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Calvin Cycle Overview

Jul 27, 2025

Overview

This lecture explains the Calvin cycle (light-independent reaction) of photosynthesis, detailing its stages and its connection to the light-dependent reaction.

Recap: Photosynthesis Basics

  • Photosynthesis is the process by which plants use light, carbon dioxide, and water to make glucose and oxygen.
  • The chemical equation: carbon dioxide + water + light energy → glucose + oxygen.
  • Photosynthesis has two stages: light-dependent and light-independent reactions.

Light-Dependent Reaction (Prior Knowledge)

  • Occurs in the leaf chlorophyll, producing ATP, NADPH, and releasing oxygen.
  • ATP and NADPH serve as energy and reducing power for the next stage.

Introduction to the Calvin Cycle (Light-Independent Reaction)

  • Also called the Calvin cycle, named after Melvin Calvin.
  • Takes place in the stroma of the chloroplast after the light-dependent reactions.
  • Uses ATP and NADPH from the light reaction to fix carbon dioxide.

Stages of the Calvin Cycle

1. Carbon Fixation

  • Carbon dioxide enters the stroma through stomata and binds to RuBP (ribulose bisphosphate/RUDP 5) to form unstable six-carbon compound (RUDP6).
  • The enzyme rubisco catalyzes this step.
  • RUDP6 splits into two molecules of 3-phosphoglycerate (3-PGA).

2. Reduction

  • 3-PGA is converted into glyceraldehyde-3-phosphate (G3P) using ATP and NADPH.
  • NADPH donates electrons; ATP provides energy.
  • G3P is a three-carbon sugar and a product used to make glucose.

3. Regeneration of RuBP

  • Some G3P molecules are recycled using ATP to regenerate RuBP, enabling the cycle to continue.
  • For every six COâ‚‚ molecules, twelve G3P form; ten regenerate RuBP, two make glucose.

Summary Diagram Explanation

  • Light reaction uses water and sunlight to create ATP, NADPH, and release oxygen.
  • Calvin cycle uses ATP, NADPH, and carbon dioxide to produce glucose.

Key Terms & Definitions

  • Photosynthesis — Process by which plants make food using light, COâ‚‚, and water.
  • Light-dependent reaction — First stage that converts sunlight to chemical energy (ATP, NADPH).
  • Calvin cycle (Light-independent reaction) — Second stage that synthesizes glucose from COâ‚‚ using ATP and NADPH.
  • Stroma — Fluid inside chloroplast where the Calvin cycle occurs.
  • RuBP (ribulose bisphosphate) — Five-carbon molecule acting as COâ‚‚ acceptor.
  • Rubisco — Enzyme that catalyzes carbon fixation.
  • 3-PGA (3-phosphoglycerate) — Three-carbon molecule formed in carbon fixation.
  • G3P (glyceraldehyde-3-phosphate) — Three-carbon sugar produced in reduction phase.

Action Items / Next Steps

  • Review the stages of the Calvin cycle and the roles of ATP and NADPH.
  • Study the summary diagram of photosynthesis.
  • Prepare for questions on Calvin cycle steps and enzyme functions.

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