Calvin Cycle Overview
The Calvin-Benson Cycle consists of three main phases that work together to produce sugars from carbon dioxide. It all begins when the enzyme RuBisCO captures CO₂ and attaches it to a 5-carbon molecule called RuBP (ribulose bisphosphate), creating a 6-carbon intermediate that quickly breaks down.
In Phase 1 (Carbon Fixation), CO₂ enters the cycle and combines with RuBP. This creates a short-lived intermediate that breaks down into 3-PGA 3−phosphoglycerate, a 3-carbon molecule. For every three CO₂ molecules that enter, six 3-PGA molecules are formed.
Phase 2 (Reduction) uses energy from ATP and NADPH (produced during the light reactions) to convert 3-PGA into G3P glyceraldehyde3−phosphate. This requires both the phosphate from ATP and the hydrogen from NADPH to reduce the molecules. Out of six G3P molecules produced, one can exit the cycle to form glucose or other compounds.
Remember This: For every three CO₂ molecules fixed by RuBisCO, only one G3P molecule (3 carbons) can leave the cycle as product, while the remaining five G3P molecules must be recycled to regenerate RuBP.
Phase 3 (Regeneration) uses the remaining five G3P molecules and more ATP to rebuild the three original RuBP molecules, allowing the cycle to continue capturing more carbon dioxide.