Biology Reference
In-Depth Information
FIGURE 2.10
Charles Ernest Overton (1855
e
1933). Courtesy of Oregon State University Libraries Special Collections.
exclusively in German. Overton's major contribution to the understanding of membranes
involved permeability studies
[12,13]
. In the late 19
th
century it was believed that membranes
were semi-permeable, meaning at that time they were only permeable to water. In sharp
contrast, Overton found that other molecules also crossed membranes, with neutral mole-
cules crossing faster than charged molecules. Overton also showed that very different types
of cells exhibited similar permeabilities, implying a common set of properties. He also found
a parallel between a solute's membrane permeability and its solubility in olive oil. He
concluded that membranes must include a lipid-like barrier. He also guessed, quite correctly,
that membranes might contain cholesterol and lecithin (phosphatidylcholine), two molecules
that were known at the time. The permeability Overton measured for most tested solutes was
'passive', meaning diffusion occurred down the solute's concentration gradient by dissolving
into and crossing the membrane (Chapter 14). He also recognized the need for 'uphill' (now
called active) transport against a gradient for some solutes that did not follow his lipid-
solubility rule (Chapter 14). Overton's work, published in 1899, was so far ahead of its
time that it was not truly appreciated for decades. As an extension of his permeability
studies, Overton later found an important relationship between an anesthetic's efficacy, its
membrane permeability, and its solubility in olive oil. This correlation is now referred to
as the Meyer-Overton Theory
[14]
.
