Photosystem transport

Electron Transport Between Photosystem I and Photosystem II Inhibitors. The interaction between PSI and PSII reaction centers (Fig. 1) depends on the thermodynamically favored transfer of electrons from low redox potential carriers to carriers of higher redox potential. This process serves to communicate reducing equivalents between the two photosystem complexes. Photosynthetic and respiratory membranes of both eukaryotes and prokaryotes contain stmctures that serve to oxidize low potential quinols while reducing high potential metaHoproteins (40). In plant thylakoid membranes, this complex is usually referred to as the cytochrome b /f complex, or plastoquinolplastocyanin oxidoreductase, which oxidizes plastoquinol reduced in PSII and reduces plastocyanin oxidized in PSI (25,41). Some diphenyl ethers, eg, 2,4-dinitrophenyl 2 -iodo-3 -methyl-4 -nitro-6 -isopropylphenyl ether [69311-70-2] (DNP-INT), and the quinone analogues,... 

Some plant HSPs are known to be associated with chloroplasts. Although chloroplasts and mitochondria do not synthesise HSP themselves (Nieto-Sotelo Ho, 1987), certain nuclear encoded HSPs synthesised in the cytosol have been shown to be transported into chloroplasts (Kloppstech et al., 1985 Vierling et al., 1986). The HSP22 of Chlamydomonas is incorporated into the thylakoid membrane without size reduction, while in pea, HSP22 is synthesised as a 26 kDa precursor. In Chlamydomonas, Schuster et al. (1988) have shown that the HSP22 is associated with the photosystem... 

Polymerase responsible for microtubule formation Electron transport system of photosystem 1... 

Unlike the photosynthetic apparatus of photosynthetic bacteria, that of cyanobacteria consits of two photosystems, PS I and II, connected by an electron transport chain. The only chlorophyll present is chlorophyll a, and, therefore, chlorophylls b—d are not of interest in this article. Chlorophyll a is the principal constituent of PS I. Twenty per cent of isolated pigment-protein complexes contain one P700 per 20—30 chlorophyll a molecules the other 80% contain only chlorophyll a20). The physical and chemical properties of chlorophyll a and its role in photosynthesis have recently been described by Meeks77), Mauzerall75), Hoch60), Butler10), and other authors of the Encyclopedia of Plant Physiology NS Vol. 5. 

These results were interpreted using the electron pool hypothesis There is an electron pool situated in the linear photosynthetic electron transport chain between photosystems II and I (Fig. 9). A phobic response is triggered by a decrease in the flow rate through the pool. This can be accomplished in two ways ... 

Atrazine enters plants primarily by way of the roots and secondarily by way of the foliage, passively translocated in the xylem with the transpiration stream, and accumulates in the apical meristems and leaves (Hull 1967 Forney 1980 Reed 1982 Wolf and Jackson 1982). The main phytotoxic effect is the inhibition of photosynthesis by blocking the electron transport during Hill reaction of photosystem II. This blockage leads to inhibitory effects on the synthesis of carbohydrate, a reduction in the carbon pool, and a buildup of carbon dioxide within the leaf, which subsequently causes closure of the stomates, thus inhibiting transpiration (Stevenson et al. 1982 Jachetta et al. 1986 Shabana 1987). 

Electron-transfer sensitization, 19 109 Electron transport, between photosystem inhibitors, 13 288 Electron-transport layer (ETL)... 

Photosynthetic electron flow, 13 287 Photosystem I (PSI), 13 286. See also PSI transport processes Photosystem I inhibitors, 13 286-288 Photosystem II (PSII), 13 286. See also PSII entries... 

Photosystem II inhibitors, 13 288-294 Photosystem inhibitors, electron transport between, 13 288... 

For the formation of one 02 molecule four electrons have to be transferred. This requires a "quantum storage device". In the photosynthetic system of green plants this is achieved with two photosystems that are linked through an electron transport chain, Fig. 10.2, and by means of the thylakoid-membrane that enables the separation of the photoproducts 02 and the reduced form of nicotinamide adenine dinucleotide phosphate, NADPH. 

Ferredoxins of the 2Fe-2S type play a role in the photosynthetic electron transport as an essential electron acceptor of photosystem I. The solution... 

Because photosystem 11 and the cytochrome b/f complex release protons from reduced plastoquinone into the lumen (via a Q. cycle), photosynthetic electron transport establishes an electrochemical gradient across the thylakoid membrane (see p. 126), which is used for ATP synthesis by an ATP synthase. ATP and NADPH+H", which are both needed for the dark reactions, are formed in the stroma. 

The photosynthetic electron transport chain in plants starts in photosystem II (PS 11 see p. 128). PS 11 consists of numerous protein subunits (brown) that contain bound pigments—i.e., dye molecules that are involved in the absorption and transfer of light energy. 

Tetranuclear iron-sulfur clusters are key relay stations in the electron flow in photosynthesis. Photosystem I comprises three subunits, PsaA, PsaB and PsaC. The latter contains two [Fe4S4] centres FA and FB. The core subunits PsaA and B, respectively, house a [Fe4S4] centre denoted FX in addition to other, organic cofactors. The role of this latter cluster was probed in preparations partially devoid of PsaC. It was concluded from the results that FX has a major role in controlling the electron transport through PS I.236 Since the final acceptor of the electrons in PS I is a ferredoxin with a [Fe2S2] cluster it was of interest to study a... 

The transport of proteins into chloroplasts also occurs by more than one mechanism. An SRP-dependent pathway may be needed only for insertion of proteins into membranes.594 Other proteins, among which are the 23-kDa and 16-kDa photosystem II proteins (Chapter 23), enter by a pathway related to the Tat pathway of bacteria. In thylakoids this pathway is directly dependent upon the large pH difference (A pH) across die thylakoid membrane. In contrast to the bacterial Sec pathway, the A pH pathway seems to be able to transport completely folded proteins. 

When electrons flow from photosystem I to photosystem II, protons are transported across the chloroplast membranes as indicated in Figure E9.1. This aspect of photosynthesis will be discussed in a later section. 

The primary process of photosynthesis (in both photosystems) is an electron transfer reaction from the electronically excited chlorophyll molecule to an electron acceptor, which is in most cases a quinone. This primary electron acceptor can then hand over its extra electron to other, lower energy, acceptors in electron transport chains which can be used to build up other molecules needed by the organism (in particular adenosine triphosphate ATP). The complete process of photosynthesis is therefore much... 

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