Sunday, October 18, 2009


In counter-transport two species of ion or other solute are pumped in opposite directions across a membrane. One of these species is allowed to flow from high to low concentration, which yields the entropic energy to drive the transport of the other solute from a low concentration region to a high one. An example is the sodium-calcium exchanger or antiporter, which allows three sodium ions into the cell to transport one calcium out.
A symporter is an integral membrane protein that is involved in movement of two or more different molecules or ions across a phospholipid membrane such as the plasma membrane in the same direction, and is therefore a type of cotransporter. Typically, the ion(s) will move down the electrochemical gradient, allowing the other molecule(s) to move against the concentration gradient. The movement of the ion(s) across the membrane is facilitated diffusion, and is coupled with the active transport of the molecule(s). Although two or more types of molecule are transported, there may be several molecules transported of each type.

SGLT1 in the intestinal epithelium transports sodium ions (Na+) and glucose across luminal membrane of the epithelial cells so that it can be absorbed into the bloodstream. This is the basis of oral rehydration therapy. If this symporter did not exist, individual sodium channels and glucose uniporters would not be able to transfer glucose against the concentration gradient and into the bloodstream.

Na+/K+/2Cl- symporter in the loop of Henle in the renal tubules of the kidney transports 4 molecules of 3 different types; a sodium ion (Na+), a potassium ion (K+) and two chloride ions (2Cl-). Loop diuretics such as furosemide (Lasix) act on this protein.

In the roots of plants, after pumping out H+, they use H+/K+ symporters to create a chemiosmotic potential inside the cell. This allows the root hairs to take up water, which moves by osmosis into the xylem so that way the root hair may stay in a hypotonic environment.

NKCC proteins are membrane transport proteins that transport sodium (Na), potassium (K), and chloride (Cl) across the cell membrane. Because they move each solute in the same direction, NKCC proteins are considered symporters. They maintain electroneutrality by moving two positively charged solutes (sodium and potassium) alongside two parts of a negatively charged solute (chloride). Thus the stoichiometry of the NKCC proteins is 1Na:1K:2Cl.

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