Photophosphorylation and chemiosmosis

Making ATP from ADP and phosphate using light is photophosphorylation. It happens in two ways.

• Non-cyclic: PS II and PS I work in series (the Z scheme). Electrons go from water all the way to NADP⁺. Both ATP and NADPH are made, and oxygen is released.
• Cyclic: only PS I works. The excited electron does not go to NADP⁺; it comes back to PS I through the carrier chain, making ATP on the way. So only ATP is made, with no NADPH and no oxygen. This may happen in the stroma lamellae, which lack PS II and NADP reductase, or when only light of wavelength beyond 680 nm is available.

How is ATP actually made? By chemiosmosis: building up a store of protons (H⁺) on one side of a membrane, then letting them flow back. Protons pile up inside the thylakoid (the lumen) in three ways:

• Water is split on the inner side, releasing protons into the lumen.
• As electrons pass along the chain, a carrier pumps protons from the stroma into the lumen.
• NADP reductase uses up protons from the stroma side when it makes NADPH.

So the lumen fills with protons and its pH falls (becomes more acidic).

The protons can only flow back to the stroma through a special enzyme, ATP synthase. Its CF₀ part sits in the membrane and forms a channel for the protons. As they rush through, the energy changes the shape of the CF₁ part, which sticks out into the stroma and makes ATP. So chemiosmosis needs four things: a membrane, a proton pump, a proton gradient, and ATP synthase.