Explain that in non-cyclic photophosphorylation: • photosystem I (PSI) and photosystem II (PSII) are both involved • photoactivation of chlorophyll occurs • the oxygen-evolving complex catalyses the photolysis of water • ATP and reduced NADP are synthesised

Sourav PanOctober 31, 2024

Answered

Non-Cyclic Photophosphorylation: A Detailed Explanation

Overview:
Non-cyclic photophosphorylation is a light-dependent process in photosynthesis involving both Photosystem I (PSI) and Photosystem II (PSII). This process generates ATP and reduced NADP (NADPH), utilizing light energy, and releases oxygen (O2) as a byproduct.

Step-by-Step Explanation:

1. Photoactivation of Chlorophyll:
   • Light Absorption (PSII): Chlorophyll a molecules in Photosystem II (PSII) absorb light energy (approximately 680 nm).
   • Excitation of Electrons (PSII): The absorbed light energy excites electrons in the chlorophyll molecules, elevating them to a higher energy state.
2. Water Photolysis (PSII):
   • Oxygen-Evolving Complex (OEC): The excited electrons in PSII are used to catalyze the photolysis of water (H2O), splitting it into:
     • Protons (H+), which contribute to the proton gradient.
     • Electrons, which reduce an electron acceptor.
     • Oxygen (O2), released as a byproduct into the atmosphere.
3. Electron Transfer Chain:
   • Electron Flow from PSII to PSI: The electrons from PSII are transferred through a series of electron carriers to Photosystem I (PSI).
   • Light Absorption (PSI): Chlorophyll a molecules in PSI absorb light energy (approximately 700 nm), further exciting the electrons.
4. Reduced NADP (NADPH) Synthesis:
   • Final Electron Acceptor: The highly energized electrons from PSI reduce NADP+ to form NADPH.
5. ATP Synthesis:
   • Proton Gradient: The protons (H+) accumulated in the thylakoid lumen during electron transfer create a proton gradient.
   • ATP Synthase: The energy from this gradient is utilized by ATP synthase to synthesize ATP from ADP + Pi.

Key Points:

• Both PSI and PSII Involved: Non-cyclic photophosphorylation requires the coordinated action of both Photosystem I and Photosystem II.
• Photoactivation of Chlorophyll: The process begins with the absorption of light by chlorophyll in PSII, initiating the electron transfer chain.
• Oxygen-Evolving Complex (OEC): Catalyzes the photolysis of water, releasing oxygen and protons.
• ATP and Reduced NADP Synthesis: The primary outcomes are the production of ATP and NADPH, essential for the Calvin Cycle (Light-Independent Reactions).