The Calvin cycle's ledger.

Turn the Calvin cycle one CO2 at a time: RuBP + CO2 → 2 PGA by RuBisCO, reduction spends 2 ATP + 2 NADPH, regeneration 1 ATP; a ledger counts carbons and energy until a glucose leaves.

Release exactly one glucose and read off the turns, ATP and NADPH it cost; then starve NADPH and see which stage stalls.

First CO2 acceptor is a 2-carbon molecule; glucose costs 6 ATP.

• Using radioactive ¹⁴C to trace carbon in algae, Melvin Calvin found that the first product of CO₂ fixation is a 3-carbon organic acid, 3-phosphoglyceric acid (PGA).
• Later, a different group of plants was found whose first stable product is a 4-carbon acid, oxaloacetic acid (OAA). Plants of the first kind are called C₃ plants and the second kind C₄ plants.
• In the Calvin cycle, CO₂ is first accepted by ribulose bisphosphate (RuBP), a ketose sugar with 5 carbons, and not by a 2-carbon molecule as was once expected.
• The Calvin cycle occurs in all photosynthetic plants, whether C₃ or C₄. It runs in three stages: carboxylation, reduction and regeneration.
• Carboxylation: CO₂ is fixed onto RuBP by RuBisCO (RuBP carboxylase-oxygenase), producing two molecules of 3-PGA. This is the most crucial step, and RuBisCO is described as the most abundant enzyme in the world.
• Reduction: 3-PGA is converted to carbohydrate using 2 ATP (for phosphorylation) and 2 NADPH (for reduction) per CO₂ fixed.
• Regeneration: RuBP is remade so the cycle can continue; this step needs one more ATP per CO₂ for phosphorylation.
• So each CO₂ fixed costs 3 ATP and 2 NADPH. Making one glucose needs six turns of the cycle: 6 CO₂ in, 18 ATP and 12 NADPH used, 1 glucose out.

Teaching this idea? Put the simulation on your own page or LMS with this code.