Cell Components (Part 2)

Figure 2–4

Schematic drawing of the molecular structure of the plasma membrane. Note the one-pass and multipass transmembrane proteins. The drawing shows a peripheral protein in the external face of the membrane, but the proteins are present mainly in the cytoplasmic face, as shown in Figure 2–3. (Redrawn and reproduced, with permission, from Junqueira LC, Carneiro J: Biologia Celular e Molecular, 6th ed. Editora Guanabara, 1997.)

Proteins, which are a major molecular constituent of membranes (about 50% w/w in the plasma membrane), can be divided into two groups. Integral proteins are directly incorporated within the lipid bilayer, whereas peripheral proteins exhibit a looser association with membrane surfaces. The loosely bound peripheral proteins can be easily extracted from cell membranes with salt solutions, whereas integral proteins can be extracted only by drastic methods that use detergents. Some integral proteins span the membrane one or more times, from one side to the other. Accordingly, they are called one-pass or multipass transmembrane proteins (Figure 2–4).

Freeze-fracture electron microscopic studies indicate that many integral proteins are distributed as globular molecules intercalated among the lipid molecules (Figure 2–3B). Some of these proteins are only partially embedded in the lipid bilayer, so that they may protrude from either the outer or inner surface. Other proteins are large enough to extend across the two lipid layers and protrude from both membrane surfaces (transmembrane proteins). The carbohydrate moieties of glycoproteins and glycolipids project from the external surface of the plasma membrane; they are important components of specific molecules called receptors that participate in important interactions such as cell adhesion, recognition, and response to protein hormones. As with lipids, the distribution of membrane proteins is different in the two surfaces of the cell membranes. Therefore, all membranes in the cell are asymmetric.

Integration of the proteins within the lipid bilayer is mainly the result of hydrophobic interactions between the lipids and nonpolar amino acids present on the outer shell of the integral proteins. Some integral proteins are not bound rigidly in place and are able to move within the plane of the cell membrane (Figure 2–5). However, unlike lipids, most membrane proteins are restricted in their lateral diffusion by attachment to the cytoskeletal components. In most epithelial cells, the tight junctions (see Chapter 4: Epithelial Tissue) prevent lateral diffusion of transmembrane proteins and even the diffusion of membrane lipids of the outer leaflet.

Figure 2–5

Experiment demonstrating the fluid nature of the cell membrane. The plasmalemma is shown as two parallel lines (representing the lipid portion) in which proteins are embedded. In this experiment, two types of cells derived from tissue cultures (one with a fluorescent marker [right] and one without) are fused (A  B) through the action of the Sendai virus. Minutes after the fusion of the membranes, the fluorescent marker of the labeled cell spreads to the entire surface of the fused cells (C). However, in many cells, most transmembrane proteins are stabilized in place by anchoring to the cytoskeleton.

The mosaic disposition of membrane proteins, in conjunction with the fluid nature of the lipid bilayer, constitutes the basis of the fluid mosaic model for membrane structure shown in Figure 2–3A. Membrane proteins are synthesized in the rough endoplasm reticulum, their molecules are completed in the Golgi apparatus, and they are transported in vesicles to the cell surface (Figure 2–6).

Figure 2–6

The proteins of the plasmalemma are synthesized in the rough endoplasmic reticulum and then transported in vesicles to the Golgi complex, where they may be modified and transferred to the cell membrane. This example shows the synthesis and transport of a glycoprotein, which is an integral protein of the membrane. (Redrawn and reproduced, with permission, from Junqueira LC, Carneiro J: Biologia Celular e Molecular, 7th ed. Editora Guanabara, 2000.)

In the electron microscope the external surface of the cell shows a fuzzy carbohydrate-rich region called the glycocalyx (Figure 2–2). This layer is composed of carbohydrate chains linked to membrane proteins and lipids and of cell-secreted glycoproteins and proteoglycans. The glycocalyx has a role in cell recognition and attachment to other cells and to extracellular molecules. The plasma membrane is the site at which materials are exchanged between the cell and its environment. Some ions, such as Na⁺, K⁺, and Ca²⁺, are transported across the cell membrane through integral membrane proteins, using energy from the breakdown of adenosine triphosphate (ATP). Mass transfer of material also occurs through the plasma membrane. This bulk uptake of material is known as endocytosis (Gr. endon, within, + kytos, cell). The corresponding name for release of material in bulk is exocytosis. However, at the molecular level, exocytosis and endocytosis are different processes that utilize different protein molecules.

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