▲ FIGURE 7-2 Overview of membrane transport proteins.

Gradients are indicated by triangles with the tip pointing toward lower concentration, electrical potential, or both. Ml Pumps utilize the energy released by ATP hydrolysis to power movement of specific ions (red circles) or small molecules against their electrochemical gradient. |2| Channels permit movement of specific ions (or water) down their electrochemical gradient. Transporters, which fall into three groups, facilitate movement into and out of cells requires the assistance of specialized membrane proteins. Even transport of molecules with a relatively large partition coefficient (e.g., water and urea) is frequently accelerated by specific proteins because their transport by passive diffusion usually is not sufficiently rapid to meet cellular needs.

All transport proteins are transmembrane proteins containing multiple membrane-spanning segments that generally are a helices. By forming a protein-lined pathway across the membrane, transport proteins are thought to allow movement of hydrophilic substances without their coming into contact with the hydrophobic interior of the membrane. Here we introduce the various types of transport proteins covered in this chapter (Figure 7-2).

ATP-powered pumps (or simply pumps) are ATPases that use the energy of ATP hydrolysis to move ions or small molecules across a membrane against a chemical concentration gradient or electric potential or both. This process, referred to as active transport, is an example of a coupled chemical reaction (Chapter 2). In this case, transport of ions or small molecules "uphill" against an electrochemical gradient, which requires energy, is coupled to the hydrolysis of ATP, which releases energy. The overall reaction—ATP hydrolysis and the "uphill" movement of ions or small molecules—is energetically favorable.

Channel proteins transport water or specific types of ions and hydrophilic small molecules down their concentration or electric potential gradients. Such protein-assisted transport sometimes is referred to as facilitated diffusion. Channel proteins form a hydrophilic passageway across the membrane through which multiple water molecules or ions move simultaneously, single file at a very rapid rate. Some ion chan-

of specific small molecules or ions. Uniporters transport a single type of molecule down its concentration gradient |3A|. Cotransport proteins (symporters, |3B|, and antiporters, |3C|) catalyze the movement of one molecule against its concentration gradient (black circles), driven by movement of one or more ions down an electrochemical gradient (red circles). Differences in the mechanisms of transport by these three major classes of proteins account for their varying rates of solute movement.

nels are open much of the time; these are referred to as non-gated channels. Most ion channels, however, open only in response to specific chemical or electrical signals; these are referred to as gated channels.

Transporters (also called carriers) move a wide variety of ions and molecules across cell membranes. Three types of transporters have been identified. Uniporters transport a single type of molecule down its concentration gradient via facilitated diffusion. Glucose and amino acids cross the plasma membrane into most mammalian cells with the aid of uniporters. In contrast, antiporters and symporters couple the movement of one type of ion or molecule against its concentration gradient with the movement of one or more different ions down its concentration gradient. These proteins often are called cotransporters, referring to their ability to transport two different solutes simultaneously.

Like ATP pumps, cotransporters mediate coupled reactions in which an energetically unfavorable reaction (i.e., uphill movement of molecules) is coupled to an energetically favorable reaction. Note, however, that the nature of the energy-supplying reaction driving active transport by these two classes of proteins differs. ATP pumps use energy from hydrolysis of ATP, whereas cotransporters use the energy stored in an electrochemical gradient. This latter process sometimes is referred to as secondary active transport.

Table 7-1 summarizes the four mechanisms by which small molecules and ions are transported across cellular membranes. In this chapter, we focus on the properties and operation of the membrane proteins that mediate the three protein-dependent transport mechanisms. Conformational changes are essential to the function of all transport proteins.

TABLE 7-1 Mechanisms for Transporting Ions and Small Molecules Across Cell Membranes

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