proportionately more non - appressed regions and a decreased thylakoid to stroma ratio.Such changes in thylakoid architecture can occur within hours and suggest that this membrane is particularly fluid in nature. Indeed, thylakoids are about 50% lipid with highconcentrations of electroneutral galactolipids, and this property permits the movement ofelectron carriers, such as plastoquinone, and light - harvesting complexes, from appressedto non - appressed areas.Each photosystem contains specific polypeptides, pigments and electron donors/acceptors, and a unique chlorophyll termed a reaction centre (p680 or p700) at which an electronis moved from low to higher energy. Functionally, the two photosystems operate in series,such that the primary reductant generated from the photolysis of water at the oxidisingside of PS II passes electrons through a series of carriers of lower reducing power to PSI. Here a second light reaction transfers electrons to their eventual natural acceptor,NADP+.When a photon of light is absorbed by the light - harvesting complex and the excitationenergy transferred to the PS II reaction centre (p680), a charge separation occurs top680+ and p680−. The species p680+ is quenched by an electron from water via a tyrosineresidue on the D1 protein, and phaeophytin, a chlorophyll molecule lacking magnesium,is the primary electron acceptor from p680−. The first stable electron acceptor is aquinone, QA, tightly bound within a particular protein environment. QA acts as a singleelectron carrier and is closely associated with the two - electron carrier, QB, located onthe D1 protein, which delivers pairs of electrons to the mobile plastoquinone pool (Figure5.2 ). Photosystem II may therefore be regarded as a water - plastoquinone oxidoreductase.Electrons then pass via a cytochrome b6 – f complex to the copper - containing proteinplastocyanin and to PS I.The PS I complex contains the reaction centre chlorophyll p700, comprising iron –sulphur centres which act as electron acceptors, several polypeptides and the electroncarriers A0 and A1. Light excitation causes charge separation at p700 and A0, a specificmonomeric chlorophyll a, is the first acceptor. A1 is thought to be a phylloquinone(vitamin k1) which donates an electron to the iron – sulphur centres and hence reducesNADP+ via ferredoxin (Figure 5.3 ). Thus PS I operates as a plastocyanin – ferredoxinoxidoredutase.Detailed structures are now known of the light - harvesting and photosystem complexesand the electron transport chain components in the thylakoid membrane. Indeed, somehave actually been crystallised and their three- dimensional structures established. Figure5.4 gives a schematic version of the thylakoid, demonstrating H+ and e− flow. The readeris referred to Blankenship (2002) and Lawlor (2001) for more details