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BiologyPhotosynthesis / Calvin Cycle
How many turns of the Calvin cycle are required for the synthesis of one molecule of glucose? For the synthesis of three glucose molecules?
Options
1
6 turns for 1 glucose; 12 turns for 3 glucose
2
3 turns for 1 glucose; 9 turns for 3 glucose
3
6 turns for 1 glucose; 18 turns for 3 glucose
4
9 turns for 1 glucose; 27 turns for 3 glucose
Correct Answer
6 turns for 1 glucose; 18 turns for 3 glucose
Solution
1

1 turn Calvin cycle = 1 CO2 fixed

Glucose has 6 carbons → needs 6 CO2 → 6 turns per glucose

2

For 3 glucose molecules: 3 × 6 = 18 turns

Answer: 6 turns for 1 glucose; 18 turns for 3 glucose

1 turn = 1 CO2 fixed | Glucose = 6C = 6 turns
3 glucose = 18 turns | Per glucose: 18 ATP + 12 NADPH needed
Theory: Photosynthesis / Calvin Cycle
1. The Calvin Cycle — Overview

The Calvin cycle (also called the dark reactions, light-independent reactions, or C3 cycle) is the metabolic pathway in the chloroplast stroma that uses the ATP and NADPH produced by the light reactions to convert CO2 into organic carbon compounds (primarily G3P, which is then used to synthesise sugars, amino acids, fatty acids, and other organic molecules). The cycle was elucidated by Melvin Calvin, Andrew Benson, and James Bassham at UC Berkeley in the 1950s using radioactive 14CO2 and chromatographic analysis — work for which Calvin received the Nobel Prize in Chemistry in 1961. The cycle has three stages: carbon fixation (CO2 attachment to RuBP), reduction (ATP and NADPH-driven reduction of 3-PGA to G3P), and RuBP regeneration (complex series of reactions using ATP to regenerate the 5-carbon RuBP acceptor molecule from 5-carbon and 3-carbon intermediates).

2. Carbon Fixation — Stage 1

The first stage of the Calvin cycle is CO2 fixation by RuBisCO: Ribulose-1,5-bisphosphate (RuBP, 5-carbon) + CO2 → unstable 6-carbon intermediate → 2 molecules of 3-phosphoglycerate (3-PGA, 3-carbon each). This reaction is catalysed by RuBisCO, the most abundant enzyme on Earth, working very slowly (approximately 3 catalytic events per second). In one turn, one CO2 molecule is incorporated into one 3-PGA molecule (the other 3-PGA comes from the remaining carbons of RuBP). The carbon backbone of RuBP provides 5 carbons + 1 CO2 = 6 carbons total → split into 2×3-PGA. Per 6 turns: 6 CO2 fixed, 12 3-PGA produced.

3. Reduction — Stage 2

Each 3-PGA molecule is reduced to G3P (glyceraldehyde-3-phosphate) in two steps. Step 1: 3-PGA + ATP → 1,3-bisphosphoglycerate (1,3-BPG) + ADP (catalysed by phosphoglycerate kinase). Step 2: 1,3-BPG + NADPH → G3P + NADP+ + Pi (catalysed by NADP-glyceraldehyde phosphate dehydrogenase). Per 12 molecules of 3-PGA (from 6 turns): 12 ATP and 12 NADPH are consumed to produce 12 G3P. G3P is a 3-carbon triose phosphate that is at the crossroads of carbohydrate, lipid, and amino acid biosynthesis — it is the primary export product of the chloroplast.

4. RuBP Regeneration — Stage 3 and Carbon Accounting

The most complex stage — 10 of the 12 G3P molecules are used in a complex series of reactions (involving C3, C4, C5, C6, and C7 sugar phosphates as intermediates) to regenerate 6 molecules of RuBP, using 6 ATP. The remaining 2 G3P molecules represent the net gain from 6 turns of the Calvin cycle — equivalent to one-half molecule of glucose (since glucose = C6 = 2×G3P). Carbon accounting per 6 turns: Input: 6 CO2 (6C) + 6 RuBP (6×5C = 30C) = 36C total. Output: 12 G3P (12×3C = 36C total). Distribution: 10 G3P (30C) → regenerate 6 RuBP (30C); 2 G3P (6C) → net output for biosynthesis. Energy consumed per 6 turns (per glucose equivalent): 18 ATP (12 for reduction + 6 for RuBP regeneration) + 12 NADPH (for reduction).

Frequently Asked Questions
1. Why do we say "6 turns of the Calvin cycle to make 1 glucose" when glucose has 6 carbons and each turn only fixes 1 CO2 — doesn't that mean only 6 of the 18 ATP and 12 NADPH are "for" glucose? ⌄
This is an astute question about carbon accounting in the Calvin cycle that gets at a common conceptual confusion. The statement "6 turns to make 1 glucose" is correct but refers specifically to the number of CO2 fixation events needed, not to a simple 6-step linear synthesis. The key is understanding that in 6 turns: 6 CO2 (6 carbons total) enter the cycle, producing 12 G3P (36 carbons total from 6C CO2 + 30C in 6 RuBP). But 10 of those 12 G3P (30 carbons) are consumed to regenerate the 6 RuBP molecules for the next round of the cycle — these carbons are essentially "recycled" within the cycle and not available for net synthesis. Only 2 G3P represent true net carbon gain from the 6 CO2 inputs (6 carbons net). These 2 G3P (6 carbons total) can then be combined to form 1 fructose-6-phosphate → 1 glucose molecule. So the energy consumed (18 ATP and 12 NADPH per 6 turns) is used for: (1) reducing all 12 G3P from 12 3-PGA (which explains the 12 ATP and 12 NADPH), and (2) regenerating 6 RuBP from 10 G3P (explaining the remaining 6 ATP). All this energy goes into making the 2 net G3P (1 glucose worth of carbon), because the 10 G3P used for RuBP regeneration are converting the energy of ATP into the chemical potential of the restored RuBP, ready for the next fixation round — so the energy is efficiently "invested" in the cycle's continuity rather than wasted.
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