Biology Reference
In-Depth Information
A biological membrane is semi-permeable, meaning it is permeable to some molecules,
most notably water, while being very impermeable to most solutes (various biochemicals
and salts) found in the bathing solution. This very important concept of unequal trans-
membrane distribution and hence permeability between water and other solutes came out
of the pioneering work of Charles Overton in the 1890s (see Chapter 2). How does a biological
membrane accomplish semi-permeability? The barrier to solutemovement is largely provided
by the membrane's hydrophobic core, a very thin (
40 ˚ thick), oily layer. The inherent
permeability of this core varies from membrane to membrane. Generally, the more tightly
packed the lipids comprising the bilayer, the lower its permeability will be. Lipid bilayers
are very impermeable to most solutes because of their tight packing. Figure 14.1 depicts
the membrane permeability of a variety of common solutes [1] . Note the data is presented
as a log scale of solute permeability (P in cm/s) and ranges fromNa þ ¼
w
10 12 cm/s to water
¼
10 2 cm/s, spanning almost 10 orders of magnitude!
0.2
high permeability
10 -2
H 2 O
10 -4
urea
glycerol
10 -6
tryptophan
glucose
10 -8
10 -10
Cl -
K +
10 -12
Na +
10 -14
low permeability
FIGURE 14.1 Log of the permeability (P in cm/s) across lipid bilayer membranes for common solutes ranging
from Na þ (10 12 cm/s) to water (0.2
10 2 cm/s). This range spans almost 10 orders of magnitude [1] .
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