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The Calvin Cycle (Light-Independent)
AP Biology · Cellular Energetics
Learning Objectives
Observe Rubisco fixing CO₂ onto a 5-Carbon RuBP molecule.
Trace the usage of ATP and NADPH from the Light Reactions to reduce 3-PGA into G3P.
Understand why it takes 3 turns of the cycle (3 CO₂) to produce ONE net G3P molecule.
Energy Cost per Net G3P
9 ATP consumed
6 NADPH consumed
Note: Making 1 full Glucose molecule requires 2 G3P (so 6 turns of the cycle total).
Tags
Biology
Photosynthesis
Calvin Cycle
Rubisco
Stroma
Chloroplast Stroma View
Current Phase
Phase 1: Carbon Fixation
Step-by-Step Cycle
1. Carbon Fixation
Rubisco attaches 3 CO₂ to 3 RuBP molecules, creating unstable 6-Carbon intermediates that instantly split into six 3-PGA.
2. Reduction
6 ATP and 6 NADPH deposit high-energy electrons to convert the six 3-PGA molecules into six G3P molecules.
3. Release & Regeneration
One net G3P exits to form glucose. The remaining 5 G3P use 3 ATP to regenerate three 5-Carbon RuBP molecules.
Quick Quiz
Why is the Calvin Cycle often called the "Light-Independent" reaction?
It happens strictly at night.
It doesn't directly use photons, but requires the ATP and NADPH produced by the Light-Dependent reactions.
It produces photons of light for the cell to use.