The Calvin cycle is a series of biochemical reactions that occur within the stroma, the fluid-filled space inside the chloroplasts of plant cells. Often referred to as the light-independent reactions, this process converts atmospheric carbon dioxide (\(\text{CO}_2\)) into organic molecules used by the plant for energy and growth. The cycle uses chemical energy stored in adenosine triphosphate (ATP) and high-energy electrons carried by nicotinamide adenine dinucleotide phosphate (NADPH), both generated during the light-dependent stage of photosynthesis. This mechanism fixes inorganic carbon, transforming it into three-carbon sugars that provide the fundamental building blocks for plant biomass. The process is divided into three main phases: carbon fixation, reduction, and the regeneration of the starting molecule.
Carbon Fixation: The First Stage
The first phase of the Calvin cycle is carbon fixation, where a carbon atom from a \(\text{CO}_2\) molecule is incorporated into an existing organic compound. This step is catalyzed by the protein Ribulose-1,5-bisphosphate carboxylase/oxygenase, nicknamed Rubisco. Rubisco is the most abundant protein on Earth.
The enzyme attaches the incoming \(\text{CO}_2\) molecule to a five-carbon sugar called ribulose-1,5-bisphosphate (RuBP). This combination results in a highly unstable six-carbon compound. The transient molecule immediately breaks down, splitting into two identical molecules of the three-carbon compound 3-phosphoglycerate (3-PGA).
This step is often the bottleneck due to Rubisco’s inefficiency, as it processes only about three to ten substrate molecules per second. The enzyme’s action is crucial because it moves carbon from an inorganic gaseous state into a usable organic form for the plant. The resulting 3-PGA molecules are the first stable organic products and are prepared for the next stage.
Converting Energy: The Reduction Stage
The second phase, reduction, utilizes the energy harvested during the light reactions to convert 3-PGA molecules into a higher-energy sugar precursor. The 3-PGA molecules must first be activated through phosphorylation, involving the transfer of a phosphate group from ATP.
Following activation, the molecules are reduced when they receive high-energy electrons from NADPH. This reaction causes the loss of one phosphate group and transforms the molecule into glyceraldehyde-3-phosphate (G3P), a three-carbon sugar. This conversion expends a total of six ATP and six NADPH molecules.
The resulting G3P is the final output of the reduction stage and serves as the raw material for carbohydrate synthesis. Only one net G3P molecule is produced for every three turns of the cycle; the majority must remain within the chloroplast to ensure the cycle continues operating.
Recycling the Molecule: The Regeneration Stage
The final stage is regeneration, which replenishes the initial RuBP acceptor molecule. For continuous carbon fixation, the five-carbon RuBP must be reformed from the remaining G3P molecules. It takes five of the G3P molecules generated in the reduction stage to rebuild the three RuBP molecules needed for the next cycle.
This complex series of reactions requires an additional input of three molecules of ATP. The ATP energy reorganizes the carbon skeletons of the G3P molecules back into the five-carbon configuration of RuBP. This maintains a steady supply of RuBP to accept new \(\text{CO}_2\) molecules. The single G3P molecule that exits the cycle is used in the cytoplasm to synthesize larger sugars like glucose and sucrose.