CALVIN CYCLE

Photophosphorylation

  • Photophosphorylation is the process by which ATP is formed utilizing the energy derived from light.

Non-Cyclic Pathway

  • The Non-Cyclic Photophosphorylation mechanism involves:

    • An electron donor, specifically water (H2O), which splits into protons (H+) and oxygen (O2) during photolysis.
    • Light energy excites electrons in photosystem II (P680), leading to their transfer down the Electron Transport Chain (ETC).
    • The energies from the electrons are used in ETC to form ATP and reduce NADP+ to NADPH through process-specific enzymes like NADP+ reductase.

  • Summary of components involved:

    • Light energy → Excites electrons → P680 → Electron Transport Chain → ATP production → NADP+ → NADPH synthesis

Energy Requirements

  • The process of making ATP involves:
    • ADP + P + energy → ATP
    • The energy is derived from the proton gradient created during the Electron Transport Chain.

Thylakoid Membrane

  • The thylakoid membrane houses critical components of the light-dependent reactions:

    • Photosystem II and Photosystem I (P-I and P-II)
    • Electron Transport Chain (ETC)
    • ATP synthase

  • Key reactions occurring in the thylakoid membrane:

    • Light absorption
    • H+ ions pumped into the thylakoid lumen
    • Chemiosmosis that powers ATP synthase for ATP production

Photolysis of Water

  • Photolysis is the splitting of water molecules initiated by light.
    • Overall reaction: H2O → 2H+ + 1/2O2
    • This contributes to two sources of H+:
    • The pumping action of the ETC.
    • The splitting of water (photolysis).

Calvin Cycle

  • The Calvin Cycle is responsible for synthesizing glucose from carbon dioxide.

    • Key components include:
    • Rubisco: Ribulose bisphosphate carboxylase/oxygenase, an enzyme involved in the initial carbon fixation step.
    • RuBP: Ribulose bisphosphate, which combines with CO2.
    • PGA: 3-phosphoglycerate, the initial stable product formed.
    • PGAL: Phosphoglyceraldehyde, which is synthesized from PGA.

  • Major Steps in the Calvin Cycle:

    • Step 1: CO2 combines with RuBP to form a 6-carbon, unstable intermediate.
    • Step 2: The unstable intermediate splits into two molecules of PGA.
    • Step 3: ATP and NADPH from light reactions convert PGA into PGAL.
    • Step 4: PGAL can be converted into glucose or recycled back into RuBP.

Chemical Reactions in the Calvin Cycle

  • The cycle uses:
    • 6 CO2 → 6 RuBP
    • Produces 12 PGA from the combination of CO2 and RuBP.
    • Uses 12 ATP and 12 NADPH to convert 12 PGA into 12 PGAL.
    • Some PGAL exits the cycle to form glucose.
    • Regeneration of RuBP requires ATP: 6 PGAL → Glucose + RuBP.

Summary of inputs and outputs:

  • Inputs of Calvin Cycle:

    • 6 CO2, 6 RuBP, 12 ATP, 12 NADPH

  • Outputs of Calvin Cycle:

    • Glucose (from PGAL), 6 RuBP, 12 ADP, 12 NADP+

Key Definitions and Concepts

  • Chemiosmosis: The process of ATP production that uses the energy from proton gradients across membranes.

  • Light Reactions: Reactions of photosynthesis that convert light energy into chemical energy in the form of ATP and NADPH.

  • Dark Reactions (Calvin Cycle): Reactions that do not require light directly but utilize the ATP and NADPH produced in the light reactions to drive the synthesis of glucose from CO2.

  • Phosphorylation: The addition of a phosphate group to a molecule, essential for ATP production.