Question

Explain how during light reaction of photosynthesis, ATP synthesis is a chemiosmotic phenomenon.

Answer 1

Chemiosmotic theory:

• Chemiosmosis is the movement of ions across a semipermeable membrane from a region of higher concentration to a lower concentration.
• During light reaction, photolysis of water generates protons which accumulate in the lumen (membrane of thylakoids).
• Movement of electrons through photosystems and reduction of $NAD{P}^{+}$ by NADP reductase removes $H^{+}$ ions from the stroma.
• Thus, a proton gradient is generated due to the accumulation of protons in the lumen and their decrease in the stroma.
• ATP synthesis is a result of the breakdown of this proton gradient during which protons move from lumen to stroma through ATPase situated in the membrane.
• Thus, ATP synthesis is a chemiosmotic phenomenon.