Light harvesting excitation and electron capture

Light is the visible part of the spectrum of electromagnetic radiation emanating from the sun. Electromagnetic energy occurs in indivisible units, called quanta, that travel along sinusoidal trajectories, at a velocity (in air) of c ~ 3 x 108 m s-1. The wavelengths of the quanta define their properties - those with wavelengths (X) between 400 and 700 nm (400 - 700 x 10-9 m) correspond with the visible wavelengths we call light (and within which waveband the quanta are called photons). The waveband of photosynthetically active radiation (PAR) coincides almost exactly with that of light. The white light of the visible spectrum is the aggregate of the flux of photons of differing wavelengths, ranging from the shorter (blue) to the longer (red) parts of the spectrum.

Relative to the solar constant (see Section 2.2.2), the PAR waveband represents some

46-48% of the total quantum flux. The corresponding photon flux density averages 1.77 x 1021 m-2 s-1. Division by the Avogadro number (1 mol = 6.023 x 1023 photons) expresses the maximum flux in the more customary units, einsteins or mols, 2.94 mmol photon m-2 s-1. The energy of a single photon, £, varies with the wavelength, e = h'c /X (3.2)

where h' is Planck's constant, having the value 6.63 x 10-34Js (e.g. Kirk, 1994). Photons at the red end of the PAR spectrum each contain about 2.84 x 10-19J, about 57% of the content of blue-light photons (4.97 x 10-19J).

While a given radiation flux of light of a single wavelength can be readily expressed in Js-1 (and vice versa), precise conversion across a spectral band does not apply. The approximate relationship proposed by Morel and Smith (1974) for the interconversion of solar radiation in the 400-700 nm band of 2.77 x 1018 quanta s-1 W-1 (equivalent to 3.62 x 10-19J per photon, or 218 kJ per mol photon) has general applicability (Kirk, 1994).

Photosynthesis depends upon the interception and absorption of photons. Both photosystems involve the photosynthetic pigment chlorophyll a (and, where applicable, other chlorophylls), which is characteristically complexed with particular proteins, and certain other pigments in many instances. These are accommodated within structures known as light-harvesting complexes (LHC) and it is these that act as antennae in picking up incoming photons. For instance, the light-harvesting complex of the eukaryotic photochemical system II (LHCII) typically comprises some 200-300 chlorophyll molecules (mostly of chlorophyll a; up to 30% may be of chlorophyll b), the specific chlorophyll-binding proteins and a variable number of xan-thophyll and carotene molecules, to a combined molecular mass of 300-400 kDa (Dau, 1994; Gous-sias et al., 2002). The prokaryotic Cyanobacte-ria lack chlorophyll b and the light-harvesting chlorophyll-proteins of PSII. They rely instead on the phycobiliproteins, assembled in bodies known as phycobilisomes (Grossman et al., 1993; Rudiger, 1994).

At the heart of the eukaryote LHCII is the antennal chlorophyll-protein known here that the reactions of PSII are initiated, when the complex is exposed to light. The energy of a single photon is sufficient to raise a P680 electron from its ground-state to its excited-state orbital. Next to the P680 is the phaeophytin acceptor molecule (usually referred to as 'Phaeo') and the two further acceptor quinones (QA and QB) that comprise the PSII reaction centre. In sequence, this acceptor chain passes the electrons to PSI. The reaction (P680 ^ P680+) is one of the most powerful biological oxidations known to science; the electrons are readily captured by the Phaeo acceptor. In its now-reduced state, Phaeo- in turn activates the QA acceptor: its reduction to Q-stimulates acceptance of the electron by QB.

In this way, the electrons are serially transported towards PSI. Once it has accepted two electrons, Q B dissociates to enter a pool of reduced plastoquinone ('PQ'). Molecules of PQH2 are eventually oxidised by the cytochrome known as b6/f, which carries the electrons to PSI.

The plastoquinone pool functions as a system capacitor, like a sort of surge tank of reductant (D. Walker, 1992; Kolber and Falkowski, 1993), whose activity can be viewed in the context of PSII light harvesting. At quiescence, the entire reaction centre is said to be ' open': P680 is in its reduced state, Phaeo and QA are oxidised. Then, photon excitation of the P680 initiates a flow of electrons to the plastoquinine pool, whence they may be removed as rapidly as PSI can accept them. At the same time, the otherwise uncomplemented positive charge of excited P+80 is balanced by the stripping of electrons from water (that is, P+80 is reduced back to P680). Note that four photochemical reactions are necessary to generate one dioxygen molecule from two molecules of water (2H2O ^ 4H+ + 4e + O2). It is now understood that P+80 is actually reduced through the action of manganese ions, via a redox-active tyrosine (Barber and Nield, 2002). However, until the P+80 molecule is re-reduced, the reaction centre is unable to accept further electrons and it is said to be 'closed'. It remains so until QA is reoxidised.

The light-harvesting complex and reaction centre of PSI are built around an analogous chlorophyll-protein complex (known as P700) and acceptor (usually denoted A). Again, photons excite the equivalent number of P700 electrons to the point where they can be accepted by A. Next in the electron transfer pathway is ferredoxin, transfer of electrons to which reoxidises A- while donation of the equivalent number from the plas-toquinone pool re-reduces P700 molecules. The electrons may be passed from ferredoxin, and beyond PSI, to bring about the reduction of NADP to NADPH that provides power to drive Calvin-cycle carboxylation. The subsequent reactions of NADPH with carbon dioxide are not directly dependent upon the photon flux and can continue in darkness (see Section 3.2.3).

The PSI generation of the carbon-reducing power nevertheless also requires the photosyn-thetic transfer of four electrons per atom of carbon. Under ideal conditions, the light reactions in photosynthesis may be summarised:

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