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Berg JM, Tymoczko JL, Stryer L. Biochemistry. 5th edition. New York: W H Freeman; 2002.

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Biochemistry. 5th edition.

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Chapter 13Membrane Channels and Pumps

The flow of ions through a single membrane channel (channels are shown in red in the illustration at the left) can be detected by the patch clamp technique, which records current changes as the channel transits between the open and closed states.

Figure

The flow of ions through a single membrane channel (channels are shown in red in the illustration at the left) can be detected by the patch clamp technique, which records current changes as the channel transits between the open and closed states. [(Left) (more...)

The lipid bilayer of biological membranes, as discussed in Chapter 12, is intrinsically impermeable to ions and polar molecules. Permeability is conferred by two classes of membrane proteins, pumps and channels. Pumps use a source of free energy such as ATP or light to drive the thermodynamically uphill transport of ions or molecules. Pump action is an example of active transport. Channels, in contrast, enable ions to flow rapidly through membranes in a downhill direction. Channel action illustrates passive transport, or facilitated diffusion.

Pumps are energy transducers in that they convert one form of free energy into another. Two types of ATP-driven pumps, P-type ATPases and the ATP-binding cassette pumps, undergo conformational changes on ATP binding and hydrolysis that cause a bound ion to be transported across the membrane. Phosphorylation and dephosphorylation of both the Ca2+-ATPase and the Na+-K+-ATPase pumps, which are representative of P-type ATPase, are coupled to changes in orientation and affinity of their ion-binding sites.

A different mechanism of active transport, one that utilizes the gradient of one ion to drive the active transport of another, will be illustrated by the sodium—calcium exchanger. This pump plays an important role in extruding Ca2+ from cells.

We begin our examination of channels with the acetylcholine receptor, a channel that mediates the transmission of nerve signals across synapses, the functional junctions between neurons. The acetylcholine receptor is a ligand-gated channel in that the channel opens in response to the binding of acetylcholine (Figure 13.1). In contrast, the sodium and potassium channels, which mediate action potentials in neuron axon membranes, are opened by membrane depolarization rather than by the binding of an allosteric effector. These channels are voltage-gated. These channels are also of interest because they swiftly and deftly distinguish between quite similar ions (e.g., Na+ and K+). The flow of ions through a single channel in a membrane can readily be detected by using the patch-clamp technique.

Figure 13.1. Acetylcholine Receptors.

Figure 13.1

Acetylcholine Receptors. An electron micrograph shows the densely packed acetylcholine receptors embedded in a postsynaptic membrane. [Courtesy of Dr. John Heuser and Dr. Shelly Salpeter.]

The chapter concludes with a view of a different kind of channel—the cell-to-cell channel, or gap junction. These channels allow the transport of ions and metabolites between cells.

  • 13.1. The Transport of Molecules Across a Membrane May Be Active or Passive
  • 13.2. A Family of Membrane Proteins Uses ATP Hydrolysis to Pump Ions Across Membranes
  • 13.3. Multidrug Resistance and Cystic Fibrosis Highlight a Family of Membrane Proteins with ATP-Binding Cassette Domains
  • 13.4. Secondary Transporters Use One Concentration Gradient to Power the Formation of Another
  • 13.5. Specific Channels Can Rapidly Transport Ions Across Membranes
  • 13.6. Gap Junctions Allow Ions and Small Molecules to Flow between Communicating Cells
  • Summary
  • Problems
  • Selected Readings

By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed.

Copyright © 2002, W. H. Freeman and Company.
Bookshelf ID: NBK21140

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