Electron transport, splitting of water and photophosphorylation

In PS II, P680 absorbs red light of 680 nm; its electrons are excited and pass to a primary electron acceptor, then along an electron transport chain of cytochromes, moving downhill in redox potential.

Those electrons reach PS I. Meanwhile P700 in PS I is also excited by 700 nm light, and its electrons pass to another acceptor with a greater redox potential and finally reduce NADP⁺ to NADPH + H⁺.

Plotted on a redox scale, this path from PS II to PS I to NADP⁺ forms a Z shape, so it is called the Z scheme.

PS II refills its lost electrons by splitting water: 2H₂O → 4H⁺ + O₂ + 4e⁻. Water splitting is tied to PS II, which sits on the inner (lumen) side of the thylakoid membrane, so protons and O₂ are released into the lumen.

ATP synthesis from ADP and inorganic phosphate using light energy is called photophosphorylation.

Non-cyclic photophosphorylation uses both photosystems working in series; electrons flow from water to NADP⁺, and both ATP and NADPH + H⁺ are produced.

In cyclic photophosphorylation only PS I works; excited electrons circle back to PS I via the transport chain rather than reaching NADP⁺, so only ATP is made, with no NADPH.

The stroma lamellae are a likely site for cyclic flow: the grana membranes have both photosystems, while stroma lamella membranes have no PS II and no NADP reductase enzyme. Cyclic photophosphorylation also occurs if only light beyond 680 nm in wavelength is available.