Ion Channels Are Ion-Selective and Fluctuate Between Open and Closed States
Two important properties distinguish ion channels from simple aqueous pores. First, they show ion selectivity, permitting some inorganic ions to pass, but not others. This suggests that their pores must be narrow enough in places to force permeating ions into intimate contact with the walls of the channel so that only ions of appropriate size and charge can pass. The permeating ions have to shed most or all of their associated water molecules to pass, often in single file, through the narrowest part of the channel, which is called the selectivity filter; this limits their rate of passage. Thus, as ion concentrations are increased, the flux of ions through a channel increases proportionally but then levels off (saturates) at a maximum rate.
The second important distinction between ion channels and simple aqueous pores is that ion channels are not continuously open. Instead, they are gated, which allows them to open briefly and then close again (Figure 11-20). In most cases, the gate opens in response to a specific stimulus. The main types of stimuli that are known to cause ion channels to open are a change in the voltage across the membrane (voltage-gated channels), a mechanical stress (mechanically gated channels), or the binding of a ligand (ligand-gated channels). The ligand can be either an extracellular mediator—specifically, a neurotransmitter (transmitter-gated channels)—or an intracellular mediator, such as an ion (ion-gated channels) or a nucleotide (nucleotide-gated channels) (Figure 11-21). The activity of many ion channels is regulated, in addition, by protein phosphorylation and dephosphorylation; this type of channel regulation is discussed, together with nucleotide-gated ion channels, in Chapter 15. Moreover, with prolonged (chemical or electrical) stimulation, most channels go into a closed “desensitized” or “inactivated” state, in which they are refractory to further opening until the stimulus has been removed, as we discuss later.
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More than 100 types of ion channels have been described thus far, and new ones are still being added to the list. They are responsible for the electrical excitability of muscle cells, and they mediate most forms of electrical signaling in the nervous system. A single neuron might typically contain 10 kinds of ion channels or more, located in different domains of its plasma membrane. But ion channels are not restricted to electrically excitable cells. They are present in all animal cells and are found in plant cells and microorganisms: they propagate the leaf-closing response of the mimosa plant, for example, and allow the single-celled Paramecium to reverse direction after a collision.
Perhaps the most common ion channels are those that are permeable mainly to K+. These channels are found in the plasma membrane of almost all animal cells. An important subset of K+ channels are open even in an unstimulated or “resting” cell and are hence sometimes called K + leak channels. Although this term covers a variety of different K+ channels, depending on the cell type, they serve a common purpose. By making the plasma membrane much more permeable to K+ than to other ions, they have a crucial role in maintaining the membrane potential across all plasma membranes.
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