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

Membranes are impermeable to most polar or charged solutes, but permeable to nonpolar compounds; they are 5 to 8 nm (50 to 80 Å) thick and appear trilaminar when viewed in cross section with the electron microscope (Fig. 11–1). The combined evidence from electron microscopy and studies of chemical composition, as well as physical studies of permeability and the motion of in dividual protein and lipid molecules within membranes, led to the development of the fluid mosaic model for the structure of biological membranes (Fig. 11–3). Phospholipids form a bilayer in which the nonpolar re gions of the lipid molecules in each layer face the core of the bilayer and their polar head groups face outward, interacting with the aqueous phase on either side. Proteins are embedded in this bilayer sheet, held by hydrophobic interactions between the membrane lipids and hydrophobic domains in the proteins. Some proteins protrude from only one side of the membrane; others have domains exposed on both sides. The orientation of proteins in the bilayer is asymmetric, giving the mem brane “sidedness”: the protein domains exposed on one side of the bilayer are different from those exposed on the other side, reflecting functional asymmetry. The in dividual lipid and protein units in a membrane form a fluid mosaic with a pattern that, unlike a mosaic of ceramic tile and mortar, is free to change constantly. The membrane mosaic is fluid because most of the interactions among its components are noncovalent, leaving individual lipid and protein molecules free to move laterally in the plane of the membrane. We now look at some of these features of the fluid mosaic model in more detail and consider the experi mental evidence that supports the basic model but has necessitated its refinement in several ways.

FIGURE 11–1 Biological membranes. Viewed in cross section, all cell membranes share a characteristic trilaminar appearance. When an erythrocyte is stained with osmium tetroxide and viewed with an electron microscope, the plasma membrane appears as a three-layer structure, 5 to 8 nm (50 to 80 Å) thick. The trilaminar image consists of two electron-dense layers (the osmium, bound to the inner and outer surfaces of the membrane) separated by a less dense central region.

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مواضيع ذات صلة

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

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