The Composition and Architecture of Membranes:- Each Type of Membrane Has Characteristic Lipids and Proteins

The relative proportions of protein and lipid vary with the type of membrane (Table 11–1), reflecting the diversity of biological roles. For example, certain neurons have a myelin sheath, an extended plasma membrane that wraps around the cell many times and acts as a pas sive electrical insulator. The myelin sheath consists primarily of lipids, whereas the plasma membranes of bacteria and the membranes of mitochondria and chloroplasts, the sites of many enzyme-catalyzed processes, contain more protein than lipid (in mass per total mass). For studies of membrane composition, the first task is to isolate a selected membrane. When eukaryotic cells are subjected to mechanical shear, their plasma mem branes are torn and fragmented, releasing cytoplasmic components and membrane-bounded organelles such as mitochondria, chloroplasts, lysosomes, and nuclei. Plasma membrane fragments and intact organelles can be isolated by centrifugal techniques described in Chapter 1 (see Fig. 1–8). Chemical analyses of membranes isolated from various sources reveal certain common properties. Each kingdom, each species, each tissue or cell type, and the organelles of each cell type have a characteristic set of membrane lipids. Plasma membranes, for example, are enriched in cholesterol and contain no detectable cardiolipin (Fig. 11–2); in the inner mitochondrial mem brane of the hepatocyte, this distribution is reversed: very low cholesterol and high cardiolipin. Cardiolipin is essential to the function of certain proteins of the inner mitochondrial membrane. Cells clearly have mechanisms to control the kinds and amounts of membrane lipids they synthesize and to target specific lipids to particular organelles. In many cases, we can surmise the adaptive advantages of distinct combinations of mem brane lipids; in other cases, the functional significance of these combinations is as yet unknown. The protein composition of membranes from different sources varies even more widely than their lipid composition, reflecting functional specialization. In a rod cell of the vertebrate retina, one portion of the cell is highly specialized for the reception of light; more than 90% of the plasma membrane protein in this region is the light-absorbing glycoprotein rhodopsin. The less specialized plasma membrane of the erythrocyte has about 20 prominent types of proteins as well as scores of minor ones; many of these are transporters, each moving a specific solute across the membrane. The plasma membrane of Escherichia coli contains hundreds of different proteins, including transporters and many enzymes involved in energy-conserving metabo lism, lipid synthesis, protein export, and cell division. The outer membrane of E. coli, which encloses the plasma membrane, has a different function (protection) and a different set of proteins. Some membrane proteins are covalently linked to complex arrays of carbohydrate. For example, in glycophorin, a glycoprotein of the erythrocyte plasma membrane, 60% of the mass consists of complex oligosaccharide units covalently attached to specific amino acid residues. Ser, Thr, and Asn residues are the most common points of attachment (see Fig. 7–31). At the other end of the scale is rhodopsin of the rod cell plasma membrane, which contains just one hexasac charide. The sugar moieties of surface glycoproteins influence the folding of the proteins, as well as their stabilities and intracellular destinations, and they play a significant role in the specific binding of ligands to glycoprotein surface receptors (see Fig. 7–37). Some membrane proteins are covalently attached to one or more lipids, which serve as hydrophobic anchors that hold the proteins to the membrane, as we shall see.

FIGURE 11–2 Lipid composition of the plasma membrane and organelle membranes of a rat hepatocyte. The functional specialization of each membrane type is reflected in its unique lipid composition. Cholesterol is prominent in plasma membranes but barely detectable in mitochondrial membranes. Cardiolipin is a major component of the inner mitochondrial membrane but not of the plasma membrane. Phosphatidylserine, phosphatidylinositol, and phosphatidylglycerol are relatively minor components (yellow) of most membranes but serve critical functions; phosphatidylinositol and its derivatives, for example, are important in signal transductions triggered by hormones. Sphingolipids, phosphatidylcholine, and phosphatidylethanolamine are present in most membranes, but in varying proportions. Glycolipids, which are major components of the chloroplast mem branes of plants, are virtually absent from animal cells.

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مواضيع ذات صلة

The Composition and Architecture of Membranes:- All Biological Membranes Share Some Fundamental Properties

Working with Lipids:- Mass Spectrometry Reveals Complete Lipid Structure

Working with Lipids:- Gas-Liquid Chromatography Resolves Mixtures of Volatile Lipid Derivatives

Working with Lipids:- Adsorption Chromatography Separates Lipids of Different Polarity

Working with Lipids:- Lipid Extraction Requires Organic Solvents

Lipids as Signals, Cofactors, and Pigments: -Plants Use Phosphatidylinositols, Steroids, and Eicosanoidlike Compounds in Signaling

Lipids as Signals, Cofactors, and Pigments:- Steroid Hormones Carry Messages between Tissues

Lipids as Signals, Cofactors, and Pigments:- Phosphatidylinositols and Sphingosine Derivatives Act as Intracellular Signals

Structural Lipids in Membranes:- Sterols Have Four Fused Carbon Rings

Structural Lipids in Membranes:- Sphingolipids at Cell Surfaces Are Sites of Biological Recognition

Structural Lipids in Membranes:- Sphingolipids Are Derivatives of Sphingosine

Structural Lipids in Membranes:- Chloroplasts Contain Galactolipids and Sulfolipids