AS and A Level Biology 5 Views 1 Answers
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SouravOctober 31, 2024

State that cyclic photophosphorylation and non-cyclic photophosphorylation occur during the light-dependent stage of photosynthesis

State that cyclic photophosphorylation and non-cyclic photophosphorylation occur during the light-dependent stage of photosynthesis

Sourav
SouravOctober 31, 2024

Answer

Photophosphorylation in the Light-Dependent Stage of Photosynthesis

During the Light-Dependent Stage of photosynthesis, light energy is converted into chemical energy in the form of ATP and NADPH. Two types of photophosphorylation processes occur in this stage:

  1. Cyclic Photophosphorylation (Cyclic Electron Flow):
    • Location: Thylakoid membranes of chloroplasts
    • Electron Flow:
      • Light energy excites electrons in Photosystem I (PSI)
      • Electrons cycle back to PSI, generating a proton gradient
    • ATP Synthesis:
      • Proton gradient drives ATP synthase to produce ATP from ADP + Pi
    • No Net Change in Electron Acceptors: No NADPH or O2 produced; only ATP generated
  2. Non-Cyclic Photophosphorylation (Non-Cyclic Electron Flow):
    • Location: Thylakoid membranes of chloroplasts
    • Electron Flow:
      • Light energy excites electrons in Photosystem II (PSII)
      • Electrons transferred to PSI, then to NADP+, reducing it to NADPH
      • Water (H2O) oxidized, releasing O2
    • ATP and NADPH Synthesis:
      • Proton gradient drives ATP synthase to produce ATP from ADP + Pi
      • Reduced NADPH produced, accepting electrons from the electron transport chain

Key Differences:

Cyclic Photophosphorylation Non-Cyclic Photophosphorylation
Electron Flow Cyclic (back to PSI) Linear (PSII → PSI → NADP+)
ATP Production Yes Yes
NADPH Production No Yes
O2 Production No Yes (from H2O oxidation)
PSI/PSII Involvement Only PSI Both PSII and PSI

Light-Dependent Stage Overview:

  • Both cyclic and non-cyclic photophosphorylation occur simultaneously in the thylakoid membranes.
  • The ATP and NADPH produced in these processes are utilized in the Light-Independent Stage (Calvin Cycle) to synthesize glucose from CO2.

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