The synthesis of ATP or phosphorylation of ADP occurs thanks to the energy provided by photons of light, which is why this process is called photophosphorylation.

The photophosphorylation occurs when, at the excited by the action of light, a flow of electrons from the photosystems are conducted through the different electronic acceptors to NADPH, while a proton gradient generated whose energy is used to synthesize ATP.

When electrons are falling from higher to lower energy levels along the transport chain, the released energy is used to pump protons (H⁺) from the stroma into the interior space of the thylakoid. Thus a proton gradient is formed between each side of the thylakoid membrane. Inside the thylakoid there is an acidic pH (pH = 5), and outside, basic (pH = 8).

Protons cannot directly cross the thylakoid membrane, so they return to the stroma, down the gradient, through proton translocating ATPases. When they pass, the gradient disappears and they have generated energy that is used to phosphorylate ADP and synthesize ATP.

Two types of photosynthesis are distinguished, depending on whether the PSI and PSII participate jointly or if only the PS I does it:

• Non-cyclic photophosphorylation. In the scheme Z PS II and PS I involved producing ATP and NADPH.
• Cyclic photophosphorylation. Only PS I is involved and only produces ATP.