Photosynthesis is the process by which plants convert light energy into chemical energy, creating their own food. The Calvin cycle is the second, light-independent stage of photosynthesis. Here, chemical energy captured from sunlight converts atmospheric carbon dioxide into sugar molecules. Its primary purpose is to fix carbon from an inorganic form into organic compounds for growth and energy storage.
Where the Calvin Cycle Occurs and Its Requirements
The Calvin cycle takes place within the chloroplasts of plant cells, specifically in the stroma. The cycle requires three main inputs. Carbon dioxide (CO2) is absorbed from the atmosphere.
Two energy-carrying molecules, ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate), are essential. These molecules are generated during the light-dependent reactions. ATP provides the chemical energy to drive steps within the cycle, while NADPH contributes electrons as a reducing agent.
Phase 1: Capturing Carbon Dioxide
The first phase, carbon fixation, incorporates atmospheric carbon dioxide into an organic molecule. This process begins when a CO2 molecule combines with the five-carbon sugar, ribulose-1,5-bisphosphate (RuBP). The enzyme RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) catalyzes this reaction.
RuBisCO is the most abundant enzyme on Earth, reflecting its important role in carbon capture. The combination of CO2 and RuBP forms an unstable six-carbon compound that immediately splits into two identical three-carbon molecules, called 3-phosphoglycerate (3-PGA). For every CO2 molecule fixed, two 3-PGA molecules are produced.
Phase 2: Building Sugar Molecules
In the second phase, reduction, 3-PGA molecules are transformed into glyceraldehyde-3-phosphate (G3P), a fundamental sugar building block. This conversion requires energy and reducing power. ATP, supplied by the light-dependent reactions, phosphorylates 3-PGA. NADPH then donates electrons, reducing the phosphorylated compound to G3P.
For every three CO2 molecules entering the cycle, six G3P molecules are produced. Only one of these six G3P molecules typically exits the cycle to synthesize glucose or other carbohydrates. The remaining G3P molecules continue within the cycle to ensure its ongoing operation.
Phase 3: Regenerating for Continuous Production
The final phase is regeneration, where remaining G3P molecules are reconfigured to restore the initial CO2 acceptor, RuBP. Five of the six G3P molecules produced are used for this. This ensures the cycle can continue to fix carbon dioxide.
Regeneration also requires ATP, which provides the energy to rearrange G3P carbon atoms back into the five-carbon RuBP. The continuous regeneration of RuBP is important because it allows the Calvin cycle to remain active, providing a constant supply of the molecule needed to capture more atmospheric carbon dioxide. This cyclical nature makes the process efficient and self-sustaining, enabling plants to continuously produce sugars for their energy and structural needs.