The Calvin cycle occurs in plants, algae, and certain bacteria during photosynthesis. It represents the “light-independent” reactions, meaning it does not directly use sunlight. Instead, it utilizes ATP and NADPH, generated during the light-dependent reactions. This cyclical process converts atmospheric carbon dioxide into organic compounds, such as sugars, for plant growth and energy storage.
Carbon Fixation
The Calvin cycle begins with carbon fixation, where atmospheric carbon dioxide (CO2) is incorporated into an organic molecule within the chloroplast’s stroma. One molecule of CO2 combines with ribulose-1,5-bisphosphate (RuBP), a five-carbon sugar, forming an unstable six-carbon intermediate compound.
This unstable compound immediately splits into two molecules of 3-phosphoglycerate (3-PGA), a three-carbon compound. The enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase, commonly known as RuBisCO, catalyzes this initial step. RuBisCO is considered the most abundant protein on Earth, important for carbon capture.
Sugar Production
Following carbon fixation, the 3-PGA molecules undergo a reduction phase to become glyceraldehyde-3-phosphate (G3P). This conversion requires energy supplied by ATP and reducing power provided by NADPH. ATP donates phosphate groups to 3-PGA, while NADPH provides electrons, transforming the molecules into G3P.
G3P is a three-carbon sugar that serves as a building block for glucose and other carbohydrates. While the Calvin cycle produces G3P, only a fraction of these molecules leaves the cycle to form sugars like glucose and starch. The remaining G3P molecules are used for the continuation of the cycle, ensuring a steady supply of the carbon-acceptor molecule.
Molecule Regeneration
The majority of the G3P molecules produced in the sugar production phase do not exit the cycle to form carbohydrates. Instead, they are used to regenerate the initial carbon dioxide acceptor molecule, RuBP. This regeneration is a series of reactions that rearranges the carbon skeletons of five G3P molecules to form three molecules of RuBP.
This regeneration process also requires additional energy in the form of ATP. The continuous regeneration of RuBP ensures that the Calvin cycle can proceed uninterrupted.
Broader Significance
The Calvin cycle holds importance for life on Earth, extending far beyond the individual plant. The sugars produced, such as glucose, provide the source of energy and carbon for the plant’s growth and survival. These organic compounds are then used to build all other molecules a plant needs, including cellulose for structural support, starches for energy storage, and components of proteins and lipids.
The cycle’s significance expands to entire ecosystems because photosynthetic organisms, primarily plants, form the base of most food chains. They convert inorganic carbon into organic forms, providing food and energy for nearly all other life forms, from herbivores to carnivores. Furthermore, the Calvin cycle plays a role in regulating atmospheric carbon dioxide levels. By continuously drawing CO2 from the air and incorporating it into organic matter, it contributes to the global carbon cycle, influencing Earth’s climate.