Chlorophyll-containing Membranes

In System 19 of Table 1 the transmembrane transport of electrons was ascribed to the electron exchange reaction  

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Membranes. Various types of chlorophyll-containing membranes have been designed for photoenergy conversion primarily on the basis of the Idea that photosynthesis Is an electronic process. The process assumes that two-dimensional chlorophyll arrays are photo-conductive. The absorption of a photon promotes one electron Into the conduction band and leaves one hole In the valence band. The excited electrons and holes are free to move around. They are also considered to be spread out over the whole unit although their lifetimes and mobilities may be different. In the second scheme. It Is assumed that the electrons can be transferred to an electron acceptor which Is thereby effectively reduced, and the holes can be transferred to an electron donor which Is thereby effectively oxldlzed. 

Chlorophyll-containing Membrane on an Electrode Surface. Photoelectrochemical and photovoltaic effects have been noted with a variety of chlorophyll assemblies deposited on metal or semiconductor electrode surfaces (Figure 6). 

THE MODIFICATION BY BILIPROTEINS OF INTENSITY AND DIRECTION OF ELECTRON FLOW ACROSS CHLOROPHYLL-CONTAINING MEMBRANES... 

Plastids are organelles of plant cells that serve a variety of purposes.45 Most important are the chloroplasts, the chlorophyll-containing sites of photosynthesis. Like mitochondria they contain folded internal membranes (see Fig. 23-19) and several small molecules of DNA. 

For the systems of A // Chi // D type the mechanism, schematically represented in Fig. 4a, has been suggested. Later on, Ford and Tollin [43, 44] have comprehensively examined the kinetics and mechanism of PET across the Chl-containing membranes. They have demonstrated that only the triplet-excited chlorophyll is involved in PET, as shown in Fig. 4a. 

Photosynthetic eubacteria are classified as filamentous, green sulfur, gram-positive linked, purple, and cyanobacteria. All contain membrane-bound RCs in which (B)Chl serves as the primary electron donor. The RCs may be divided into two main types RC-1, in which the initial electron acceptor is a (B)Chl molecule and the secondary acceptor is an Fe-S center, and RC-2, in which the initial acceptor is a (B)Ph molecule and the secondary acceptor is a quinone. RC-1 centers are found in green sulfur and gram-positive linked bacteria, while RC-2 centers are found in filamentous bacteria and purple bacteria. Cyanobacteria contain both RC-1 and RC-2 centers in which the chlorophyll is Chi a. BChl a is found in filamentous, green sulfur and purple bacteria, while BChl g is characteristic of the grampositive line. BChl b is found in certain purple bacteria instead of BChl a. 

Stage 1 Absorption of Light The initial step in photosynthesis is the absorption of light by chlorophylls attached to proteins in the thylakoid membranes. Like the heme component of cytochromes, chlorophylls consist of a porphyrin ring attached to a long hydrocarbon side chain (Figure 8-31). In contrast to the hemes, chlorophylls contain a central... 

The nucleophilic reactivity of cysteine has been exploited in Michael reactions with quinones. One example is a water-soluble naphthoquinone, which has been entrapped in chlorophyll-containing vesicles in order to study light-induced electron transfer through a membrane from glutathione to the quinone (Fore, 1983). Another example is an asymmetrical vesicle membrane made of a cysteine quinone carboxylate bolaamphiphile, where all the quinone is localized on the outer surface of the vesicle (see Scheme 7.2.6 Scheme 9.5.1). 

The specific surface o is defined as the area of one surface of the thylakoid membrane per total chlorophyll contained therein. By means of this quantity the specific volume of the membrane from its thickness or the average distance between the plane thylakoid membranes from specific volumes can be estimated. Such distances are required for the computation of the electrical potential profile across thylakoid stacks (If 2). The specific surface has not yet been measured but only inferred from indirect evidence. The value most frequently used is 1.5 m /yumol as estimated by Barber (3). 

FIGURE 3. The effect of cytochrome c concentration on the rate of its photooxidation in continuous light. The reaction medium contained membranes equivalent to 10 yg chlorophyll/ml (80 nM P700), 25 yg/ml superoxide dismutase,... 

The interiors of rhodopseudomonad bacteria are filled with photosynthetic vesicles, which are hollow, membrane-enveloped spheres. The photosynthetic reaction centers are embedded in the membrane of these vesicles. One end of the protein complex faces the Inside of the vesicle, which is known as the periplasmic side the other end faces the cytoplasm of the cell. Around each reaction center there are about 100 small membrane proteins, the antenna pigment protein molecules, which will be described later in this chapter. Each of these contains several bound chlorophyll molecules that catch photons over a wide area and funnel them to the reaction center. By this arrangement the reaction center can utilize about 300 times more photons than those that directly strike the special pair of chlorophyll molecules at the heart of the reaction center. 

The structure of the LH2 complex of R. acidophila is both simple and elegant (Figure 12.17). It is a ring of nine identical units, each containing an a and a P polypeptide of 53 and 41 residues, respectively, which both span the membrane once as a helices (Figure 12.18). The two polypeptides bind a total of three chlorophyll molecules and two carotenoids. The nine heterodimeric units form a hollow cylinder with the a chains forming the inner wall and the P chains the outer wall. The hole in the middle of the cylinder is empty, except for lipid molecules from the membrane. 

Detergent treatment of a suspension of thylakoids dissolves the membranes, releasing complexes containing both chlorophyll and protein. These chlorophyll-protein complexes represent integral components of the thylakoid membrane, and their organization reflects their roles as either light-harvesting com-... 

The photosynthetic apparatus in green plants and algae is located in the chloroplast, which is a flattened, double-membraned structure about 150-200 A thick/4,5 The two flat membranes lie one above the other and are united at their peripheries. These double-membraned structures have been termed thylakoids (from the Greek sacklike )/ Each membrane of the thylakoid consists of a water-insoluble lipoprotein complex which contains the light-absorbing chlorophyll and other pigments utilized in photosynthesis. 

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