Food Lipids: Chemistry, Nutrition, and Biotechnology

nonelectrolytes, however, the permeability coefficients are at least two orders of
magnitude smaller than those of water. Furthermore, for a given homologous series
of compounds, the permeability increases as the solubility in a hydrocarbon environment
increases, indicating that the rate-limiting step in diffusion is the initial
partitioning of the molecule into the lipid bilayer [5].
Measures of the permeability of membranes to small ions are complicated,
since for free permeation to proceed, a counterflow of other ions of equivalent charge
is required. In the absence of such a counterflow, a membrane potential is established
that is equal and opposite to the chemical potential of the diffusing species. A remarkable
impermeability of lipid bilayers exists for small ions with permeability
coefficients of less than 10 �10 cm/s commonly observed. While permeability coefficients
for Na � and K � may be as small as 10 �14 cm/s, lipid bilayers appear to be
much more permeable to H � or OH � ions, which have been reported to have permeability
coefficients in the range of 10 �4 cm/s [6]. One of the hypotheses put forth
to explain this anomaly involves hydrogen-bonded wires across membranes. Such
water wires could have transient existence in lipid membranes, and when such structures
connect the two aqueous phases, proton flux could result as a consequence of
H–O–H���O–H bond rearrangements. Such a mechanism does not involve physical
movement of a proton all the way across the membrane; hence, proton flux occurring
by this mechanism is expected to be significantly faster when compared with the
flux of other monovalent ions which lack such a mechanism. As support for the
existence of this mechanism, an increase in the level of cholesterol decreased the
rate of proton transport that correlated to the decrease in the membrane’s water
content [7].
Two alternative mechanisms are frequently used to describe ionic permeation
of lipid bilayers. In the first, the solubility-diffusion mechanism, ions partition and
diffuse across the hydrophobic phase. In the second, the pore mechanism, ions traverse
the bilayer through transient hydrophilic defects caused by thermal fluctuations.
Based on the dependence of halide permeability coefficients on bilayer thickness and
on ionic size, a solubility-diffusion mechanism was ascribed to these ions [8]. In
contrast, permeation by monovalent cations, such as potassium, has been accounted
for by a combination of both mechanisms. In terms of the relationship between lipid
composition and membrane permeability, ion permeability appears to be related to
the order in the hydrocarbon region, where increased order leads to a decrease in
permeability. The charge on the phospholipid polar head group can also strongly
influence permeability by virtue of the resulting surface potential. Depending on
whether the surface potential is positive or negative, anions and cations could be
attracted or repelled to the lipid–water interface.
B. Membrane Fluidity
The current concept of biological membranes is a dynamic molecular assembly characterized
by the coexistence of structures with highly restricted mobility and components
having great rotational freedom. These membrane lipids and proteins comprising
domains of highly restricted mobility appear to exist on a micrometer scale
in a number of cell types [9,10]. Despite this heterogeneity, membrane fluidity is
still considered as a bulk, uniform property of the lipid phase that is governed by a
complex pattern of the components’ mobilities. Individual lipid molecules can display
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