Rucete ✏ Campbell Biology In a Nutshell
Unit 2 THE CELL — Concept 10.4 The Calvin Cycle Uses the Chemical Energy of ATP and NADPH to Reduce CO₂ to Sugar
The Calvin cycle is the second stage of photosynthesis. It uses ATP and NADPH from the light reactions to convert CO₂ into sugar through a series of enzyme-driven reactions in the chloroplast stroma.
Overview of the Calvin Cycle
An anabolic cycle that builds sugars from smaller molecules
Carbon enters as CO₂ and leaves as a three-carbon sugar (G3P)
Requires energy (ATP) and reducing power (NADPH)
For one G3P molecule: uses 3 CO₂, 9 ATP, and 6 NADPH
Must cycle three times to produce one net G3P
Phase 1: Carbon Fixation
Each CO₂ molecule is attached to ribulose bisphosphate (RuBP), a 5-carbon sugar
Enzyme: rubisco (most abundant protein on Earth)
Forms an unstable 6-carbon intermediate → splits into two 3-phosphoglycerate (3-PGA) molecules
Phase 2: Reduction
Each 3-PGA is phosphorylated by ATP → becomes 1,3-bisphosphoglycerate
Reduced by NADPH → becomes glyceraldehyde-3-phosphate (G3P)
For every 3 CO₂, 6 G3P are formed, but only 1 exits as product
The remaining 5 G3P molecules continue through the cycle
Phase 3: Regeneration of RuBP
5 G3P molecules are rearranged into 3 RuBP molecules
Uses 3 more ATP molecules
Cycle restarts, ready to fix more CO₂
Importance of G3P
G3P is the direct output of the Calvin cycle
Can be used to synthesize glucose, sucrose, and other carbohydrates
Neither the Calvin cycle nor the light reactions alone can make sugar—both are essential
In a Nutshell
The Calvin cycle fixes CO₂ into sugar using ATP and NADPH from the light reactions. Through three phases—carbon fixation, reduction, and RuBP regeneration—it produces G3P, the sugar precursor for plant metabolism.