lecture2_notes2 - 38 LECTURE#2-2 Symports and Antiports The...

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38 LECTURE #2-2: Symports and Antiports The Na + -glucose cotransporter we described in the previous section is an example of a symport . A symport is a membrane protein that transports two or more substances in the same direction across the plasma membrane. In most cases, the transport rate is appreciable only when all of the substances are transported together in some synchronous fashion, probably simultaneously. Epithelial cells contain a large variety of symports, for transporting metabolites as well as ions. In some cases, symports use the energy of the Na + gradient to move some other substance against a concentration gradient, as we have seen for the Na + - glucose co-transporter. In other cases, the energy of the Na + gradient is not used in this way, and the symport seems merely to be a device to increase membrane permeability selectively to some ion or molecule. Epithelial cells also contain another group of transporters, called antiports or countertransporters. An antiport transports two or more substances in different directions across the plasma membrane. Antiports are widely distributed among epithelia and many other tissues in the body, where they serve essential roles in the regulation of pH and intracellular Cl - and Ca 2+ concentration. We shall not attempt to describe all of the different symport and antiport proteins in epithelial cells. Rather we shall give a few examples of some of the most important of these proteins and briefly describe their roles in epithelial cell function. We will start by discussing the Fundamental Law of Symports (FLS). The free energy due to the presence of an ion gradient is given by, (1) ] [P ] [P RT zFV E o i m ln (Notice that at equilibrium, i.e. when E = 0 , equation (1) is reduced to the Nernst equation). Equation (1) can be rearranged as follows, ] [P ] [P V RT zF RT E o i m } ln { } ] [ ] [ ln ) {ln( o i V RT zF P P e RT E m ' ) ] [ ] [ ln( (1) e P P RT E m V RT zF o i Coupled transport is a form of protein-mediated transport in which the free energy is transferred from one or more ion gradients to other ion gradient(s), without any change (gain or loss) of the total energy. Thus, for a symport we can write, n 1 E 1 + n 2 E 2 +  = 0 (2) where n is the number of each ion type transported per cycle. Substituting E from equation (1)’ in the equation (2), and rearranging the terms, we will obtain the equation for FLS (see slide #1). A Symport: The Na + -K + -Cl - Cotransporter Many cells contain a symport that simultaneously transports one Na + , one K + , and two Cl - . This protein is usually referred to as the Na-K-Cl cotransporter. First discovered in the small intestine, the Na-K- Cl cotransporter has now been described in kidney and a large number of other epithelia, including those of
39 trachea, salivary gland, lacrimal gland, and sweat gland, and the retinal pigment epithelium.

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