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Life depends on a membrane’s ability to precisely control the level of solutes in the aqueous compartments inside and outside bathing it. The membrane determines what solutes enter and leave a cell.
Sodium-Potassium Pump
One of the most familiar forms of active transport is the pump that moves sodium and potassium ions in opposite directions across a cell membrane, building up concentration gradients for each substance. Such gradients are important in nerve cells because they allow electrical signals to propagate along nerve axons (see this diagram of how a nerve cell works).
Like other pumps, the sodium-potassium pump requires energy to work. It gets this energy from adenosine triphosphate (ATP), the primary energy-carrying molecule of living organisms. One of the phosphate groups on ATP is split off by an enzyme called sodium-potassium-ATPase, and this release of energy powers the pump.
The pump goes back and forth between two forms, one that is open to the inside of the cell and another that is closed. When the sodium-potassium pump binds sodium ions it shifts to its closed form. This binding and release of sodium ions requires energy, so it gets that energy from adenosine triphosphate again.
Glucose Transporter
As the primary energy substrate of most cells, glucose is essential for life. It is absorbed in the small intestine through a process of active transport. Larger carbohydrates, like fructose, cannot be absorbed through this route because they are unable to cross the plasma membrane.
The transport is accomplished by members of the GLUT family (SLC2A) of facilitative transport proteins. Fourteen GLUT isoforms are known in humans; they vary in tissue specificity, kinetic properties and affinity for glucose.
Glucose is a highly polar molecule, but the plasma membrane is hydrophobic (literally afraid of water). To overcome this obstacle, glucose binds to a protein in the plasma membrane through facilitated diffusion. The resulting bidirectional movement of the sugar across the membrane creates the concentration gradient that drives active transport. The GLUT family members whose affinity for glucose is highest are GLUT1, GLUT3 and GLUT4. They are found in tissues with high energy demands such as skeletal muscle, heart, placenta, kidney and adipose tissue cells.
Sodium-Potassium Receptor
A key aspect of cell membranes is that they allow some molecules to move across them freely (by diffusion or osmosis) but restrict the movement of others. The restricted movement of some molecules is dependent on the concentration gradient of those molecules, whereas for others it is dependent on the voltage of the cell.
Cells are equipped with proteins that allow them to transport substances down their concentration gradients using energy from the breakdown of ATP. One such protein, the sodium-potassium pump (Na+/K+ ATPase), exports three sodium ions out of the cell for two potassium ions, maintaining normal concentration gradients of these ions across the cell membrane (1).
The sodium-potassium pump system also allows other substances to be transported “uphill” against their gradients, for example glucose molecules through a cotransporter protein that binds them to sodium ions. The energy from the movement of sodium ions down their gradient is used to bring glucose molecules into the cell (2).
Sodium-Potassium Channels
The potassium channels that let the flow of sodium and other ions through are sensitive to voltage changes and open and close in response. They’re found along the entire excitable cell membrane. During rest, more of the potassium pores are open than the sodium ones, so potassium ions flow out and sodium ions stream into the cell. This helps establish the resting membrane potential.
The structural features of a potassium channel, like the one shown here (PDB entry 1bl8) from bacteria, are well-understood: it has a central cavity and pore that selects for potassium and rejects sodium, with the ion entering the pore hydrated. It then dehydrates at the selective filter to pass through.
Some of these channels also have a distinct conductance for hyperpolarized versus depolarized cell states and are called inward rectifying potassium channels. Mutations in these channels are associated with the long QT syndrome. Another class of inward potassium channels, also called KCNQ channels, are found in support cells around outer hair cells and spiral ganglion neurons; mutations in these channels cause hearing loss.
