Calvin Cycle
Calvin Cycle
The Calvin cycle is a fundamental component of the photosynthesis process, specifically categorized as the light-independent reactions, or sometimes referred to as the "dark reactions," though this is a misnomer since these reactions do not specifically occur in darkness. The Calvin cycle is responsible for converting carbon dioxide and other substrates into glucose, which serves as an energy source for the plant and other organisms.
Historical Context
The cycle is named after Melvin Calvin, an American biochemist who, alongside colleagues Andrew Benson and James Bassham, elucidated the pathway of carbon reduction during photosynthesis using the radioactive isotope carbon-14. The pathway is sometimes referred to as the Calvin-Benson-Bassham (CBB) cycle.
Mechanism and Function
The Calvin cycle operates in the stroma of chloroplasts in photosynthetic organisms. It consists of a series of biochemical redox reactions that produce glucose using the energy derived from the light-dependent reactions of photosynthesis. These reactions utilize the ATP and NADPH produced in the light-dependent reactions.
Phases of the Calvin Cycle
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Carboxylation: This phase involves the fixation of carbon dioxide into an organic molecule. The enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) catalyzes the reaction between CO2 and ribulose bisphosphate (RuBP) to form 3-phosphoglycerate (3-PGA).
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Reduction: In this phase, the 3-PGA molecules are phosphorylated by ATP and then reduced by NADPH to form glyceraldehyde-3-phosphate (G3P). Some of the glyceraldehyde-3-phosphate molecules continue through the cycle to regenerate RuBP, while others are siphoned off to form glucose and other carbohydrates.
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Regeneration of RuBP: The remaining G3P molecules undergo a series of reactions to regenerate RuBP, thus allowing the cycle to continue. This step requires additional ATP.
Biological Significance
The Calvin cycle is crucial not only for plants but also for ecosystems and the biosphere as a whole. It is the primary pathway through which inorganic carbon is fixed into organic matter, providing the foundation for the food web.
Variations and Adaptations
Apart from the typical Calvin cycle, plants have evolved variations like C4 carbon fixation and CAM photosynthesis to improve efficiency under specific environmental conditions, such as high temperatures or water scarcity.
Related Topics
In summary, the Calvin cycle is a pivotal biochemical pathway that not only sustains plant life but also supports the energy requirements of nearly every organism on Earth through the production of glucose and other carbohydrates.