In the presence of light, chloroplasts are capable of driving thermodynamically unfavorable reactions.

Two kinds of photosystems are involved in photosynthesis in plants. Photosystem I (PSI) absorbs light around 700 nm and photosystem II (PSII) absorbs light only to a wavelength of about 680 nm. In algae, cyanobacteria, and all higher plants, these two photosystems are linked in series to carry out the complete sequence of the light reactions.

Figure 17.12 illustrates the two photosystems and the paths taken by electrons through them.

In each of the two photosystems, the primary step is transfer of a light-excited electron from a reaction center (P680 or P700) into an electron transport chain.

The ultimate source of the electrons is water molecules. In this part of the process, the destination of the electrons is NADP⁺, to form NADPH.

At two stages in the process of transporting electrons, protons are released into the thylakoid lumen. The transfer of protons into the lumen produces a pH gradient across the thylakoid membrane. The resulting proton gradient is used to drive the synthesis of ATP in a manner very similar to that used in oxidative phosphorylation. In oxidative phosphorylation, mitochondria establish a proton gradient that allows the synthesis of ATP to be coupled to electron transport.

Thus, ATP and the reducing power of NADPH are the products of the light reactions.