Photosystems

In energy terms, carbon reduction and water oxidation are separated by about 1.2 V. This means that electrons have to move uphill against an energy gradient of 1.2 V in order to reduce CO2 to carbohydrates. This unique process is driven by two inputs of light energy at huge pigment-protein complexes termed photosystems, which span the thylakoid membrane. The thylakoid itself is now known to contain two functional regions, namely appressed and non-appressed membranes (Figure 5.1). The appressed region shows close membrane interaction and is enriched in Photosystem II (PS II) and the non-appressed region contains stroma-exposed areas and is enriched in Photosystem I (PS I) and the ATP synthase complex. The relative proportions of appressed versus non- appressed regions depends on the chemical environment of the chloroplast, and is very sensitive to available photon flux density. Thus, leaves growing in shade conditions will contain chloroplasts with more appressed areas (i.e. granal stacks), whereas leaves in full sun will exhibit

Herbicides and Plant Physiology, Second Edition Andrew H. Cobb and John P.H. Reade © 2010 A.H. Cobb and J.P.H. Reade. ISBN: 978-1-405-12935-0

Figure 5.1 Leaf structure in Galium aparine (cleavers).

A. Leaf arrangement in whorls.

B. Leaf section to show epidermal (e), palisade (p) and mesophyll (m) cells.

C. Palisade cell detail.

D. Cell ultrastructure featuring chloroplast (c), stroma (s), vacuole (v) and mitochondrion (mi).

E. Detail of thylakoids. Appressed thylakoids (a) are enriched in Photosystem II (II) and light-harvesting complexes (L); non-appressed thylakoids (na) are enriched in Photosystem I (I) and ATP synthase (as).

Figure 5.1 Leaf structure in Galium aparine (cleavers).

A. Leaf arrangement in whorls.

B. Leaf section to show epidermal (e), palisade (p) and mesophyll (m) cells.

C. Palisade cell detail.

D. Cell ultrastructure featuring chloroplast (c), stroma (s), vacuole (v) and mitochondrion (mi).

E. Detail of thylakoids. Appressed thylakoids (a) are enriched in Photosystem II (II) and light-harvesting complexes (L); non-appressed thylakoids (na) are enriched in Photosystem I (I) and ATP synthase (as).

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 high concentrations of electroneutral galactolipids, and this property permits the movement of electron carriers, such as plastoquinone, and light-harvesting complexes, from appressed to 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 electron is 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 oxidising side of PS II passes electrons through a series of carriers of lower reducing power to PS I. 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 excitation energy transferred to the PS II reaction centre (p680), a charge separation occurs to p680+ and p680-. The species p680+ is quenched by an electron from water via a tyrosine residue on the D1 protein, and phaeophytin, a chlorophyll molecule lacking magnesium, is the primary electron acceptor from p680-. The first stable electron acceptor is a quinone, QA, tightly bound within a particular protein environment. QA acts as a single electron carrier and is closely associated with the two-electron carrier, QB, located on the D1 protein, which delivers pairs of electrons to the mobile plastoquinone pool (Figure 5.2). Photosystem II may therefore be regarded as a water-plastoquinone oxidoreductase. Electrons then pass via a cytochrome b6 - f complex to the copper-containing protein plastocyanin 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 electron carriers A0 and A1 , Light excitation causes charge separation at p700 and A-, a specific monomeric 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 reduces NADP+ via ferredoxin (Figure 5.3- . Thus PS I operates as a plastocyanin-ferredoxin oxidoredutase.

Detailed structures are now known of the light-harvesting and photosystem complexes and the electron transport chain components in the thylakoid membrane. Indeed, some have actually been crystallised and their three-dimensional structures established. Figure 5.4 gives a schematic version of the thylakoid, demonstrating H+ and e- flow. The reader is referred to Blankenship (2002) and Lawlor (2001) for more details.

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