Figure

(a) Electron micrograph of a human red-cell plasma membrane. Cell membranes are 6 to 10 nm thick, too thin to be seen without the aid of an electron microscope. In an electron micrograph, a membrane appears as two dark lines separated by a light interspace. The dark lines correspond to the polar regions of the proteins and lipids, whereas the light interspace corresponds to the nonpolar regions of these molecules. (b) Arrangement of the proteins and lipids in a membrane.

From J. D. Robertson in Michael Locke (ed.), "Cell Membranes in Development," Academic Press, Inc., New York, 1964.

the nonpolar regions are in the interior in association with nonpolar fatty acid chains (Figure 3-7). Like the membrane lipids, many of the integral proteins can move laterally in the plane of the membrane, but others are immobilized because they are linked to a network of peripheral proteins located primarily at the cytosolic surface of the membrane.

Most integral proteins span the entire membrane and are referred to as transmembrane proteins. Most

Carbohydrate portion of glycoprotein Channel

Extracellular fluid

Transmembrane proteins

Nonpolar regions

Carbohydrate portion of glycoprotein Channel

Extracellular fluid

Transmembrane proteins

Nonpolar regions

Polar regions

Bimolecular Layer

Peripheral proteins

Phospholipids

Intracellular fluid

FIGURE 3-7

Arrangement of integral and peripheral membrane proteins in association with a bimolecular layer of phospholipids.

Polar regions

Peripheral proteins

Phospholipids

Intracellular fluid

FIGURE 3-7

Arrangement of integral and peripheral membrane proteins in association with a bimolecular layer of phospholipids.

of these transmembrane proteins cross the lipid bilayer several times (Figure 3-8). These proteins have polar regions connected by nonpolar segments that associate with the nonpolar regions of the lipids in the membrane interior. The polar regions of transmembrane proteins may extend far beyond the surfaces of the lipid bilayer. Some transmembrane proteins form channels through which ions or water can cross the membrane, whereas others are associated with the transmission of chemical signals across the membrane or the anchoring of extracellular and intracellular protein filaments to the plasma membrane.

Peripheral membrane proteins are not amphi-pathic and do not associate with the nonpolar regions of the lipids in the interior of the membrane. They are located at the membrane surface where they are bound to the polar regions of the integral membrane proteins (see Figure 3-7). Most of the peripheral proteins are on the cytosolic surface of the plasma membrane where they are associated with cytoskeletal elements that influence cell shape and motility.

The extracellular surface of the plasma membrane contains small amounts of carbohydrate covalently linked to some of the membrane lipids and proteins. These carbohydrates consist of short, branched chains of monosaccharides that extend from the cell surface into the extracellular fluid where they form a fuzzy, "sugar-coated" layer known as the glycocalyx. These surface carbohydrates play important roles in enabling cells to identify and interact with each other.

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

I. Basic Cell Functions

3. Cell Structure

© The McGraw-Hill Companies, 2001

Cell Structure CHAPTER THREE

Cell Structure CHAPTER THREE

Extracellular

Extracellular

Phospholipid Bilayer

FIGURE 3-8

A typical transmembrane protein with multiple hydrophobic segments traversing the lipid bilayer. Each transmembrane segment is composed of nonpolar amino acids spiraled in an alpha helical conformation.

Phospholipid bilayer

FIGURE 3-8

A typical transmembrane protein with multiple hydrophobic segments traversing the lipid bilayer. Each transmembrane segment is composed of nonpolar amino acids spiraled in an alpha helical conformation.

The lipids in the outer half of the bilayer differ somewhat in kind and amount from those in the inner half, and, as we have seen, the proteins or portions of proteins on the outer surface differ from those on the inner surface. Many membrane functions are related to these asymmetries in chemical composition between the two surfaces of a membrane.

All membranes have the general structure described above, which has come to be known as the fluid-mosaic model in which membrane proteins float in a sea of lipid (Figure 3-9). However, the proteins and, to a lesser extent, the lipids (the distribution of cholesterol, for example) in the plasma membrane are different from those in organelle membranes. Thus, the special functions of membranes, which depend primarily on the membrane proteins, may differ in the various membrane-bound organelles and in the plasma membranes of different types of cells.

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Responses

  • joseph
    Are peripheral proteins are nonpolar?
    6 years ago

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