Passive transport carriers.
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Some carriers are not linked to the Na+ gradient and move substances across the membrane passively. The glucose transporters in many cell types belong to this class of transporters. In β cells of the pancreas, for example, the glucose transporters (Glut-1)
Extracellular fluid
Cytoplasm ATP
ADP Pi
ATP-binding cassette
FIG 3.23 The ABC transporters are the largest known family of membrane transport proteins. They are characterized by an ATP-binding domain that causes a substrate pocket to be exposed first on one side of the membrane and then on the other as ATP is bound and hydrolyzed to ADP and Pi.
are always present in the plasma membrane and let glucose into the cell according to its concentration in the extracellular fluid. In this way the pancreas detects blood glucose levels and releases an appropriate amount of insulin. In insulin-sensitive cells, such as muscle, liver, and adipose cells, the glucose carriers are sequestered inside the cell until insulin binds to its receptor at the cell surface. Receptor activation causes the glucose carriers (Glut-4) to move to the cell surface, where they allow passive influx of glucose (Fig. 3.24).
Glucose
Signal from insulin
receptor
Translocation to cell
surface
Sequestered Glut–4
transporters
Insulin
Insulin receptor
FIG 3.24 In response to insulin binding to its receptor on the cell surface, carrier proteins that transport glucose (Glut-4) are moved to the cell surface where they passively transport glucose into the cell (facilitated diffusion).
44 UNIT II Cellular Function
Membrane Channel Proteins In contrast to carrier proteins, which bind molecules and move them across the membrane by a conformational change, channel proteins form water-filled pores in the membrane. Nearly all channel proteins are involved in transport of ions and may be referred to as ion channels. Ions can flow through the appropriate channel at very high rates (100 million ions/sec); this is much faster than carrier-mediated transport. However, channels are not linked to an energy source, so ions must flow passively down an electrochemical gradient. The channel proteins in the plasma membranes of animal cells are highly selective, permitting only a particular ion or class of ions to pass. Humans have about 400 genes that encode channel proteins. Ion channels are particularly important in allowing the cell to respond rapidly to a variety of external stimuli. Most channels are not continuously open, but they open and close according to membrane signals. Ion channels may be stimulated to open or close in three principal ways: (1) voltage-gated channels respond to a change in membrane potential; (2) mechanically gated channels respond to mechanical deformation; and (3) ligand-gated channels respond to the binding of a signaling molecule (a hormone or neurotransmitter) to a receptor on the cell surface (Fig. 3.25). In addition, some channels open without apparent stimulation and are referred to as leak channels. Ion channels are responsible for the
FIG 3.25 Gating of ion channels. A, Voltage-gated channel. B, Ligand-gated channel. C, Mechanically gated channel.
development of membrane potentials and are of vital importance in nerve and muscle function, as discussed in the next section.
KEY POINTS • Large, lipid-insoluble molecules are transported across the plasma membrane
by endocytosis and exocytosis. • Small, lipid-insoluble molecules are transported across the plasma membrane
by three kinds of membrane proteins: adenosine triphosphate (ATP)-driven pumps, carriers, and channels.
• Pumps use the energy of ATP to move solutes against a gradient. Examples of ATP-driven active transport include proton pumps, Na+–K+ pumps, Ca2+ pumps, and ATP-binding cassette (ABC) transporters.
• Carriers may be passive or use the Na+ gradient for secondary active transport. Neurotransmitter reuptake carriers and those that transport glucose and amino acids across the gut and renal tubules are examples of Na+-driven carriers. Passive carriers include those that allow glucose entry into insulin- sensitive cells.
• Channels are always passive and allow ions to move down their electrochemi- cal gradients when open. Channels open and close in response to specific signals, such as voltage changes, ligand binding, and mechanical pressure.