Light-Dependent Reactions of Photosynthesis: Photophosphorylation and Electron Transport

Key processes

1. Absorption of light. Pigment-protein complexes (photosystems) in the thylakoid membrane absorb photons and raise electrons to higher energy states. The photosystem associated with water splitting (photosystem II) provides the initial excited electrons.

2. Water splitting (photolysis). Enzymes associated with photosystem II split water molecules, releasing molecular oxygen, protons (often described as hydrogen ions), and free electrons. The released oxygen is not used by the photosynthetic apparatus and typically leaves the cell.

3. Electron transport and proton pumping. Excited electrons travel through an electron transport chain in the thylakoid membrane. As they move, energy is used to pump protons into the thylakoid lumen, building an electrochemical gradient.

4. ATP synthesis by chemiosmosis. The proton gradient drives chemiosmosis: protons flow back across the membrane through ATP synthase, producing ATP from ADP and inorganic phosphate.

5. Formation of reducing power. Electrons and protons are ultimately used to reduce NADP+ to NADPH. The produced NADPH, together with ATP, supplies the energy and reducing power for the subsequent light-independent reactions.

6. Gas release. The O₂ generated by water splitting typically leaves the cell and the surrounding environment by diffusion (diffuses), where it can enter the atmosphere or be used by other organisms.

All of these events are localized in the stacked thylakoid membranes (often referred to as the grana thylakoid) within the chloroplasts. The ATP and NADPH produced feed into the carbon-fixation reactions that do not require light, allowing the organism to synthesize sugars even when photons are not being absorbed.