Where does the carbon go?
Track carbon through the Calvin cycle in the chloroplast stroma.
STROMA · three turns shown together
One turn fixes one CO₂. For clear carbon accounting, each animated circuit groups three turns of the Calvin cycle: 3 CO₂ enter together, 1 net G3P (3C) leaves, and 3 RuBP are regenerated. Two animated circuits represent six turns and supply two net G3P for one 6C sugar equivalent outside the cycle.
Net carbohydrate products
0 six-carbon sugar equivalents
Two net G3P (3C + 3C) contribute to one six-carbon carbohydrate. This downstream assembly is shown separately from the Calvin cycle; it is not a single direct cycle reaction.
G3P waiting for a partner
Predict → observe → explain
Can incoming CO₂ carbon appear in regenerated RuBP? What happens if ATP runs out before regeneration? How many CO₂ molecules are needed for one net six-carbon sugar equivalent?
Teacher notes & model limits
Fixation: 3 RuBP (15C) + 3 CO₂ (3C) form 6 three-carbon compounds (18C). Reduction uses 6 ATP and 6 NADPH. One net G3P (3C) leaves; regeneration uses 3 ATP to rearrange the remaining 15 carbons into 3 RuBP. Six CO₂, 18 ATP and 12 NADPH support two net G3P, equivalent to six net carbohydrate carbons.
CED scope (topic 3.4.B.6): ATP and NADPH power carbohydrate production from CO₂ in the stroma. Memorising Calvin-cycle steps, molecule structures and enzyme names is excluded from the AP Exam. The turn counts and stoichiometry here are explanatory enrichment, not a required memorisation list.
The simulation conserves individual carbon identities. Each animated circuit groups three turns, with three separate CO₂-fixation events shown together. It is not a literal single turn using three CO₂. The grouped sequence uses 6 ATP and 6 NADPH for reduction, exports one of six G3P, and uses 3 further ATP to regenerate 3 RuBP from the five retained G3P. Regeneration uses a simplified deterministic carbon rearrangement, not the actual enzymatic atom mapping or isotope-labeling kinetics. Each dot represents carbon only; oxygen, hydrogen, phosphate and unstable intermediates are omitted. Orange marks carbon origin, not a different chemical element. Initial RuBP carbon can leave in product as incoming carbon replaces it in the cycle.
ATP is consumed, not produced, here. ADP and NADP⁺ accumulate; no light reactions replenish ATP/NADPH in this closed experiment. Phosphate, proton and water accounting is omitted. Reaction timing is illustrative. This extension supports AP Biology topic 3.4; detailed cycle intermediates are beyond required memorization.