Where does the Calvin cycle take place

Calvin cycle (CO2 fixation)

The Calvin cycle is a cyclical sequence of chemical reactions in which carbon dioxide (CO2) is reduced to glucose and water.

The CO diffusing through the stomata into the leaves2 is fixed (carboxylated) in the stroma of the chloroplasts by the enzyme Rubisco (ribulose-1,5-bisphosphate carboxylase) and attached to a C5 body (a molecule with 5 carbon atoms, here the ribulose bisphosphate, also ribulose diphosphate) transfer. A C6 body is created for a short time, but it quickly breaks down into two C3 bodies (the 3-phosphoglycerate). So it is created by the fixation of a CO2-Molecule two molecules of 3-phosphoglycerate (under ATP consumption, ATP becomes ADP).

These are converted to glyceraldehyde-3-phosphate (GAP) by consuming NADPH (which is oxidized to NADP). To the CO2-Fixation can continue to operate, the C5 body, the ribulose bisphosphate, must be regenerated. For this purpose, 5 GAP molecules are converted into 3 C5 bodies (xylulose-5-phosphate), which are ultimately converted back into ribulose bisphosphate, using ATP. This cycle is called the Calvin cycle.

If one assumes that one molecule of CO2 is fixed, one GAP molecule is created per three cycles. It is used to build up assimilates (strength). But since two GAP molecules were created, the other one remains in the cycle.

The overall equation of the Calvin cycle is therefore:

3 CO2 + 6 NADPH + 6 H + 9 ATP -> Triose (GAP) + 3 H20 + 6 NADP + 9 ADP + 9 P

Glucose is built up from the resulting GAP. Two GAP molecules are required for each glucose molecule. Starch is then formed from glucose, a ring-shaped molecule. It consists of long chains of glucose molecules and is initially stored in small starch grains (granules) in the chloroplast. If necessary, starch is converted into the disaccharide sucrose (“transport sugar”) and transported to the destination via the phloem.