Which photosystem has a reaction center that absorbs light at 680 nm?

Photosystem II has a reaction center that absorbs light at 680 nm. Its reaction-center pigment is called P680, with “P” referring to pigment and “680” indicating its characteristic absorption maximum.

When P680 absorbs light—or receives excitation energy from surrounding antenna pigments—it loses an energized electron to a nearby acceptor molecule called pheophytin. This begins the electron-transfer chain of oxygenic photosynthesis. The resulting positively charged P680 is restored by electrons extracted from water by the oxygen-evolving complex, releasing oxygen as a by-product.

A common mix-up is Photosystem I, whose reaction-center pigment is P700 and absorbs most strongly near 700 nm. The numbers are labels for the reaction-center absorption properties, not the total range of wavelengths each photosystem can use. Antenna complexes absorb across broader regions and pass that energy to the reaction center. Photosystem II acts first in the linear electron-flow pathway, sending electrons onward through plastoquinone and cytochrome b6f.