Essential membrane structure

In biological systems, one often observes membrane structures with nonzero spontaneous curvatures, e.g. in mitochondria. This type of bilayer structure is also essential in various transport related processes such as endo- and exocy-tosis (see Chapter 8 of this volume). These curved membrane systems may be stabilised by protein aggregation in the bilayer, or may be the result of the fact that biological membranes are constantly kept off-equilibrium by lipid transport and/or by (active) transport processes across the bilayer. These interesting... 

The formalism sketched above has been used in the literature in more or less the same detail by many authors [87-92]. The predicted membrane structure that follows from this strategy has one essential problem the main gel-to-liquid phase transition known to occur in lipid membranes is not recovered. It is interesting to note that one of the first computer models of the bilayer membrane by Marcelja [93] did feature a first-order phase transition. This author included nematic-like interactions between the acyl tail, similar to that used in liquid crystals. This approach was abandoned for modelling membranes in later studies, because this transition was (unfortunately) lost when the molecules were described in more detail [87]. 

In spite of the variety of appearances of eukaryotic cells, their intracellular structures are essentially the same. Because of their extensive internal membrane structure, however, the problem of precise protein sorting for eukaryotic cells becomes much more difficult than that for bacteria. Figure 4 schematically illustrates this situation. There are various membrane-bound compartments within the cell. Such compartments are called organelles. Besides the plasma membrane, a typical animal cell has the nucleus, the mitochondrion (which has two membranes see Fig. 6), the peroxisome, the ER, the Golgi apparatus, the lysosome, and the endosome, among others. As for the Golgi apparatus, there are more precise distinctions between the cis, medial, and trans cisternae, and the TGN trans Golgi network) (see Fig. 8). In typical plant cells, the chloroplast (which has three membranes see Fig. 7) and the cell wall are added, and the lysosome is replaced with the vacuole. 

Figure 5.1 The structure of a glycerophospholipid. A simple diagram showing the charges on the head group. In this struction, palmitic and oleic acids, provide the hydrophobic component of the phospholipids and choline (and four bases) and the phosphate group provide the hydrophilic head. The unsaturated fatty acid, oleic acid, provides a kink in the structure and therefore some flexibility in the membrane structure which allows for fluidity. The more unsaturated the fatty acid, the larger is the kink and hence more fluidity in the membrane. Cholesterol molecules can fill the gaps left by the kink and hence reduce flexibility. Hydroxyl groups on the bases marked are those that form phosphoester links. Choline and inositol may sometimes be deficient in the diet so that they are, possibly, essential micronutrients (Chapter 15).

Lead Inorganic lead oxides and salts Gastrointestinal, respiratory Soft tissues redistributed to skeleton (> 90% of adult body burden) CNS deficits peripheral neuropathy anemia nephropathy hypertension reproductive toxicity Inhibits enzymes interferes with essential cations alters membrane structure Renal (major) feces and breast milk (minor)... 

Hou, S.Y., et al. 1991. Membrane structures in normal and essential fatty acid-deficient stratum corneum Characterization by ruthenium tetroxide staining and x-ray diffraction. J Invest Dermatol 96 215. 

Because erythrocytes (red blood cells) do not contain any subcellular organelles (they are essentially a membranous sac for carrying hemoglobin) their plasma membrane is a convenient model system for studies of membrane structure as it can readily be isolated from other membranes and intracellular components. One of the major glycoproteins in the plasma membrane of erythrocytes is glycophorin A a 131 amino acid protein that was the first integral protein to be sequenced (see Topic B9). This revealed that the polypeptide chain of glycophorin consists of three domains ... 

Lipids are characterized by a predominantly hydrocarbon structure, are very soluble in organic solvents and sparingly soluble in water, and have physical properties that are in agreement with their hydrophobicity. Lipids are divided into several classes or families, some having a polar part they have important biological functions besides being essential membrane components. Links with other molecules are via covalent bonding or van der Waals forces. 

Fats and oils have major roles in human nutrition. They are concentrated dietary sources of energy, providing approximately 9 kcal/g when metabolized compared with 4 kcal/g for carbohydrates and proteins, and account for about 36 percent of domestic caloric intake per capita.19 Dietary lipids also can provide essential molecular structures that are synthesized by the body into compounds required for selective functioning of cell membranes and regulation of life processes. 

The conclusion that mitochondria may be essentially inactive in the intracellular Ca " homeostasis during normal conditions, and only become activated when emergency situations must be controlled (or, possibly, in response to pharmacological influences), may sound excessively negative. Conceptually, it would seem difficult to justify the development of such a sophisticated machinery as that for the transport of Ca " in and out of mitochondria if its use would only be limited to the improbable cases where cytosolic Ca is allowed to fluctuate widely out of the normal limits. It may well be, however, that the primary reason for the development of the systems that constitute the Ca cycle of mitochondria has not been the control of cytosolic Ca under normal physiological conditions. The kinetic limitations of the mitochondrial system, and the existence in cells of other membrane structures capable of transporting Ca back and forth more efficiently than mitochondria under normal in situ conditions, are undisputable facts. Even the role... 

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