Solute Transport across Membranes:-Ion-Selective Channels Allow Rapid Movement of Ions across Membranes

Ion-selective channels—first recognized in neurons and now known to be present in the plasma membranes of all cells, as well as in the intracellular membranes of eukaryotes—provide another mechanism for moving in organic ions across membranes. Ion channels, together with ion pumps such as the Na⁺K⁺ ATPase, determine a plasma membrane’s permeability to specific ions and regulate the cytosolic concentration of ions and the membrane potential. In neurons, very rapid changes in the activity of ion channels cause the changes in mem brane potential (the action potentials) that carry signals from one end of a neuron to the other. In myocytes, rapid opening of Ca²⁺ channels in the sarcoplasmic reticulum releases the Ca²⁺ that triggers muscle con traction. We discuss the signaling functions of ion channels in Chapter 12. Here we describe the structural basis for ion-channel function, using as examples a bacterial K⁺ channel, the neuronal Na⁺ channel, and the acetylcholine receptor ion channel.

Ion channels are distinguished from ion trans porters in at least three ways. First, the rate of flux through channels can be several orders of magnitude greater than the turnover number for a transporter— 10⁷ to 10⁸ ions/s for an ion channel, near the theoretical maximum for unrestricted diffusion. Second, ion channels are not saturable: rates do not approach a maximum at high substrate concentration. Third, they are “gated”—opened or closed in response to some cellular event. In ligand-gated channels (which are generally oligomeric), binding of an extracellular or intracellular small molecule forces an allosteric transition in the protein, which opens or closes the channel. In voltage gated ion channels, a change in transmembrane electrical potential (Vm) causes a charged protein domain to move relative to the membrane, opening or closing the ion channel. Both types of gating can be very fast. A channel typically opens in a fraction of a millisecond and may remain open for only milliseconds, making these molecular devices effective for very fast signal transmission in the nervous system.

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مواضيع ذات صلة

Solute Transport across Membranes:- ABC Transporters Use ATP to Drive the Active Transport of a Wide Variety of Substrates

Solute Transport across Membranes:- P-Type Ca2+ Pumps Maintain a Low Concentration of Calcium in the Cytosol

Solute Transport across Membranes:-Active Transport Results in Solute Movement against a Concentration or Electrochemical Gradient

Solute Transport across Membranes:-The Chloride-Bicarbonate Exchanger Catalyzes Electroneutral Cotransport of Anions across the Plasma Membrane

Solute Transport across Membranes:- The Glucose Transporter of Erythrocytes Mediates Passive Transport

Solute Transport across Membranes:- Transporters Can Be Grouped into Superfamilies Based on Their Structures

Solute Transport across Membranes:- Passive Transport Is Facilitated by Membrane Proteins

Membrane Dynamics:- Membrane Fusion Is Central to Many Biological Processes

The Composition and Architecture of Membranes:- Sphingolipids and Cholesterol Cluster Together in Membrane Rafts

The Composition and Architecture of Membranes:- Peripheral Membrane Proteins Are Easily Solubilized

The Composition and Architecture of Membranes:- A Lipid Bilayer Is the Basic Structural Element of Membranes

Reversible Binding of a Protein to a Ligand: Oxygen-Binding Proteins:- Cooperative Ligand Binding Can Be Described