Explain that in cyclic photophosphorylation: • only photosystem I (PSI) is involved • photoactivation of chlorophyll occurs • ATP is synthesised

Sourav PanOctober 31, 2024

Answered

Cyclic Photophosphorylation: A Detailed Explanation

Overview:
Cyclic photophosphorylation is a light-dependent process in photosynthesis where only Photosystem I (PSI) is involved. This process generates ATP from ADP + Pi, utilizing light energy.

Step-by-Step Explanation:

1. Photoactivation of Chlorophyll:
   • Light Absorption: Chlorophyll a molecules in Photosystem I (PSI) absorb light energy (approximately 700 nm).
   • Excitation of Electrons: The absorbed light energy excites electrons in the chlorophyll molecules, elevating them to a higher energy state.
2. Electron Transfer and Cycle:
   • Primary Electron Acceptor: The excited electrons are transferred to a primary electron acceptor, ferredoxin, in the PSI complex.
   • Electron Transport Chain: These electrons then pass through a series of electron carriers in the thylakoid membrane, ultimately returning to the PSI complex. This creates a cyclic flow of electrons.
3. Proton Gradient Formation:
   • Proton Pumping: During the electron transport, protons (H+ ions) are pumped across the thylakoid membrane, creating a proton gradient (pH gradient).
4. ATP Synthesis:
   • ATP Synthase: The energy from the proton gradient is utilized by ATP synthase, an enzyme embedded in the thylakoid membrane.
   • ATP Production: As protons flow back across the membrane through ATP synthase, the energy is harnessed to synthesize ATP from ADP + Pi.

Key Points:

• Only PSI Involved: Cyclic photophosphorylation exclusively involves Photosystem I, distinguishing it from non-cyclic photophosphorylation, which involves both PSI and PSII.
• Photoactivation of Chlorophyll: The process begins with the absorption of light by chlorophyll in PSI, initiating the electron transfer chain.
• ATP Synthesis: The primary outcome of cyclic photophosphorylation is the production of ATP, which is essential for the Calvin Cycle (Light-Independent Reactions).