10 10 10"' Oil-Water Partition Coefficient (Kj)

FIGURE 5 Relation between permeability coefficients and oil-water partition coefficients for a number of solutes. The sizes of the circles are proportional to the molecular diameters of the solutes.

the basis of their lipid solubilities alone. This astute observation led Collander to conclude:

It seems therefore natural to conclude that plasma membranes of the Chara cells contain lipoids, the solvent power of which is on the whole similar to olive oil. But, while medium sized and large molecules penetrate the plasma membrane only when dissolved in the lipoids, the smallest molecules can also penetrate in some other way. Thus, the plasma membrane acts both as a selective solvent and as a molecular sieve.

In short, he proposed that small, highly water-soluble molecules can penetrate biologic membranes through two parallel pathways: (1) they can dissolve in the lipid matrix and diffuse through the membrane just as lipid-soluble molecules do but to a very much lesser extent, or (2) they can pass through channels or pores that perforate the lipid envelope.

During the years that have elapsed since Collander's suggestion of a mosaic lipid membrane (i.e., a discontinuous lipid layer perforated by water-filled channels), a compelling body of evidence has accrued for the presence of proteinaceous channels that span the lipid bilayer and serve as the routes for the diffusional flows of highly water-soluble solutes across biologic membranes. These flows are often referred to as restricted diffusion inasmuch as their rates are dramatically influenced by molecular size.

The permeability coefficients of large lipid-soluble molecules may be quite high due to partition coefficients that strongly favor their entry into the lipid phase. These movements are often referred to as simple diffusion.

2. Small water-soluble molecules, which have little affinity for the lipid phase, may penetrate the membrane through pores. This restricted diffusion is, however, limited to small molecules; most cell membranes are essentially impermeable to water-soluble molecules having five or more carbon atoms. Most, if not all, essential metabolic substrates (e.g., glucose, essential amino acids, water-soluble vitamins) cannot enter the cell to any appreciable extent by this mechanism. As we shall see, other mechanisms (carrier-mediated processes) are present that mediate the entry of these substances into cells at rates that are sufficiently rapid to sustain essential metabolic processes.

3. Inasmuch as water and small water-soluble uncharged molecules have finite lipid solubilities, it is often difficult to determine experimentally the extent to which they traverse biologic membranes by diffusing through the lipid phase and/or by restricted diffusion through pores. It is generally accepted that the permeation rates of water and other small, polar nonelectrolytes (e.g., urea, glycerol) through biologic membranes (where determined) are too fast to be accounted for by diffusion through the lipid phase and that pores must be invoked. There is no doubt that inorganic ions traverse biologic membranes virtually exclusively via pores inasmuch as their solubilities in lipid are minute. Indeed, a large family of integral membrane proteins that serve as selective channels for water movements across biological membranes, named aquaporins, have been identified, isolated, and extensively studied at the molecular level. Some of these channels are almost exclusively permeable to water, while others are also permeable to small uncharged solutes such as urea. Further, highly selective channels have been identified, as have clones and sequences for virtually all major ions of physiological significance (e.g., Na+, K+, Ca2+), and they will be discussed later in this book.

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