Ion Exchange across Membranes

A different form of ion exchange is that which occurs across a membrane separating two aqueous phases (Figure 2). Once again, the simplest exchange is of two ions, cations such as Na+/K + , Mg2 + /Ca2+ or of anions such as OH/Cl, where there is equal charge maintenance but change of concentration on both sides of the membrane. There are other possibilities than exchange of equally charged ions, such as the exchange of 2Na+/Ca2+ or 2Cl~/HPO4~, when only concentration changes are again involved on exchange. The further possibility is that the exchange process is electrogenic, when charge differences are built up across a membrane. Simple examples are the exchanges of H + /Ca2+ or OH_/HPO2~.

In the case of electrogenic exchange, an electrical potential develops across the membrane. Such potentials exist across most membranes of all cells but to different degrees. The development of such ion exchange potentials has led to the successive evolution from simple cells to nerves and then to the brain. In these cells Na + /K + exchange carries the electrolytic current.

Examples of exchange of ions across membranes are numerous in biological systems. One of the best known is the 2Na + /Ca2 + exchange in many slow

Figure 2 The distribution of the major anions, CI and RPO4 and cations, Na +, K + , Ca2+ and Mg2+ across a biological cell membrane. Filled circles, charged (negative) phospholipids; open circles, uncharged lipids.

muscle cells. This exchange occurs after muscle action is triggered by the invasion of Ca2 + and is used to restore the resting state of a muscle such as that of the heart. An example of electrogenic exchange is found in the nerve when 2K + ions enter the cell in exchange for 3Na +, thus creating a membrane potential (Figure 3). However, this process costs energy and was a later development in evolution. We turn to the energetics of such exchange processes below.

Figure 3 The adenosine triphosphate (ATP) pump in mammalian cells. It exchanges3Na+ for2K+ and iselectrogenic. The concentration of ions inside and outside human cells is shown below.
Solar Panel Basics

Solar Panel Basics

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