5.3—Light-dependent reactions

Light excites chlorophyll electrons in the thylakoid membrane. Their movement through electron carriers drives proton pumping and chemiosmosis to make ATP; non-cyclic flow also reduces NADP, while photolysis of water replaces electrons and releases oxygen.

Non-cyclic photophosphorylation uses photosystems II and I and produces ATP, reduced NADP and oxygen. Cyclic photophosphorylation uses photosystem I and returns electrons to the chain, producing ATP but not reduced NADP or oxygen.

In non-cyclic flow, excited electrons leave photosystem II, water supplies replacement electrons, protons accumulate in the thylakoid space, and ATP synthase uses their return flow to phosphorylate ADP.

“Light-independent” does not mean the Calvin cycle can run without ATP and reduced NADP. Also keep photosystem II’s name separate from its order in the pathway: it acts before photosystem I.