The Photochemical Reaction Center

A photochemical reaction center is a pigment-protein complex located at the core of each photosystem.

The reaction centers are grouped into two categories depending on their terminal electron acceptors.

Type I reaction center has a ferredoxin and type II has a plastoquinone molecule as the terminal electron acceptor.

Each reaction center also has a special pair of chlorophyll a molecules that absorb specific wavelengths of light.

The PSI reaction center absorbs photons with a wavelength of 700 nanometers and is, therefore, called P700.

In contrast, the PSII reaction center absorbs a wavelength of 680 nanometers and is called P680.

Despite their differences, both PSI and PSII work on a very similar principle.

When the chlorophyll pair in a reaction center absorbs a photon, one of its ground state electrons attains an excited energy level.

Consequently, the chlorophyll molecule becomes excited and must lose the excess energy it possesses for stability. Thus, it emits the energized electron to a nearby electron acceptor.

The positively charged chlorophyll ion pair then withdraws electrons from a donor, like a water molecule, to return to its original state.