Photosystem architecture

The electron transfer that this equation represents is readily facilitated by the physical arrangement of the two main components (PSII, PSI) and the intercoupling plastoquinone pool, the b6/f cytochrome complex and, in most algae and plants, the soluble electron carrier plasto-cyanin (in Cyanobacteria, cytochrome c may substitute). The basic architecture and the location of the biochemical functions of the photosyn-thetic units seems to be extremely well conserved among eukaryotic algae, plants and their ancestral cyanobacterial lines. The best-known features were revealed long ago, through light microscopy and early transmission electron microscopy. The granule-like units, comprising LHC antennae and the reaction centres, are strung on proteinaceous membranes, called thylakoids. In the cells of eukaryotes, stacks of thylakoids are contained within one or more separate membrane-bound envelopes, the chromophores (also called plastids or, where they occur in chlorophyte algae and all higher plants, chloroplasts) whose shape and arrangement is often taxon-specific. Cyanobacte-ria lack separate chromophores; the thylakoids

Figure 3.1

Diagram of the configuration of the structure and the flow of excitation energy through the photsystems. Electrons are extracted from water in photosystem II and transported through the quinone cycle and released to photosystem I. Electrons are accepted by ferredoxin, to bring about the reduction of NADP to NADPH that enables the cell to synthesise its molecular components. Redrawn, with permission from Kuhlbrandt (2001).

are rather loosely dispersed through the body of the cell. Apart from anchoring the various transmembrane structures (including, in the case of the Cyanobacteria, the phycobilisomes), the thy-lakoid also maintains a regulatory charge gradient, down which the electrons are passed.

The molecular structure of the energy-harvesting apparatus has become clearer as a result of the recent application of electron crystallography. Since Kuhlbrandt and Wang (1991) published the three-dimensional structure of a light-harvesting complex, other investigative studies have followed, showing, at increasingly fine resolution, the organisation and interlink-ages of the major sub-units of PSII in plants and Cyanobacteria (McDermott et al., 1995; Zouni et al., 2001; Barber and Nield, 2002) and also of PSI (Jordan et al., 2001). The recent overview and model of Fromme et al. (2002) upholds that proposed by Kuhlbrandt et al. (1994) and updates it in several respects.

The cited literature should be consulted for more of the fascinating details of the structures and organisational patterns of light harvesting and the electron-transport chain. Here, we should emphasise the generalised configuration and functional dynamics of the various sub-units involved in photon absorption and electron capture, for it is these which impinge upon their physiological performance and their adaptability to operation under sub-ideal conditions. As will be seen (in Section 3.3), the relevant outputs of an adequate carbon-reducing capacity relate to system performance under ambient light fluxes, how it behaves in poor light (low photon fluxes)

and, equally, its reactions to damagingly high light levels. The relevant ultrastructural and biochemical input parameters concern how much light-harvesting capacity there is present in an alga and how much reductant it can deliver per unit time.

The arrangement and linkage of the photosystems are schematised in Fig. 3.1. The size of the LHCII structures studied by Kuhlbrandt et al. (1994) averaged 13 nm in area and 4.8 nm in thickness. The PSII complexes from Synechococcus measured roughly 19 x 10 nm across and 12 nm thick (Zouni et al., 2001). The LHCI complexes from Synechococcus revealed by Jordan et al. (2001) are apparently of similar size. On the basis of there being 200-300 chlorophyll molecules in a typical LHC, Reynolds (1997a) calculated that 1 g chlorophyll could be organised into 2.2 to 3.4 x 1018 LHCs. Because the area that 1 g of chlorophyll subtends in the light field can be as great as 20 m2 (see Section 3.3.3), each LHC contributes an average photon absorption of up to 10 x 10-18 m-2 (i.e. 10 nm2).

It was also supposed that the photon absorption is in inverse proportion to the product of the area of the LHC and the aggregate time for the electron transport chain to accept photons and clear electrons, ready for the next photon. Kolber and Falkowski (1993) approximated the aggregate time of reactions linking initial excitation (occupying less than 100 fs, or 10-13: Knox, 1977) to the re-oxidation of Q- to be 0.6 ms. The principal rate-limiting step is the onward passage of electrons from the plastoquinone pool, which, depending upon temperature, needed between 2 and 15 ms.

Thus, a single pathway might accommodate up to 66 reactions per second at 0 °C and some 500 s-1 at 30 °C, with a matching carbon-reducing power.

As a rough approximation indicates that, at 30 °C, 1 g chlorophyll containing >2 x 1018 active LHCs has the capacity to deliver >1021 electrons every second and a theoretical potential to reduce more than 1.25 x 1020 atoms of carbon [i.e. > ~200 |imol C (g chla)-1 s-1].

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