Light-Dependant Reaction

What are the products of the light-dependent reaction?

ATP, reduced NADP, and oxygen (as a by-product).

What is photoionisation?

Light excites electrons in chlorophyll, which are emitted from the molecule. The chlorophyll becomes positively charged (ionised). (LIGHT HARVESTING)

What is photolysis and why is it needed?

Light splits water: H₂O → 2H⁺ + 2e⁻ + ½O₂. The electrons replace those lost from chlorophyll in PSII, the H⁺ ions help form reduced NADP and the proton gradient, and oxygen is released.

Describe non-cyclic photophosphorylation.

Light excites electrons in PSII, which leave and pass along the electron transport chain. Energy released pumps H⁺ into the thylakoid space, creating a proton gradient. H⁺ flow back through ATP synthase into the stroma (chemiosmosis), making ATP from ADP and Pᵢ. The electrons then reach PSI, are re-excited, and are accepted by NADP along with H⁺, forming reduced NADP.

What is chemiosmosis?

The movement of H⁺ down their electrochemical gradient through ATP synthase, which drives ATP synthesis.

How does cyclic photophosphorylation differ from non-cyclic?

Only PSI is involved. Excited electrons return to PSI via the electron transport chain rather than going to NADP, so only ATP is made, with no reduced NADP or oxygen.

Why is cyclic photophosphorylation useful?

It makes extra ATP when the Calvin cycle needs more ATP than reduced NADP, and it works even if photolysis or NADP supply is limited.