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Calvin Cycle Overview

Aug 17, 2025

Overview

This lecture explains the Calvin cycle, the second stage of photosynthesis, detailing its phases and how it uses ATP and NADPH to produce sugars from carbon dioxide.

The Calvin Cycle Overview

  • The Calvin cycle occurs after the light-dependent reactions in photosynthesis.
  • It is called the light-independent or dark reactions because it does not require light directly.
  • The cycle takes place in the stroma, the fluid area surrounding the thylakoids within the chloroplast.
  • The Calvin cycle depends on ATP and NADPH, products of the light-dependent reactions.

Phase 1: Carbon Fixation

  • Rubisco, an enzyme, adds COâ‚‚ to a five-carbon compound called RuBP (ribulose bisphosphate).
  • This results in an unstable six-carbon compound that splits into two molecules of 3-phosphoglycerate (3-PGA).
  • For every three COâ‚‚ molecules, six molecules of 3-PGA are made.

Phase 2: Reduction

  • ATP provides energy to modify 3-PGA into a reactive intermediate.
  • NADPH donates electrons and hydrogens to reduce this intermediate, forming glyceraldehyde 3-phosphate (G3P).
  • For every three COâ‚‚ entering the cycle, one G3P is produced as a net gain.

Phase 3: Regeneration

  • Only one G3P leaves the cycle; the other five are used to regenerate RuBP.
  • Five G3P molecules (each with three carbons) are rearranged to make three RuBP molecules (each with five carbons).
  • ATP provides the energy needed for RuBP regeneration.

The Calvin Cycle and Glucose Production

  • The cycle must turn twice (fixing six COâ‚‚ total) to produce two G3P, which combine to form one glucose molecule.
  • Glucose and other organic compounds produced support plant growth and, by extension, nearly all life.

Key Terms & Definitions

  • Calvin Cycle — The set of light-independent reactions in photosynthesis that convert COâ‚‚ into sugars.
  • Stroma — The fluid surrounding thylakoids in chloroplasts where the Calvin cycle occurs.
  • ATP — A molecule that supplies energy for the Calvin cycle.
  • NADPH — A molecule that donates electrons and hydrogens in the Calvin cycle.
  • Rubisco — The enzyme that catalyzes COâ‚‚ fixation in the cycle.
  • RuBP — Ribulose bisphosphate, a five-carbon starting compound for carbon fixation.
  • 3-PGA — 3-phosphoglycerate, a three-carbon product of COâ‚‚ fixation.
  • G3P — Glyceraldehyde 3-phosphate, the main sugar product of the Calvin cycle.

Action Items / Next Steps

  • Practice the Calvin cycle using the photosynthesis interactive at Bioman Biology.

Full transcript