13.1.12—Calvin cycle intermediates

Calvin-cycle intermediates change form and carbon number as carbon dioxide is fixed, reduced and either exported or recycled. The key boundary is that a five-carbon RuBP acceptor leads to three-carbon GP and TP intermediates, while TP can contribute to larger organic molecules or regenerate RuBP.

• RuBP — 5C acceptor: in the stroma, rubisco combines carbon dioxide with the five-carbon acceptor ribulose bisphosphate (RuBP). The unstable six-carbon product splits into two three-carbon molecules.
• GP — 3C product of fixation: glycerate 3-phosphate (GP) is the immediate three-carbon product after carbon dioxide has been incorporated. GP is not yet a carbohydrate.
• TP — 3C reduced product: ATP and reduced NADP from the light-dependent stage drive reduction of GP to triose phosphate (TP), a phosphorylated three-carbon sugar.
• TP destinations: some TP is used to form larger organic products, including hexose phosphates that can contribute to starch, sucrose or cellulose; other TP is retained for cycle regeneration.
• Regeneration: retained TP is rearranged using ATP to reform the five-carbon RuBP acceptor. This restores the starting acceptor so further carbon dioxide can be fixed.
• Energy-transfer boundary: ATP and reduced NADP supply energy and reducing power to the intermediate conversions; this card tracks intermediate identity and carbon-number changes, not limiting-factor experiments or exact cycle counts.

Do not call GP a carbohydrate, treat TP as the final product of every cycle turn, or omit that most TP is recycled to regenerate RuBP. Carbon-number labels describe intermediates, not separate photosynthesis stages.