In photosynthesis, 18 molecules of ATP are used for the synthesis of one molecule of glucose. This requirement stems from the Calvin cycle, which uses ATP and NADPH produced during the light-dependent reactions to fix carbon dioxide into glucose.
The Calvin Cycle and ATP Consumption
The Calvin cycle, also known as the light-independent reactions, can be broken down into three main stages:
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Carbon Fixation: Carbon dioxide combines with ribulose-1,5-bisphosphate (RuBP), a five-carbon molecule. This initial reaction doesn't directly use ATP.
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Reduction: The resulting six-carbon molecule is unstable and immediately splits into two molecules of 3-phosphoglycerate (3-PGA). 3-PGA is then phosphorylated by ATP to form 1,3-bisphosphoglycerate. Each 3-PGA molecule consumes one ATP molecule, so this step requires 2 ATP molecules per carbon dioxide molecule fixed. The 1,3-bisphosphoglycerate is then reduced by NADPH to glyceraldehyde-3-phosphate (G3P).
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Regeneration: To keep the Calvin cycle running, RuBP must be regenerated. This process is complex and requires ATP. For every six molecules of G3P produced, only one molecule exits the cycle to be used for glucose synthesis. The remaining five molecules are used to regenerate three molecules of RuBP. This regeneration step consumes 5 ATP molecules per three RuBP molecules regenerated, which translates to consuming 3 ATP molecules for each carbon dioxide molecule used to produce 1/6th of a glucose molecule.
ATP Usage Summary
To synthesize one molecule of glucose, the Calvin cycle must run six times, fixing six carbon dioxide molecules. Thus, the ATP usage is as follows:
- Reduction Phase: 2 ATP are required per CO2 fixed. Since 6 CO2 molecules are required to make one glucose molecule, 2 ATP/CO2 * 6 CO2 = 12 ATP
- Regeneration Phase: 3 ATP are required per CO2 fixed. Since 6 CO2 molecules are required to make one glucose molecule, 3 ATP/CO2 * 6 CO2 = 6 ATP
Total ATP Required: 12 ATP + 6 ATP = 18 ATP.
Therefore, a total of 18 ATP molecules are used during photosynthesis for the synthesis of one glucose molecule.
