A photosystem combines a light-harvesting antenna with a reaction center that converts electronic excitation into directed electron transfer.

When excitation reaches the reaction-center chlorophyll pair, an electron is promoted to a higher-energy state. In that excited state, electron transfer to a nearby primary electron acceptor becomes favorable.

The reaction center therefore undergoes photochemical charge separation:

reaction-center chlorophyll + acceptor
        ↓ photon-driven excitation
excited chlorophyll* + acceptor
        ↓ electron transfer
oxidized chlorophyll+ + reduced acceptor−

The positive and negative charges are now located on different molecular components.

Charge separation converts transient excitation into redox chemistry

An isolated excited pigment normally relaxes quickly. A reaction center captures part of that excited-state free energy by moving an electron away from the oxidized chlorophyll before the excitation is lost.

The reduced acceptor can pass the electron into an electron-transport pathway. The oxidized reaction-center chlorophyll must then receive a replacement electron from an electron donor.

Thus every productive reaction-center cycle has two sides:

1. acceptor side: the excited reaction center donates an electron to a downstream acceptor;
2. donor side: another species supplies an electron to restore the oxidized reaction center.

Light changes redox capability

The ground-state reaction center may not be able to reduce the primary acceptor strongly enough. Photon absorption raises the electronic free energy of the reaction center, making the excited state a much stronger electron donor.

Photosynthesis therefore does not obtain chemical reducing power merely because chlorophyll is green. Its key photochemical step is light-driven charge separation that converts photon energy into a higher-energy redox state.

Different photosystems use different donors and acceptors, but this reaction-center logic is broadly reusable across photosynthetic systems.