Membrane Proteins

Although proteins are only about 2% of the molecules of the plasma membrane, they are larger than lipids and constitute about 50% of the membrane weight. Some of them, called integral (transmembrane) proteins, pass through the membrane. They have hydrophilic regions in contact with the cytoplasm and extracellular fluid, and hydrophobic regions that pass back and forth through the lipid of the membrane (fig. 3.7). Most integral proteins are glycoproteins, which are conjugated with oligosaccharides on the extracellular side of the membrane. Many of the integral proteins drift about freely in the phospholipid film, like ice cubes floating in a bowl of water. Others are anchored to the cytoskele-ton—an intracellular system of tubules and filaments discussed later. Peripheral proteins do not protrude into the phospholipid layer but adhere to the intracellular face of the membrane. A peripheral protein is typically associated with an integral protein and tethered to the cytoskeleton.

The functions of membrane proteins include the following:

  • Receptors (fig. 3.8a). The chemical signals by which cells communicate with each other (epinephrine, for example) often cannot enter the target cell, but bind to surface proteins called receptors. Receptors are usually specific for one particular messenger, much like an enzyme that is specific for one substrate.
  • Second-messenger systems. When a messenger binds to a surface receptor, it may trigger changes within the cell that produce a second messenger in the cytoplasm. This process involves both transmembrane proteins (the receptors) and peripheral proteins. Second-messenger systems are discussed shortly in more detail.
  • Enzymes (fig. 3.8b). Enzymes in the plasma membranes of cells carry out the final stages of starch and protein digestion in the small intestine, help produce second messengers, and break down hormones and other signaling molecules whose job is done, thus stopping them from excessively stimulating a cell.
  • Channel proteins (fig. 3.8c). Channel proteins are integral proteins with pores that allow passage of water and hydrophilic solutes through the membrane. Some channels are always open, while others are gates that open and close under different circumstances, thus determining when solutes can pass through
  • fig. 3.8d). These gates open or close in response to three types of stimuli: ligand-regulated gates respond to chemical messengers, voltage-regulated gates to changes in electrical potential (voltage) across the plasma membrane, and mechanically regulated gates to physical stress on a cell, such as stretch and
Physiology Gates

Oligosaccharide

Integral protein Hydrophilic region Hydrophobic region

Phospholipid

— Cytoskeletal protein

Anchoring peripheral protein

Figure 3.7 Transmembrane Proteins. A transmembrane protein has hydrophobic regions embedded in the phospholipid bilayer and hydrophilic regions projecting into the intracellular and extracellular fluids. The protein may cross the membrane once (left) or multiple times (right). The intracellular regions are often anchored to the cytoskeleton by peripheral proteins.

Oligosaccharide

Integral protein Hydrophilic region Hydrophobic region

Phospholipid

— Cytoskeletal protein

Anchoring peripheral protein

Figure 3.7 Transmembrane Proteins. A transmembrane protein has hydrophobic regions embedded in the phospholipid bilayer and hydrophilic regions projecting into the intracellular and extracellular fluids. The protein may cross the membrane once (left) or multiple times (right). The intracellular regions are often anchored to the cytoskeleton by peripheral proteins.

Saladin: Anatomy & I 3. Cellular Form and I Text I © The McGraw-Hill

Physiology: The Unity of Function Companies, 2003 Form and Function, Third Edition pressure. By controlling the movement of electrolytes through the plasma membrane, gated channels play an important role in the timing of nerve signals and muscle contraction (see insight 3.1).

  • Carriers (see figs. 3.18 and 3.19). Carriers are integral proteins that bind to glucose, electrolytes, and other solutes and transfer them to the other side of the membrane. Some carriers, called pumps, consume ATP in the process.
  • Molecular motors (fig. 3.8e). These proteins produce movement by changing shape and pulling on other molecules. They move materials within a cell, as in transporting molecules and organelles to their destinations; they enable some cells, such as white blood cells, to crawl around in the body's tissues; and they make cells change shape, as when a cell surrounds and engulfs foreign particles or when it divides in two. Such processes depend on the action of fibrous proteins, especially actin and myosin, that pull on the integral proteins of the plasma membrane.
  • Cell-identity markers (fig. 3.8/). Glycoproteins contribute to the glycocalyx, a carbohydrate surface coating discussed shortly. Among other functions, this acts like an "identification tag" that enables our bodies to tell which cells belong to it and which are foreign invaders.

Chapter 3 Cellular Form and Function 101

• Cell-adhesion molecules (fig. 3.8g). Cells adhere to one another and to extracellular material through certain membrane proteins called cell-adhesion molecules (CAMs). With few exceptions (such as blood cells and metastasizing cancer cells), cells do not grow or survive normally unless they are mechanically linked to the extracellular material. Special events such as sperm-egg binding and the binding of an immune cell to a cancer cell also require CAMs.

Insight 3.1 Clinical Application

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Responses

  • Angelika Mueller
    Which of the following would NOT be found in the plasma membrane of a human cell?
    6 years ago

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