THE MOLECULAR LAB · PHOTOSYNTHESIS

Carbon to life.

The Calvin cycle · light-independent reactions
VIEW
CHLOROPLAST STROMAA cycle of enzyme-controlled reactions
Select a molecule or reaction to explore it.
Zoom
0%

Read the diagram first

The main loop shows carbon fixation by Rubisco, reduction of GP to TP, and regeneration of RuBP. Rubisco is a catalyst: it stays at the fixation step and is not consumed. ATP and reduced NADP come from the light-dependent reactions.

Simple keeps just the pathway and its inputs. Carbon counting adds the quantities for three CO₂: six GP form, six TP form, five TP regenerate three RuBP, and one TP is the net gain. Molecular opens the 3D viewer without changing the cycle layout.

What is simplified?

Arrows represent a sequence of chemical reactions, not a physical track. Moving markers indicate reaction order, not individual atoms. Carbon dots count atoms within molecules; their arrangement is not a bond diagram or an isotope-tracing result. Successive stages do not represent simultaneous molecule stocks.

Water, H⁺ and inorganic phosphate are omitted from the overview. It is a pathway map and carbon/energy budget, not a fully balanced chemical equation. Net TP withdrawal need not mean transport out of the chloroplast. TP supplies carbon for other molecules through further reactions.

The molecular shapes are calculated conformers, not experimentally measured structures or reaction dynamics. Neutral acid forms are used for consistent drawings; many groups are ionised in the stroma. Glyceraldehyde 3-phosphate is the TP shown; GP is glycerate 3-phosphate, a different compound.

The experiment is a qualitative, carbon-conserving model. Pool levels are normalised to their own initial values. Rates, temperature responses and time units are illustrative, not measured plant data. Stomata, photorespiration and light-dependent enzyme activation are not modelled.

Primary references & data

  1. Photosynthesis curriculum reference · section 5.2.1, photosynthesis.
  2. IUBMB: Calvin cycle — fixation and reduction.
  3. IUBMB: regeneration of ribulose bisphosphate.
  4. Rubisco, EC 4.1.1.39; phosphoglycerate kinase, EC 2.7.2.3; NADP-dependent GAPDH, EC 1.2.1.13.
  5. ChEBI chemical connectivity: RuBP, GP, D-glyceraldehyde 3-phosphate, Ru5P. Extension intermediates follow IUBMB reaction chemistry.
  6. Companion cofactor models calculated from wwPDB CCD stereochemical structures: ATP, NADP⁺ (NAP), NADPH (NDP).
  7. RDKit · conformer generation, valence checks, stereochemistry and 2D drawings.
  8. PDB-101: Rubisco and experimental spinach Rubisco structure 1RCX for further exploration. A protein structure is not embedded in this app.
  9. OpenStax Biology: using light energy to make organic molecules.

Using it in class

Use the stage buttons or Play all to follow the pathway. Carbon counting is optional. Predict the effect of a change before testing it in Experiment.

Keyboard: Space plays/pauses; ← and → change the guided stage; F toggles presentation view; Escape closes a dialog or presentation view. No external scripts, fonts, analytics or network downloads are used.

C · carbonO · oxygenN · nitrogenP · phosphorusH · hydrogen
Drag to rotate · scroll / pinch to zoom

2D STRUCTURE · SAME CONNECTIVITY

Molecular diagrams use neutral acid forms. In the stroma, many phosphate and carboxyl groups carry negative charges. Conformers are calculated, not crystallographic structures; the reaction pathway is not an atom-by-atom dynamics simulation.

Through several enzyme-catalysed rearrangements, not by turning each three-carbon molecule into a five-carbon molecule. Here is a carbon-balanced route. GAP and DHAP are interconvertible triose phosphates.

1
5 TP → 3 GAP + 2 DHAP
Five three-carbon molecules; 15 carbons altogether.
Triose phosphate isomerase establishes the mixture.
2
GAP (3C) + DHAP (3C) → F6P (6C)
Via fructose 1,6-bisphosphate; phosphate is released.
Aldolase, then fructose-bisphosphatase.
3
F6P (6C) + GAP (3C) → Xu5P (5C) + E4P (4C)
A two-carbon unit moves from the 6C sugar to the 3C sugar.
Transketolase.
4
E4P (4C) + DHAP (3C) → S7P (7C)
Via sedoheptulose 1,7-bisphosphate; phosphate is released.
Aldolase, then sedoheptulose-bisphosphatase.
5
S7P (7C) + GAP (3C) → R5P (5C) + Xu5P (5C)
A second two-carbon transfer. Three pentoses now remain.
Transketolase.
6
2 Xu5P + 1 R5P → 3 Ru5P
All three molecules still have five carbons.
Pentose-phosphate epimerase and isomerase.
7
3 Ru5P + 3 ATP → 3 RuBP + 3 ADP
One extra phosphate is transferred to each Ru5P.
Phosphoribulokinase.

Core idea: five TP contain 15 carbons, enough to regenerate three RuBP; regeneration requires ATP. The individual rearrangement enzymes are extension detail here.

GAP = glyceraldehyde 3-phosphate; DHAP = dihydroxyacetone phosphate; F6P = fructose 6-phosphate; E4P = erythrose 4-phosphate; S7P = sedoheptulose 7-phosphate; Xu5P = xylulose 5-phosphate; R5P = ribose 5-phosphate; Ru5P = ribulose 5-phosphate. Source: IUBMB Calvin-cycle regeneration scheme, linked in About.

This is a carbon and energy budget, not a balanced chemical equation. TP is the net cycle product. Hexose phosphates form through subsequent reactions; glucose does not pop straight out of one turn of the Calvin cycle. The main diagram always uses a three-CO₂ batch.