The lipid bilayer that constitutes the cell membrane forms a selective barrier that prevents diffusion of molecules that are charged - like ions, or big - like proteins or glucose. Such molecules are only able to cross the cell membrane by active transport. Active transport is the movement of a substance against its electrochemical gradient. It is mediated by membrane-spanning proteins which use an energy source to transport the molecules across the cell membrane.
Active transport is necessary to maintain the correct concentration of small molecules inside the cell. Compared to the surrounding environment, cells maintain high concentrations of certain ions - such as potassium - and low concentrations of other ions - for example sodium. These steep concentration gradients are maintained by transport proteins (transporters), which translocate (move) specific molecules. The specificity of the transporter is mediated by a binding site that recognises a single solute. The solute binds to the transporter and is translocated across the membrane by reversible conformational changes in the transporter, resulting in the solute binding site becoming exposed on the opposite side of the membrane to which it started.
There are three main types of transporters each requiring a different energy source;
ATP-driven pumps maintain the electrochemical gradients of several ions across the cell membrane. These gradients are used in coupled transport to drive the translocation of other solutes. Therefore ATP-driven transporters can be said to mediate primary transport, while coupled transporters mediate secondary transport.
These pumps use the energy released from the hydrolysis of ATP to ADP and phosphate to translocate solutes across the cell membrane. ATP-driven pumps are divided into three main classes that are all found in prokaryotic and eukaryotic cells:
These pumps use a phosphate group from ATP to phosphorylate themselves during translocation of ions. P-type pumps maintain the gradients of Na+, K+, H+ and Ca2+ ions across the cell membrane.
A well known example of P-type pumps is the sodium-potassum pump. This pump translocates three Na+ ions out of the cell, and two K+ ions into the cell. Additionally it contributes to the action potential over the membranes of nerve cells. To see this pump in action, watch this animation: http://www.youtube.com/watch?v=GTHWig1vOnY
The ATP binding casettes (ABC) transporters pump small molecules across the cell membrane. Increased production of these pumps is linked to resistance to chemotherapeutic drugs in cancer cells.
F-type pumps resemble a turbine. They are found in bacterial membranes and the inner membrane of mitochondria and chloroplasts. Here, they produce ATP using energy derived from the concentration gradient of H+ ions across the membrane. A similar, yet distinct family, are the V-type ATP-driven pumps, which use ATP to drive translocation of H+ ions.
This animation explains how these pumps function to generate ATP: http://www.youtube.com/watch?v=3y1dO4nNaKY
These transporters utilise energy derived from the transportation of one solute down its electrochemical gradient, to transport another solute against its electrochemical gradient.
There are two types of coupled transporters; symporters and antiporters.
Symporters transport both solutes in the same direction.
In the human gut, glucose is taken up by the sodium-glucose transport protein (SGLT)1 symporter along with Na+. The concentration gradient of Na+ over the cell membrane provides the driving force for uptake of glucose by this symporter.
Antiporters transport the solutes in different directions across the membrane.
The sodium-calcium exchanger employs the concentration gradient of Na+ across the cell membrane to move Ca2+ out of the cell. Three Na+ ions enter the cell for each Ca2+ ion that is transported out of the cell. This transporter functions in maintaining the correct concentration of Ca2+ in several different cell types such as cardiac cells and photoreceptor cells.
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