The Light Reaction: Photosystems I and II

The photochemical (light) phase of photosynthesis begins when light energy is absorbed by pigment molecules embedded in the thylakoid membrane. These pigments are not scattered at random across the membrane but are organised into large, discrete functional units called photosystems, of which there are two distinct kinds, named simply Photosystem I (PS I) and Photosystem II (PS II).

Each photosystem is itself built from two functionally distinct parts. The first is a light-harvesting complex (sometimes called the antenna complex), consisting of several hundred pigment molecules -- a mixture of chlorophyll a, chlorophyll b and carotenoids -- clustered together and arranged to maximise the capture of incoming photons across a broad range of wavelengths. The second, much smaller part is the reaction centre, a single, specialised pair of chlorophyll a molecules to which all of the antenna pigments ultimately funnel their absorbed energy by a rapid, non-chemical process of resonance energy transfer, somewhat like a bucket brigade passing energy hand to hand until it reaches the one molecule capable of using it chemically.

The two photosystems are distinguished, historically and functionally, by the precise wavelength of light their reaction-centre chlorophyll a molecule absorbs best. The reaction centre of Photosystem I is designated P700, because it absorbs maximally around 700 nm, while the reaction centre of Photosystem II is designated P680, absorbing maximally around 680 nm -- both numbers referring to the wavelength at which each pigment-protein complex shows its characteristic absorption peak. Within the thylakoid membrane, the two photosystems also tend to be spatially separated to some degree: PS II is concentrated predominantly in the tightly stacked granal thylakoid membranes, while PS I, together with the ATP synthase enzyme, is found predominantly in the unstacked stroma lamellae and the stroma-exposed margins of the grana. …