Explain that in cyclic photophosphorylation: • only photosystem I (PSI) is involved • photoactivation of chlorophyll occurs • ATP is synthesised
Explain that in cyclic photophosphorylation: • only photosystem I (PSI) is involved • photoactivation of chlorophyll occurs • ATP is synthesised
Answer
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:
- 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.
- 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.
- Proton Gradient Formation:
- Proton Pumping: During the electron transport, protons (H+ ions) are pumped across the thylakoid membrane, creating a proton gradient (pH gradient).
- 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).