Photosystem and Reaction Centre

The photosynthetic machinery of the thylakoid membrane is organised into two distinct photosystems, Photosystem I (PS I) and Photosystem II (PS II), and each occupies a different physical location within the membrane system: PS I sits in the unstacked region of the thylakoid membrane, facing the chloroplast stroma, while PS II sits in the stacked (grana) region, facing the thylakoid lumen instead. Each photosystem is built from two functional parts. The core complex (CC) houses the actual reaction-centre pigment together with its associated proteins and its immediate electron donor and acceptor molecules - this is the site where captured light energy is genuinely converted into a chemical electron-transfer event. The light-harvesting complex (LHC), or antenna complex, is built from a larger pool of accessory pigment molecules - additional chlorophylls, carotenoids and xanthophylls - whose only function is to intercept photons over a broad area and funnel the resulting excitation energy inward to the core complex's reaction centre, rather than carrying out any electron chemistry themselves. In PS I, the core complex (CC I) is built around the reaction centre P700, paired with the antenna complex LHC I; in PS II, the core complex (CC II) is built around the reaction centre P680, paired with the antenna complex LHC II. Beyond their different reaction centres and locations, PS I and PS II differ in several other functional respects, summarised in Table 13.2: PS I participates in both cyclic and non-cyclic electron transport, plays no role in water photolysis or oxygen evolution, and receives its electrons from PS II during non-cyclic transport, whereas PS II participat …