The Calvin Cycle: Carboxylation, Reduction and Regeneration
The Calvin cycle is where the light reactions finally pay for something. The ATP and NADPH made in the thylakoid membranes are spent in the stroma to turn carbon dioxide into carbohydrate — and, remarkably, to rebuild the very acceptor that started the process.
This quiz tests the three stages of the cycle as worked out by Calvin and his co-workers:
- Carboxylation — fixation of CO2 onto RuBP by RuBisCO, giving two molecules of 3-PGA.
- Reduction — ATP and NADPH convert 3-PGA towards glucose.
- Regeneration — one ATP rebuilds RuBP so the cycle can continue.
You will also be asked about the cyclic nature of the pathway, its universal occurrence in C3 and C4 plants, and the energy arithmetic of the whole cycle. Work carefully through the stoichiometry — the numbers are the heart of this lesson.
Q1. Calvin and his co-workers worked out the whole pathway and demonstrated one key feature of its operation. What was that feature?
Calvin and his co-workers worked out the whole pathway and showed that it operated in a cyclic manner, with ##\ce{RuBP}## regenerated rather than consumed.
Q2. The Calvin pathway occurs in which of the following groups of plants?
The Calvin pathway occurs in all photosynthetic plants; it does not matter whether they have C3 or C4 (or any other) pathways.
Q3. For ease of understanding, the Calvin cycle is described under three stages. Which sequence is correct?
The Calvin cycle is described under three stages in the order carboxylation, reduction and regeneration.
Q4. Carboxylation is best defined as:
Carboxylation is the fixation of ##\ce{CO2}## into a stable organic intermediate, and it is the most crucial step of the Calvin cycle.
Q5. Which step of the Calvin cycle is described as the most crucial step, where ##\ce{CO2}## is actually utilised?
Carboxylation is the most crucial step of the Calvin cycle, where ##\ce{CO2}## is utilised for the carboxylation of ##\ce{RuBP}##.
Q6. Which enzyme catalyses the carboxylation of ##\ce{RuBP}## in the Calvin cycle?
The reaction is catalysed by the enzyme ##\ce{RuBP}## carboxylase, which results in the formation of two molecules of 3-PGA.
Q7. Carboxylation of ##\ce{RuBP}## by RuBisCO results in the formation of:
The carboxylation reaction catalysed by ##\ce{RuBP}## carboxylase results in the formation of two molecules of 3-PGA.
Q8. Why is the enzyme of carboxylation more correctly called ##\ce{RuBP}## carboxylase-oxygenase (RuBisCO)?
Since this enzyme also has an oxygenation activity, it would be more correct to call it ##\ce{RuBP}## carboxylase-oxygenase or RuBisCO.
Q9. The reduction stage of the Calvin cycle is best described as:
Reduction is a series of reactions that lead to the formation of glucose, using ATP and NADPH.
Q10. Per ##\ce{CO2}## molecule fixed, how many molecules of ATP and NADPH are used in the reduction stage?
The reduction steps involve utilisation of 2 molecules of ATP for phosphorylation and two of NADPH for reduction per ##\ce{CO2}## molecule fixed.
Q11. How many molecules of ##\ce{CO2}## and how many turns of the cycle are required for the formation of one molecule of glucose from the pathway?
The fixation of six molecules of ##\ce{CO2}## and 6 turns of the cycle are required for the formation of one molecule of glucose from the pathway.
Q12. Why is the regeneration of ##\ce{RuBP}## crucial for the Calvin cycle?
Regeneration of the ##\ce{CO2}## acceptor molecule ##\ce{RuBP}## is crucial if the cycle is to continue uninterrupted.
Q13. How many ATP molecules are required in the regeneration steps for phosphorylation to form ##\ce{RuBP}##?
The regeneration steps require one ATP for phosphorylation to form ##\ce{RuBP}##.
Q14. For every ##\ce{CO2}## molecule entering the Calvin cycle, how many molecules of ATP and NADPH are required in total?
For every ##\ce{CO2}## molecule entering the Calvin cycle, 3 molecules of ATP and 2 of NADPH are required.
Q15. To make one molecule of glucose through the Calvin pathway, what are the total inputs and outputs?
The balance sheet of the Calvin cycle is: in — six ##\ce{CO2}##, 18 ATP, 12 NADPH; out — one glucose, 18 ADP and 12 NADP.