Facilitated Diffusion of Ions
Facilitated diffusion of ions takes place through proteins, or assemblies of proteins, embedded in the plasma membrane. These transmembrane proteins form a water-filled channel through which the ion can pass down its concentration gradient. The transmembrane channels that permit facilitated diffusion can be opened or closed. They are said to be "gated"; some types of gated ion channels:ligand-gated mechanically-gated voltage-gated light-gated
Ligand-gated ion channels
Many ion channels open or close in response to binding a small signaling molecule or "ligand". Some ion channels are gated by extracellular ligands; some by intracellular ligands. In both cases, the ligand is not the substance that is transported when the channel opens.
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External ligands (shown here in green) bind to a site on the extracellular side of the channel.
Examples:Acetylcholine (ACh). The binding of the neurotransmitter acetylcholine at certain synapses opens channels that admit Na+ and initiate a nerve impulse or muscle contraction. Gamma amino butyric acid (GABA). Binding of GABA at certain synapses — designated GABAA — in the central nervous system admits Cl- ions into the cell and inhibits the creation of a nerve impulse
Mechanically-gated ion channels
Sound waves bending the cilia-like projections on the hair cells of the inner ear open up ion channels leading to the creation of nerve impulses that the brain interprets as sound. Mechanical deformation of the cells of stretch receptors opens ion channels leading to the creation of nerve impulses.
The Patch Clamp Technique
The properties of ion channels can be studied by means of the patch clamp technique. A very fine pipette (with an opening of about 0.5 µm) is pressed against the plasma membrane of either an intact cell or the plasma membrane can be pulled away from the cell and the preparation placed in a test solution of desired composition. Current flow through a single ion channel can then be measured.
The ligand-binding domain is usually restricted to a single type of molecule.
The ATP bound to its domain provides the energy to pump the ligand across the membrane.
The human genome contains 48 genes for ABC transporters. Some examples:CFTR — the cystic fibrosis transmembrane conductance regulator TAP, the transporter associated with antigen processing The transporter that liver cells use to pump the salts of bile acids out into the bile. ABC transporters that pump chemotherapeutic drugs out of cancer cells thus reducing their effectiveness.
ABC transporters must have evolved early in the history of life. The ATP-binding domains in archaea, eubacteria, and eukaryotes all share a homologous structure, the ATP-binding "cassette".
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Indirect Active Transport
Indirect active transport uses the downhill flow of an ion to pump some other molecule or ion against its gradient. The driving ion is usually sodium (Na+) with its gradient established by the Na+/K+ ATPase.
In antiport pumps, the driving ion (again, usually sodium) diffuses through the pump in one direction providing the energy for the active transport of some other molecule or ion in the opposite direction. Example:
Ca2+ ions are pumped out of cells by sodium-driven antiport pumps. Antiport pumps in the vacuole of some plants harness the outward facilitated diffusion of protons (themselves pumped into the vacuole by a H+ ATPase) to the active inward transport of sodium ions. This sodium/proton antiport pump enables the plant to sequester sodium ions in its vacuole. Transgenic tomato plants that overexpress this sodium/proton antiport pump are able to thrive in saline soils too salty for conventional tomatoes. Antiport pumps to the active inward transport of nitrate ions (NO3−)