Biology · Introductory biology · Worked example
Count what the Calvin cycle spends on a sugar
How many CO₂, ATP and NADPH does the Calvin cycle use to make one G3P that leaves the cycle? How many does it use to make one glucose?
Fix three CO₂
Rubisco joins 3 CO₂ to 3 RuBP. Each six-carbon product splits, giving 6 molecules of 3-PGA.
Reduce to G3P
Each 3-PGA takes 1 ATP and 1 NADPH to become G3P, so 6 ATP and 6 NADPH make 6 G3P.
Regenerate RuBP
Five G3P, with 15 carbons, are rearranged into 3 RuBP, also with 15 carbons, spending 3 more ATP. One G3P is left over to leave the cycle.
Total for one G3P
3 CO₂, 6 + 3 = 9 ATP and 6 NADPH.
Scale up to glucose
Glucose has six carbons, two G3P, so everything doubles: six turns of the cycle use 6 CO₂, 18 ATP and 12 NADPH.
Result
One exported G3P costs 3 CO₂, 9 ATP and 6 NADPH; one glucose costs 6 CO₂, 18 ATP and 12 NADPH.
Your turn
Sucrose has 12 carbons. How many CO₂, ATP and NADPH does the Calvin cycle use to make the G3P for one sucrose?
Show the answer and explanation
12 CO₂, 36 ATP and 24 NADPH.
Twelve carbons are 4 G3P, and each G3P costs 3 CO₂, 9 ATP and 6 NADPH, so multiply each by 4.
Keep exploring
The Calvin cycle opens in Biology with the ATP spent on regeneration and the G3P that leaves; step through it to follow CO₂, ATP and NADPH into sugar.
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