During the first quarter of this century, compelling evidence began to surface that life would not be possible if diffusion were the only mechanism available for the exchange of solutes across cell boundaries. There are two sets of observations that most strongly implied the necessity for additional transport mechanisms:
1. Most biologic membranes are virtually impermeable to hydrophilic molecules having molecular radii significantly greater than 4 A or that have five or more carbon atoms. Thus, virtually no essential nutrients and building blocks (e.g., glucose, amino acids) can penetrate biologic membranes to any significant extent by diffusion, so other mechanisms are necessary to provide for their entry into cells. Similarly, biologic membranes are generally imper-meant to essential multivalent ions such as phosphate, so their movements across cell membranes must also be mediated by mechanisms other than diffusion.
2. The intracellular concentrations of many water soluble solutes differ markedly from their concentrations in the extracellular medium bathing the cells. For example, as discussed in Chapter 1, a characteristic of virtually every cell in the animal and plant kingdoms is that the intracellular K+ concentration greatly exceeds that in the extracellular fluid (in some cases by a factor of more than 1000 to 1), and in cells from higher animals the intracellular Na+ concentration is much less than that in the bathing media (often by a factor greater than 10). This ionic asymmetry is essential for a number of vital processes and, as we shall see, is the basis of many bioelectric phenomena that play an essential role in nerve conduction and muscle contraction. Diffusional processes alone cannot be responsible for the production and maintenance of these asymmetries.
To accommodate these two sets of observations, a concept referred to as carrier-mediated transport or the carrier hypothesis evolved. This hypothesis is generally attributed to Osterhout, who, in 1933, suggested that biologic membranes contain components ("carriers") that are capable of binding a solute molecule at one side of the membrane to form a carrier-solute complex, which then crosses the membrane, dissociates, and discharges the transported solute on the other side.
Since then an overwhelming body of evidence for the role of membrane components, or carriers, in biologic transport processes has accumulated. Carriers have been implicated in the transport of a wide variety of solutes, and the specific properties of numerous carrier systems have been described in considerable detail. Needless to say, a comprehensive discussion of biologic carriers is beyond the scope of this presentation, and we will limit ourselves to a brief consideration of some of the general characteristics of carrier-mediated transport processes. Specific systems will be considered in later sections of this volume dealing with specific tissues or organs.
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