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Lodish H, Berk A, Zipursky SL, et al. Molecular Cell Biology. 4th edition. New York: W. H. Freeman; 2000.

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Molecular Cell Biology. 4th edition.

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Section 15.2Overview of Membrane Transport Proteins

Very few molecules enter or leave cells, or cross organellar membranes, unaided by proteins. Even transport of molecules, such as water and urea, that can diffuse across pure phospholipid bilayers is frequently accelerated by transport proteins. The three major classes of membrane transport proteins are depicted in Figure 15-3a. All are integral transmembrane proteins and exhibit a high degree of specificity for the substance transported. The rate of transport by the three types differs considerably owing to differences in their mechanism of action.

Figure 15-3. Schematic diagrams illustrating action of membrane transport proteins.

Figure 15-3

Schematic diagrams illustrating action of membrane transport proteins. Gradients are indicated by triangles with the tip pointing toward lower concentration, electrical potential, or both. (a) The three major types of transport proteins. Pumps utilize (more...)

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. 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 a concentration gradient or electric potential across a membrane, which requires energy, is coupled to the hydrolysis of ATP to ADP and Pi, which releases energy. The overall reaction — ATP hydrolysis and the “uphill” movement of ions or small molecules — is energetically favorable. Such pumps maintain the low calcium (Ca2+) and sodium (Na+) ion concentrations inside virtually all animal cells relative to that in the medium, and generate the low pH inside animal-cell lysosomes, plant-cell vacuoles, and the lumen of the stomach.

Channel proteins transport water or specific types of ions down their concentration or electric potential gradients, an energetically favorable reaction. They form a protein-lined passageway across the membrane through which multiple water molecules or ions move simultaneously, single file at a very rapid rate — up to 108 per second. As discussed in a later section, the plasma membrane of all animal cells contains potassium-specific channel proteins that are generally open and are critical to generating the normal, resting electric potential across the plasma membrane. Many other types of channel proteins are usually closed, and open only in response to specific signals. Because these types of ion channels play a fundamental role in the functioning of nerve cells, they will be discussed in detail in Chapter 21.

Transporters, a third class of membrane transport proteins, move a wide variety of ions and molecules across cell membranes. In contrast to channel proteins, transporters bind only one (or a few) substrate molecules at a time; after binding substrate molecules, the transporter undergoes a conformational change such that the bound substrate molecules, and only these molecules, are transported across the membrane. Because movement of each substrate molecule (or small number of molecules) requires a conformational change in the transporter, transporters move only about 102 – 104 molecules per second, a lower rate than that associated with channel proteins.

Three types of transporters have been identified (Figure 15-3b). Uniporters transport one molecule at a time down a concentration gradient. This type of transporter, for example, moves glucose or amino acids across the plasma membrane into mammalian cells. In contrast, antiporters and symporters couple the movement of one type of ion or molecule against its concentration gradient to the movement of a different ion or molecule down its concentration gradient. Like ATP pumps, antiporters and symporters mediate coupled reactions in which an energetically unfavorable reaction is coupled to an energetically favorable reaction. Because symporters and antiporters catalyze “uphill” movement of certain molecules, they are often referred to as “active transporters,” but unlike pumps, they do not hydrolyze ATP (or any other molecule) during transport. A better term for these proteins is cotransporters, referring to their ability to transport two different solutes simultaneously.

To study the functional properties of the different kinds of membrane-transport proteins, researchers need experimental systems in which a particular transport protein predominates. In one common approach, a specific transport protein is extracted and purified; the purified protein then is reincorporated into pure phospholipid bilayer membranes, such as liposomes (Figure 15-4). Alternatively, the gene encoding a transport protein can be expressed at high levels in a cell normally not expressing it; the difference in transport of a substance by the transfected and nontransfected cells will be due to the expressed transport protein. In these systems, the functional properties of the various membrane proteins can be examined without ambiguity.

Figure 15-4. Liposomes containing a single type of transport protein can be used to investigate properties of the transport process.

Figure 15-4

Liposomes containing a single type of transport protein can be used to investigate properties of the transport process. Here, all the integral proteins of the erythrocyte membrane are solubilized by a nonionic detergent, such as octylglucoside. The glucose transport (more...)


  •  The plasma membrane regulates the traffic of molecules into and out of the cell.
  •  Gases and small hydrophobic molecules diffuse directly across the phospholipid bilayer at a rate proportional to their ability to dissolve in a liquid hydrocarbon.
  •  Ions, sugars, amino acids, and sometimes water cannot diffuse across the phospholipid bilayer at sufficient rates to meet the cell’s needs and must be transported by a group of integral membrane proteins including channels, transporters, and ATP-powered ion pumps (see Figure 15-3).
  •  Two common experimental systems for studying the functions of transport proteins are liposomes containing a purified transport protein (see Figure 15-4) and cells transfected with the gene encoding a particular transport protein.

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Copyright © 2000, W. H. Freeman and Company.
Bookshelf ID: NBK21592


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