Chloroplasts lamellar membranes

The values of A,max for Chi a in vivo result from interactions between a chlorophyll molecule and the surrounding molecules, such as the proteins and the lipids in the chloroplast lamellar membranes (Fig. 1-10) as well as adjacent water molecules. Indeed, all Chi a molecules are associated with proteins in chlorophyll-protein complexes. Hydrophobic interactions among... 

Figure 6-6. Proton energy differences across chloroplast lamellar membranes and the mitochondrial inner membrane for various ApH s and AZs s. Data are for 25°C and are calculated using Equation 6.17c.

In summary, the site of oleate is most likely the endoplasmic reticulum. This organelle contains, in addition, all the enzymes involved in phospholipid biosynthesis. Whether or not a specific polar lipid or an acyl-CoA or an acyl-ACP is directly or indirectly involved for linoleic synthesis remains for further investigations to clarify. There is indirect evidence suggesting that the monogalactosyldiglyceride in the outer envelope of the chloroplast may be involved in the conversion of linoleic acid to linolenic acid. Once linoleic acid and linolenate are formed, these acyl moieties must be transported to their specific sites. In the leaf cell, the principal site of these acids is the chloroplast lamellar membrane. At present, there is no direct evidence for the occurrence of polyunsaturation in these specific membranes or even in chloroplasts themselves (see Table II). Thus these acyl moieties must be presumably transferred directly or indirectly to their final site from their synthesizing site (see Section III for a discussion of transport mechanisms). 

Carotenoids involved in photosynthesis are bound to and help stabilize chlorophyll-protein complexes, of which various types occur in the lamellar membranes of chloroplasts (Fig. 1-10). Carotenoids also are found in organelles known as chromoplasts, which are about the size of chloroplasts and are often derived from them. For instance, lycopene (red) is in tomato fruit chromoplasts, and a- and pi-carotenes (orange) occur in carrot root chromoplasts. A great diversity of carotenoids occurs in the chromoplasts of flower petals, which is important for attracting pollinators, and fruits, which aids in seed dispersal by attracting other animals. 

The probability for resonance transfer of electronic excitation decreases as the distance between the two molecules increases. If chlorophyll molecules were uniformly distributed in three dimensions in the lamellar membranes of chloroplasts (Fig. 1-10), they would have acenter-to-center spacing of approximately 2 nm, an intermolecular distance over which resonance transfer of excitation can readily occur (resonance transfer is effective up to about 10 nm for chlorophyll). Thus both the spectral properties of chlorophyll and its spacing in the lamellar membranes of chloroplasts are conducive to an efficient migration of excitation from molecule to molecule by resonance transfer. 

Like chlorophyll, plastoquinone A has a nonpolar terpenoid or isoprenoid tail, which can stabilize the molecule at the proper location in the lamellar membranes of chloroplasts via hydrophobic reactions with other membrane components. When donating or accepting electrons, plastoquinones have characteristic absorption changes in the UV near 250 to 260, 290, and 320 nm that can be monitored to study their electron transfer reactions. (Plastoquinone refers to a quinone found in a plastid such as a chloroplast these quinones have various numbers of isoprenoid residues, such as nine for plastoquinone A, the most common plastoquinone in higher plants see above.) The plastoquinones involved in photosynthetic electron transport are divided into two categories (1) the two plastoquinones that rapidly receive single electrons from Peso (Qa and Qb) and (2) a mobile group or pool of about 10 plastoquinones that subsequently receives two electrons (plus two H+ s) from QB (all of these quinones occur in the lamellar membranes see Table 5-3). From the plastoquinone pool, electrons move to the cytochrome b f complex. 

The chlorophyll-protein complexes are oriented in the lamellar membranes in such a way that the electron transfer steps at the reaction centers lead to an outward movement of electrons. For instance, the electron donated by Photosystem II moves from the lumen side to the stromal side of a thylakoid (see Figs. 1-10 and 5-19). The electron that is donated back to the trap chi (Pgg0) comes from H20, leading to the evolution of 02 by Photosystem II (Eq. 5.8). The 02 and the H+ from this reaction are released inside the thylakoid (Fig. 5-19). Because 02 is a small neutral molecule, it readily diffuses out across the lamellar membranes into the chloroplast stroma. However, the proton (H+) carries a charge and hence has a low partition coefficient (Chapter 1, Section 1.4A) for the membrane, so it does not readily move out of the thylakoid lumen. 

Although the ratio of reduced to oxidized forms of species j affects its redox potential [Ej = EfH - (RT/qF)ln(reduced))(oxidized)) Eq. 6.9], the actual activities of the two forms are usually not known in vivo. Moreover, the value of the local pH (which can affect h7) is also usually not known. Consequently, midpoint (standard) redox potentials determined at pH 7 are usually compared to predict the direction for spontaneous electron flow in the lamellar membranes of chloroplasts. We will assume that free energy is required to transfer electrons to a compound with a more negative midpoint redox potential, whereas electrons spontaneously flow toward higher midpoint redox potentials. 

In the previous chapter we indicated that the components involved with electron flow are situated in the lamellar membranes of chloroplasts such that they lead to a vectorial or unidirectional movement of electrons and protons (see Fig. 5-19). We now return to this theme and focus on the gradients in H+ (protons) thus created. In the light, the difference in the chemical potential of H+ from the inside to the outside of a thylakoid acts as the energy source to drive photophosphorylation. This was first clearly recognized in the 1960s by Peter Mitchell, who received the 1978 Nobel Prize in chemistry for his enunciation of what has become known as the chemiosmotic hypothesis for interpreting the relationship among electron flow, proton movements, and ATP formation. 

The electrical term in the chemical potential of H+ can also power ATP formation. For instance, when an EM of 0.16 V is artificially created across lamellar membranes, ATP formation can be induced in the dark. This is consistent with our prediction that an electrical potential difference of at least 0.13 V is necessary (Fig. 6-6). In chloroplast thylakoids, EM in the light is fairly low, e.g., near 0.02 V in the steady state (see Fig. 6-5). However, the electrical term can be the main contributor to A/xh for the first 1 or 2 seconds after chloroplasts are exposed to a high photosynthetic photon flux (PPF). The electrical component of the H+ chemical potential difference can be large for the chromatophores of certain photosynthetic bacteria such as Rhodopseudomonas spheroides, for which Em can be 0.20 V in the light in the steady state. 

Most of the components involved in electron transport in mitochondria are contained in four supramolecular protein complexes that traverse the inner mitochondrial membrane. Complex I, which contains FMN and various iron-sulfur clusters as active sites, transfers electrons from NADH to ubiquinone (Fig. 6-8). Complex II, which contains FAD, various iron-sulfur clusters, and a Cyt >, transfers electrons from succinate also to a ubiquinone. Ubiquinone functions as a pool of two-electron carriers, analogous to the function of plastoquinone A in the lamellar membranes of chloroplasts, which accepts electrons from Complexes I and II and delivers them to the... 

Fig. 2. Electron micrograph of spinach lamellae with quantasomes. Lamellae were prepared from broken spinach chloroplasts according to the method of Park and Pon (1961). After air-drying of the lamellae on a screen, the preparation was chromium-shadowed, and its image in the electron microscope was photographed. The smooth layer may be predominantly lipid material on one surface of the top lamellar membrane. Where the top membrane has been torn away, the inner side of the next, opposing membrane, consisting of an almost crystalline array of quantasomal particles, is revealed. By close inspection of individual quantasomes, one can see what appears to be substructure. Permission to publish this photograph was kindly granted by Professor Roderic B. Park.

Recently, Kannangara and Jenson (1975) elegantly showed that [ CJbio-tin, when added to aseptically germinating barley embryos, was rapidly incorporated into the lamellar membranes of chloroplasts from newly formed leaves. 

Kannangara and Stumpf (1973) have observed that all isolated chloro-plasts, obtained from leaf tissue of a large number of plants, have BCCP associated with the lamellar membranes. In general, then, functional biotin in the chloroplast is membrane-bound, although both biotin carboxylase and transcarboxylase, which catalyze the formation of COj—biotin and utilization of the charged biotin, are in the stroma phase (see Fig. 1). 

The problem of the adequate determination of internal pH is complicated in the heterogeneous populations of vesicles. Chloroplasts are a typical example of such systems. According to electron microscopy data, there are at least two morphologically different membrane systems stroma- and grana-exposed thylakoids (see the sketch in Fig. 3.7). The numerous data in the last two decades brought evidence of the lateral heterogeneity of the chloroplasts lamellar system (see references [60-69]). Stroma- and grana-exposed thylakoids differ with respect to the content of the electron transport and the ATP-synthase complexes. 

Below we consider the evidence that the structural and functional heterogeneity of a chloroplast lamellar system may be the reason for different pH gradients across the membranes of grana- and stroma-exposed thylakoids. 

The phase structure of total polar lipid extracts of chloroplast thylakoid membranes when dispersed in physiological salt solutions is characterised by non-lamellar arrangements of lipid C4D. This contrasts markedly with the general belief that the dominant form of the lipids in the photosynthetic membrane is in a bilayer configuration. The conclusion that must be drawn from this is that the interation of the different polar lipids with the other membrane components imposes a bilayer structure and prevents phase separation of non-bilayer forming lipids into separate domains within the membrane. 

Ji TH, Hess JL and Benson AA (1968) Studies on chloroplast membrane structure. I.Association of pigments with chloroplast lamellar protein, Biochim.Biophys.Acta 150, 676-685. 

Biophysical chemical studies have suggested that the thylakoid membrane of the chloroplast consists of an ultrathln layer of lipids (presumably a lipid bllayer) with sorbed proteins nd pigments organized in a lamellar structure approximately 100 A thick (Figure 4). Although the precise functions of the thylakoid membrane are still obscure, it is believed that the membrane is the locus of the primary photophyslcal and photochemical processes. 

In addition to the assays described in this section, other assays involving the separation of substrates have been used to quantitate lipid transfer activity. These assays differ primarily in their choice of substrates and the method of separation. Additional biological membranes, which have been used to quantitate transfer activity, include chloroplasts (Tanaka et al., 1980), retinol rod disc membranes (Dudley and Anderson, 1978), intra-cytoplasmic membrane derived from Rhodopseudomonas sphaeroides (Cohen et al., 1979), myelin (Ruenwongsa et al., 1979), and lamellar bodies (Tsao, 1979). Biological membranes have been isolated by sucrose... 

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