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Ion gradients across the cell plasma membrane
All membrane potentials depend on the existence of ion gradients across the membrane in question. The major ion species that shape the form of both resting potentials and action potentials are K+, Na+, Ca++, and Cl-. The ion gradi-ents result from the activities of three types of membrane proteins:
1.Ion pumps. These proteins use metabolic energy in the form of ATP to transport ions against their concentra tion gradients. Quantitatively the most important ion pump is Na+/K+-ATP'-ase (Figure 4.1), which transports both sodium and potassium against their respective gra-dients (table 4.1). In addition, various types of calcium pumps are found in the cytoplasmic, ER and mitochon-drial membranes; the direction of Ca++ transport is al-ways from the cytosol to the other compartment.
2. Exchange- and co-transporters. These link the gradients of different ion species to one another, so that gradients can be established for ions for which specific pumps do not exist (or have insufficient capacity). Important ex-amples are the sodium/calcium exchanger and the potas-sium/chloride co-transporter in the cytoplasmic mem-brane (Figure 4.1).
3. Ion channels. These proteins simply facilitate the diffu-sion of ions downhill their concentration gradients, i.e. they tend to dissipate the concentration gradients estab-lished by the transporters.
Most, but not all channels can switch between open and closed states. Switching can be accomplished by a ligand binding to the channel or by changes in the surrounding electrical field. Therefore, we have the following major functional groups of ion channels:
1. Ligand-gated channels, which may either open or close
in response to ligand binding. Important examples are the nicotinic acetylcholine receptor, which allows Na+ into the cell in response to acetylcholine, and the sul-fonylurea receptor-associated Kir channel, which ceases to permit efflux of K + in response to ATP.
2. Voltage-gated channels. The voltage-gated channels for K+, Na+ and Ca++ are all involved in cell excitability.
3. `Leak' channels, which seem to be fairly simple-mind-ed and just continuously permit flux of the cognate ion. The most important ones are those for K+; they are re-sponsible for the fact that K+ permeability dominates the resting potential.
The continuous flux of ions through leak channels (and ex-change transporters) requires continuous operation of the ion pumps. Therefore, a sizeable fraction of our metabolic energy is expended just to keep up the ion gradients across our cell membranes.
The major ion transport processes that are responsible for maintaining the ion gradients and the resting potential across the cytoplasmic membrane are summarized in Figure 4.1:
1. Na+/K+-ATPase (top) exchanges 3 Na+ with 2 K+ ions for each molecule of ATP consumed.
2. Ca++ is transported in exchange for Na+, so that the Ca++ gradient depends in part on the Na+ gradient. In addition, Ca++ is also extruded by specific pumps (not shown).
3. Chloride is expelled from the cytosol by co-transport with K+. The chloride gradient therefore is dependent on1and sustained by the K+ gradient.
4. The leak channels cause the permeability of K+ to be higher than for any other ion species, so that the resting potential is kept close to the K+ equilibrium potential (see below).
Note that the extra Na+ ion that is extruded by Na+/K+-ATP'ase is not the immediate cause of the negative-inside membrane potential. Instead, extra sodium ions are allowed back in during active transport of metabo-lites such as amino acids and (with some cells) glucose, and in exchange for Ca++; any remaining intracellular cation deficit would be balanced by K + flowing back in across its own channels.
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