Biomembrane plants

Transporters, particularly those carrying nonlipophilic species across biomembranes or model membranes, can be regarded as vectorial catalysts (and are also called carriers, translocators, permeases, pumps, and ports [e.g., symports and antiports]). Many specialized approaches and techniques have been developed to characterize such systems. This is reflected by the fact that there are currently twenty-three volumes in the Methods in Enzymology series (vols. 21,22,52-56,81,88,96-98,125-127,156-157, 171-174, and 191-192) devoted to biomembranes and their constituent proteins. Chapters in each of these volumes will be of interest to those investigating transport kinetics. Other volumes are devoted to ion channels (207), membrane fusion techniques (220 and 221), lipids (14, 35, 71, and 72), plant cell membranes (148), and a volume on the reconstitution of intracellular transport (219). See Ion Pumps... 

Figure F4.3.2 Absorption spectra of the major carotenoids of the photosynthetic biomembranes of green leaves of higher plants in diethyl ether (pure solvent). The carotenoids were freshly isolated from a pigment extract by TLC following Lichtenthaler and Pfister (1978) and Lichtenthaler (1987). P-C, p-carotene Lut, lutein Neo, neoxanthin Viola, violaxanthin.

Lipophilic compounds will interfere not only with the biomembranes of microbes and herbivores, but also with those of the producing plant. In order to avoid autotoxicity, plants cannot store these compounds in the vacuole but usually sequester them on the cuticle, in dead resin ducts or cells, which are lined not by a biomembrane but by an impermeable solid barrier (Fig. 1.5). In some cases, the compounds are combined with a polar molecule, so that they can be stored as more hydrophilic chemicals in the vacuole. 

Genes of protoberberine biosynthesis are abundantly expressed in rhizomes of Thalictrum flavum, but were also active in roots and other organs (Samanani et al. 2005). In roots, transcripts were localized in the immature endodermis and root pericycle. In rhizomes transcripts were found in the protoderm of leaf primordial. As known from other plants, these data show that the sites of synthesis are not identical with the sites of accumulation. In many instances, a long-distance transport must occur. If this is the case, alkaloids have to pass several biomembranes. ABC-transporters and H+-alkaloid antiporters can be involved (see Chapter 1). 

Even though cholesterol (substituted by other sterols in plants) is chemically classified as a lipid, the molecule is better grouped it into a special category, which is distinct from membrane proteins and (phospho- and sphingo-) lipids, when discussing its role in biomembrane stmcture and function. Cholesterol constitutes between 25% and 40% of the total lipid plus cholesterol fraction of most typical cell membranes. These numbers translate into a 33-66% fraction of the noncholesterol lipids, or considering its smaller size, about a 10-20%... 

Cholesterol is the main sterol in tissues of animal origin tissues. Plant cells or bacteria on the other hand do not contain cholesterol in their biomembranes. However, they produce other sterols such as sitosterol, stigmasterol, campesterol etc, while lanosterol and ergosterol are predominant in bacteria. Human skin contains derivatives of cho e.stcrol and cholesterol sulfate has been identified as the main cholesterol... 

The influence of plant sterols on the phase properties of phospholipid bilayers has been studied by differential scanning calorimetry and X-ray diffraction [206]. It is interesting that the phase transition of dipalmitoylglycerophosphocholine was eliminated by plant sterols at a concentration of about 33 mole%, as found for cholesterol in animal cell membranes. However, less effective modulation of lipid bilayer permeability by plant sterols as compared with cholesterol has been reported. The molecular evolution of biomembranes has received some consideration [207-209]. In his speculation on the evolution of sterols, Bloch [207] has suggested that in the prebiotic atmosphere chemical evolution of the sterol pathway if it did indeed occur, must have stopped at the stage of squalene because of lack of molecular oxygen, an obligatory electron acceptor in the biosynthetic pathway of sterols . Thus, cholesterol is absent from anaerobic bacteria (procaryotes). 

Phosphatidylcholines (PC), (l,2-diacyl-s -glycerol-3-phosphoryl-cholinc) are predominant in animal cell membranes [7]. They are often called lecithins (Greek name "lekilhos the egg yolk) and their content in the biomembranes or animal tissues is very high. They are the main lipid compounds of plant membranes but their proportion in bacterial... 

It appears evident therefore that the main function of most nutrient elements is as a constituent of the enzymes required to build up the organic matter within plants. Many such reactions run side by side in cells and tissues, and this is made possible by the presence of biomembranes that allow the build-up and decomposition of compounds, without mixing the components. Biomembranes subdivide cells into reaction spaces (e.g., nucleus, plastids, mitochondria, ribosomes, vacuoles, cytosol), and they permit well-ordered substance exchange between the compartments. Such processes are also responsible for ion uptake by root cells from the soil solution. Despite certain differences, all biomembranes have a similar chemical structure, the basic components being double lamellae of P-containing lipids (Figure 2.2) such as phosphatidylserine and glycolipids (Table 2.4). Proteins are movably incorporated into these double lamellae (see... 

I 2 Essential and Toxic Effects of Macro-, Trace and Ultratrace Elements for Higher Plants Tab. 2.4 Some components of biomembranes... 

Infrared spectroscopy has been used to study quite a wide range of biological systems and a number of these are examined in this chapter in order to illustrate the various approaches to the use of infrared spectroscopy in the life sciences. These systems include proteins, peptides, lipid, biomembranes, animal tissues, carbohydrates, pharmaceuticals, foods, and plant materials. 

Lipid-soluble metal complexes such as copper xanthates (from mineral flotation plants), copper 8-hydroxyquinolinate (agricultural fungicide) or alkyl-mercury compounds are particularly toxic forms of heavy metals because they diffuse rapidly through a biomembrane and carry both metal and ligand into the cell. ... 

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