbon fixation, PGA reduction and RuBP regeneration in the C3 Calvin cycle.Syllabus
Syllabus 9477, Core 3(d). Explain carbon fixation, PGA reduction and RuBP regeneration in the C3 Calvin cycle.
About 8 min
Make a guess. You are not marked.
The Calvin cycle fixes 3 CO2 and makes 6 triose phosphates. How many can leave to make sugar?
Show the answer
Only 1; the other 5 regenerate RuBP.
Fifteen carbons are needed to rebuild three RuBP. Only one triose phosphate is net output.
The key idea
Rubisco fixes CO2; ATP and reduced NADP support reduction; most carbon regenerates RuBP so fixation can continue.
Explore the idea
Keep the carbon acceptor available
3 CO2 bring 3 new carbons into the cycle. The 5 recycled triose phosphates contain 15 carbons, enough to regenerate 3 five-carbon RuBP. Only 1 triose phosphate is the net output.
TP = triose phosphate; PGA = glycerate 3-phosphate; NADPH = reduced NADP. Reduction uses 6 ATP and 6 NADPH; regeneration uses another 3 ATP. Two net TP can contribute to a six-carbon sugar; glucose is not released directly at every fixation event.
Explanation
In carbon fixation, rubisco catalyses addition of CO2 to five-carbon ribulose bisphosphate, RuBP. The unstable six-carbon product splits into two three-carbon molecules of glycerate 3-phosphate, also called 3-phosphoglycerate or PGA. This is why the pathway is described as C3.
In the reduction phase, ATP supplies energy and reduced NADP supplies reducing power to convert PGA into triose phosphate. ADP, phosphate and oxidised NADP return to support the light-dependent reactions. The cycle does not obtain its energy simply by absorbing a photon directly at rubisco.
For three CO2 fixed, three RuBP produce six PGA and then six triose phosphates. One triose phosphate is a net carbon output; the other five, containing fifteen carbons, regenerate three five-carbon RuBP using additional ATP. This regeneration keeps the carbon acceptor available.
The net production of one three-carbon triose phosphate requires nine ATP and six reduced NADP in the standard accounting. Two net triose phosphates can contribute to a six-carbon sugar. Light-independent means no direct photon step, not independence from the light reactions: ATP and reduced NADP are continually required.
Step by step
- 1
Balance the carbons
Three CO2 plus fifteen RuBP carbons give eighteen in intermediates.
- 2
Separate output from recycling
Only three carbons leave per three CO2 in the net model.
- 3
Account for energy
Nine ATP and six NADPH support one net triose phosphate.
Worked example
Work through the evidence
How many CO2, ATP and reduced NADP are needed for two net triose phosphates in this model?
One way to explain it
Six CO2, eighteen ATP and twelve reduced NADP.
Why this answer works
- Double the three-CO2 accounting.
- Most intermediate carbon is recycled into RuBP, not exported each turn.
Watch out for this
A student says: "All six triose phosphates leave after three CO2 are fixed." What is wrong with this?
Show the answer
Five are needed to regenerate three RuBP; only one is net output in that accounting.
Check your understanding
What is the direct role of rubisco?
Show the answer
Catalysing CO2 fixation to RuBP
This begins the Calvin-cycle carbon-fixation phase.
Exam-style questions
Check your understanding
A plant in light is suddenly put in the dark. What happens to the amounts of RuBP and GP in its chloroplasts over the next minute?
Show the answer
RuBP falls and GP rises.
Without ATP and reduced NADP, GP cannot be reduced and RuBP cannot be regenerated, but RuBP is still fixed into GP for a while.
Quick recall
Why do most triose phosphates stay in the Calvin cycle?