Answer
Symporters and antiporters are plasma membrane proteins that facilitate the movement of substances across the membrane by utilizing the energy of the concetration gradients of Na+ or K+ --which energy is indirectly derived from the hydrolysis of ATP.
Through active transport process the transporter proteins of the membrane expel some ions ( Na+ , H+) ions and bring in some ions ( K+ and Ca++). This so-called Na/K pump (Na/K ATPase) is powered by the hydrolysis of ATP. Each cell membrane has many of these pumps and they establish and maintain steep Na+ and K+ gradients across the plasma.membrane. However, the plasma membrane is not leak- proof, and a predominantly extracellular ion like Na+ will leak back into the cytosol down its concentration gradient ; its leakage is facilitated by certain membrane transport protein molecules -- the symporters and the antiporters.
Work Step by Step
Symporters and antiporters are powered by the PE stored in the concentration gradient of an ion, such as the sodium ion(Na+). The transproteins provide channels for the ion to leak back into the cytosol down its concentration gradient. The movement of the Na+ ions converts the stored PE of the gradient into the KE of the movement of the ions down their concentration gradient. This is the energy used in secondary transport processes and it is derived indirectly from ATP which provides the energy for setting up the gradients in the first place.
The two types of membrane transporter proteins, symporters and antiporters, function slightly differently: In general, a membrane transporter protein molecule binds to an Na+ ion ( for example) , then it binds to another ion, say an intracellular ion like Ca++. The transporter protein then changes its shape; this allows both substances to cross the plasma membrane at the same time. The difference is that symporters move two substances, for example Na+ and glucose ( or Na and amino acids), in the same direction, in this case, into the cytosol. On the other hand, antiporters move two substance , say Na/Ca++ (or Na+and Ca++),in opposite directions across the plasma membrane; in this case Na+ moves into and Ca++ moves out of the cytosol. These processes are vital because symporters are important in the absorption of nutrients into the epithelial cells of the small intestines, and antiporters function in the maintenance of the proper pH of the cytosol.
