The light-dependent reaction occurs on the thylakoid membranes. It uses four protein complexes: PSI, PSII, the cytochrome complex and the ferredoxin complex.

Water is split by light energy to produce ATP and hydrogen. Oxygen is released as a waste product.

First of all, photons of light are absorbed by the chlorophyll molecules in PSII. This excites the chlorophyll electrons, raising them to a higher energy level. This leads to PSII undergoing a charge separation which causes water molecules to split into oxygen (O₂), electrons (e^–) and protons (H⁺). This process is known as photolysis and, due to water’s high molecular stability, requires four light photons.

2H₂O ? O₂ + 4H₂ + 4e^–

The oxygen diffuses out and is released into the atmosphere. As the splitting continues, a proton gradient is formed in the thylakoid lumen due to a build-up of protons while the electrons take the place of the excited electrons.

The excited electrons don’t simply escape but are passed down a proton complex chain. Firstly, from PSII to the cytochrome complex. Here, the energy is used in order to pump four protons from the stroma into the lumen. The electrons then pass to PSI. Here, the chlorophyll molecules absorb more light photons, providing the electrons with more energy after which they move onto the ferredoxin complex.

In the ferredoxin complex hydrogen atoms are formed when one electron combines with a proton. This is then collected by NADP, a hydrogen carrier.

The proton gradient created inside the thylakoid lumen is what’s used to create ATP. As in respiration, photosynthesis also uses the ATP synthase enzyme. However, the process is known as photophosphorylation as ADP is phosphorylated by light energy to make it.