Historical Perspectives The Dawning of Membrane Biology

The first suggestion of the existence of biological membranes is generally attributed to the botanist Carl Wilhelm Nageli (1817-1891), who became a pioneer in the application of microscopic techniques to the study of cell detail through his attempts to relate structure and function. In his classic work Primordialschlauch, published in 1855, he pointed out that the region immediately adjacent to the inner surfaces of the walls of plant cells appeared to differ from the underlying protoplasm and conjectured that this may be because the protoplasm becomes a firmer gel on contact with the extracellular fluid. Two decades later, another botanist, W. Pfeffer, noted that, when plant cells were placed in concentrated solutions, the protoplasm shrank away from the wall and appeared to be surrounded by a distinct structure, or "membrane." In his treatise dealing with these studies (Osmotische Untersuchungen), he argued that these membranes were discrete structures that could serve as selective barriers for the passage of substances into and out of cells. This was merely a guess, but one that had the virtue of being correct; that guess is sometimes referred to as Pfeffer's postulate.

It was not until 1890 that Ernst Overton confirmed Pfeffer's guess. Overton compared the solubility of a large number of solutes in olive oil with the ease with which they permeate (enter) cells and, as is discussed in greater detail in the following sections, concluded that

Pfeffer's membrane behaved very much as if it were made up of lipids similar to olive oil and thus differed from the aqueous protoplasm.

This first clue to the composition of biological membranes takes us back many years, and, indeed, it would not be too much of an exaggeration to say that the earliest contribution in this area can be attributed to Pliny the Elder (A.D. 23-79) who noted, ''Everything is soothed by oil, and this is the reason why divers send out small quantities of it from their mouths, because it smoothes every part which is rough'' (Natural History, Book II, Section 234). Apparently, pearl divers made a habit of diving with a mouthful of oil when the surface of the water was rippled by winds; when they ejected the oil, it would ''soothe the troubled waters'' and increase underwater visibility.

In 1762, Benjamin Franklin was purportedly reminded of this calming effect of oil by an old sea captain, and, in 1765, when he was the American ambassador to the Court of St. James, he performed an experiment that is a landmark in the science of surface chemistry. Franklin poured olive oil onto a pond in Clapham Common (near London) and noted that the ripples caused by the wind were indeed calmed. He also astutely noted that a given quantity of oil would only spread over a definite area of the pond; the same quantity of oil would always cover the same area, and twice that volume of oil would cover twice that area. Knowing the volume of oil poured on the pond and the area covered, Franklin calculated that the thickness of the layer formed by the oil was about 25 A (2.5 nm) and that it ''could not be spread any thinner.'' Current measurements of the thickness of a monomolecular

3. Membrane Transport

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