The Calvin Cycle

In autotrophic plants, the Calvin Cycle starts when atmospheric carbon dioxide eventually diffuses into the stroma of the chloroplast.

Here, one carbon atom from the carbon dioxide is added or fixed to a five-carbon acceptor sugar molecule, ribulose bisphosphate, or RuBP, in a reaction catalyzed by the enzyme Ribulose 1,5-bisphosphate-carboxylase-oxygenase, or RuBisCo for short. The resulting six-carbon molecule is highly unstable and splits into two three-carbon molecules of 3-phosphoglyceric acid, 3-PGA.

With ATP providing the energy, and NADPH affixing one hydrogen to each, the three PGA chains are converted into another three-carbon intermediate called glyceraldehyde-3-phosphate. One G3P then exits the cycle and waits for another one to build glucose with six carbon atoms.

Meanwhile, the remaining G3P must wait for four more cycles as carbons accumulate and ATP provides more energy to regenerate the RuBP acceptors. Overall, six turns of the Calvin Cycle fix six carbon dioxides from the atmosphere using the energy and reducing power of 18 ATPs and 12 NADPHs, respectively, to generate one molecule of glucose and rebuild RuBP to continue the loop.