Photosynthesis occurs within the chloroplast. A cellular organelle found in plants and some algae. Photosynthetic reactions also happen within cyanobacteria, which do not have chloroplasts.

The photosynthetic electron transport chain is embedded in the chloroplast's thylakoid membrane. It consists of four large protein complexes, one small protein complex, and three smaller mobile carrier proteins. The space within the thylakoid membrane is called the lumen. The space surrounding the membrane is called the stroma.

Photosystem II contains an antenna structure known as the light harvesting complex (LHC). This complex contains chlorophyll molecules that transform the energy contained in photons of light into resonance energy.

The resonance energy is tranferred from one cholorphyll molecule to another within the LHC until it is funneled into the reaction center of Photosystem II.

At the heart of the reaction center is a special pair of cholorophyll molecules known as P680. Within P680 are electrons that can be excited by resonance energy. One photon of light is needed to excite each electron. Once excited, each electron is transferred to Qb (magenta), a mobile carrier protein.

Qb accepts two electrons from P680 and also picks up two hydrogen ions (protons) from the stroma.

The electrons that are lost from P680 are replaced by the splitting of water molecules at the oxygen evolving complex (OEC). Each water molecule provides two electrons to P680.

After two water molecules have been split, the remaining oxygen atoms are combined to create one molecule of molecular oxygen (O2).