The Calvin Benson Cycle

RuBisCo catalyzes carbon fixation, the first stage of the Calvin-Benson cycle, by adding CO2 to ribulose 1,5-bisphosphate (RuBP), a five-carbon sugar. This reaction produces an unstable six-carbon intermediate that cleaves into two three-carbon molecules called 3-phosphoglycerate (3-PGA). Although RuBisCo is the most abundant enzyme on Earth, it is inefficient, fixing only about three CO2 molecules per second.

ATP and NADPH, produced during the light reactions, power the reduction stage of the Calvin-Benson cycle. NADPH transfers electrons to 1,3-bisphosphoglycerate, producing glyceraldehyde-3-phosphate (G3P), while ATP regenerates ribulose 1,5-bisphosphate (RuBP). Overall, the cycle requires 12 ATP and 12 NADPH molecules to produce one six-carbon sugar from six CO2 molecules.

Photorespiration occurs when oxygen binds to RuBisCo's CO2 binding site due to poor substrate specificity, producing an abnormal molecule and releasing CO2. This wasteful process increases under high intracellular oxygen levels. Every photosynthetic organism experiences some photorespiration, but it becomes problematic in conditions that favor oxygen over carbon dioxide fixation.

C4 plants have evolved a special mechanism that temporarily fixes CO2 by forming four-carbon intermediates such as oxaloacetate and malate before the Calvin cycle's carbon fixation step. This process increases intracellular CO2 levels, reducing photorespiration. C4 plants are particularly common in tropical environments where high oxygen levels would otherwise trigger excessive photorespiration.

The CAM pathway, or crassulacean acid metabolism, is used by succulent plants in hot, arid environments. These plants keep stomata closed during the day to conserve water, absorbing CO2 at night when conditions are cool and moist. CO2 is temporarily stored as malate and released during daytime by NADP-linked malic enzymes for use in the Calvin cycle.

The Calvin-Benson cycle consists of three stages: carbon fixation, where RuBisCo adds CO2 to RuBP; reduction, where ATP and NADPH convert 3-phosphoglycerate into glyceraldehyde-3-phosphate (G3P); and regeneration, where ATP regenerates ribulose 1,5-bisphosphate (RuBP). One G3P exits to form plant metabolites while the other continues the cycle.

The light reactions generate ATP and NADPH, which are essential energy carriers required by the Calvin-Benson cycle to fix CO2 into sugar. Without the electron transport photosynthesis provides during light reactions, the Calvin-Benson cycle cannot proceed. Together, these two phases convert light energy and inorganic carbon into chemical energy stored in glucose.