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.