The input of solar energy into photosynthesis, and thence into the biospher
e, occurs via chlorophyll-containing proteins known as reaction centres. Th
ere are two kinds of reaction centre in oxygenic photosynthesis: photosyste
m I(PSI)and photosystem II(PSII). The PSII reaction centre, alias the oxyge
n-evolving enzyme, the water-oxidizing complex or the water-plastoquinone p
hoto-oxidoreductase, has now been crystallized and its structure solved to
a resolution of 3.8 Angstrom.