The Calvin cycle is a series of biochemical reactions in photosynthesis that take place in the stroma of chloroplasts in plants and photosynthetic organisms. It is also called the light-independent reactions, Calvin-Benson cycle, or dark reactions, though the latter term is misleading because the cycle depends indirectly on light
Purpose
The Calvin cycle uses chemical energy from ATP and reducing power from NADPH, both produced by the light-dependent reactions, to fix carbon dioxide (CO2) from the atmosphere and convert it into organic molecules like glucose and other sugars that plants can use for energy and growth
Main Stages
The Calvin cycle consists of three main stages:
- Carbon Fixation: CO2 is attached to a 5-carbon sugar called ribulose-1,5-bisphosphate (RuBP) by the enzyme RuBisCO, producing an unstable 6-carbon compound that quickly splits into two molecules of 3-phosphoglycerate (3-PGA)
- Reduction: ATP and NADPH are used to convert 3-PGA into glyceraldehyde-3-phosphate (G3P), a 3-carbon sugar. G3P is the key product that can be used to form glucose and other carbohydrates
- Regeneration: Most G3P molecules are used to regenerate RuBP, allowing the cycle to continue fixing more CO2. A net gain of one G3P molecule occurs for every three CO2 molecules fixed
Summary
- The Calvin cycle is essential for converting inorganic CO2 into organic compounds.
- It occurs in the chloroplast stroma.
- It relies on ATP and NADPH from light-dependent reactions.
- The key enzyme is RuBisCO.
- The cycle produces G3P, which can be used to synthesize glucose and other biomolecules
This cycle is fundamental to the biosphere as it forms the basis of the food chain by producing sugars used by plants and, indirectly, by animals.
