Structure of a cyclic nucleotide-gated ion channel. A, The secondary structure of a single subunit has six membrane-spanning regions and a binding site for cyclic nucleotides on the cytosolic side of the membrane. B, Four identical subunits (I—IV) assemble together to form a functional channel that provides a hydrophilic pathway across the plasma membrane.

lular messengers also abound in nature. This type of gating mechanism allows the channel to open or close in response to events that occur at other locations in the cell. For example, a sodium channel gated by intracellular cyclic GMP is involved in the process of vision (see Chapter 4). This channel is located in the rod cells of the retina and it opens in the presence of cyclic GMP. The generalized structure of one subunit of an ion channel gated by cyclic nucleotides is shown in Figure 2.9. There are six membrane-spanning regions and a cyclic nucleotide-binding site is exposed to the cytosol. The functional protein is a tetramer of four identical subunits. Other cell membranes have potassium channels that open when the intracellular concentration of calcium ions increases. Several known channels respond to inositol 1,4,5-trisphosphate, the activated part of G proteins, or ATP. The gating of the epithelial chloride channel by ATP is described in the Clinical Focus Box 2.1 in this chapter.

Solutes Are Moved Against Gradients by Active Transport Systems

The passive transport mechanisms discussed all tend to bring the cell into equilibrium with the extracellular fluid. Cells must oppose these equilibrating systems and preserve intracellular concentrations of solutes, particularly ions, that are compatible with life.

Lipid bilayer-

Lipid bilayer-

Sodium Potassium Pump Tetramer

ft M

The possible sequence of events during one cycle of the sodium-potassium pump. The functional form may be a tetramer of two large catalytic subunits and two smaller subunits of unknown function. Binding of intracellular Na+ and phosphorylation by ATP inside the cell may induce a conformational change that transfers Na+ to the outside of the cell (steps 1 and 2). Subsequent binding of extracellular K+ and dephosphoryla-tion return the protein to its original form and transfer K+ into the cell (steps 3, 4, and 5). There are thought to be three Na+ binding sites and two K+ binding sites. During one cycle, three Na+ are exchanged for two K+, and one ATP molecule is hydrolyzed.

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Essentials of Human Physiology

Essentials of Human Physiology

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