Food Lipids: Chemistry, Nutrition, and Biotechnology

diffusion of three different types: lateral, rotational, and transversal [11]. Lateral
diffusion of lipids in biological membranes refers to the two-dimensional translocation
of the molecules in the plane of the membrane. Rotational diffusion of lipid
molecules is restricted to the plane of biological membranes, whereas transverse
diffusion is the out-of-plane rotation or redistribution of lipid molecules between the
two leaflets of the bilayer. Transverse diffusion or ‘‘flip-flop’’ motion is very low in
lipid bilayers, and specific enzymes are required to mediate the process.
There are two major components of membrane fluidity. The first component is
the order parameter (S), also called the structural, static, or range component of
membrane fluidity. This is a measure of angular range of rotational motion, with
more tightly packed chains resulting in a more ordered or less fluid bilayer. The
second component of membrane fluidity is microviscosity and is the dynamic component
of membrane fluidity. This component measures the rate of rotational motion
and is a more accurate reflection of membrane microviscosity.
There are many physical and chemical factors that regulate the fluidity properties
of biological membranes, including temperature, pressure, membrane potential,
fatty acid composition, protein incorporation, and Ca 2� concentration. For example,
calcium influenced the structure of membranes containing acidic phospholipids by
nonspecifically cross-linking the negative charges. Consequently, increasing the calcium
concentration in systems induced structural rearrangements and a decrease in
membrane fluidity [12]. Similarly, changes in microfluidity and lateral diffusion fluidity
were exhibited when polyunsaturated fatty acids oxidized [13].
Fluidity is an important property of membranes because of its role in various
cellular functions. Activities of integral membrane-bound enzymes, such as Na � ,K � -
ATPase, can be regulated to some extent by changes in the lipid portions of biological
membranes. In turn, changes in enzyme activities tightly connected to ion transport
processes could affect translocations of ions.
C. Phase Transitions
As is the case for triacylglycerols, phospholipids can exist in a frozen gel state or
in a fluid liquid crystalline state, depending on the temperature [14] as illustrated in
Fig. 3. Transitions between the gel and liquid crystalline phases can be monitored
by a variety of techniques, including nuclear magnetic resonance (NMR), electron
spin resonance, fluorescence, and differential scanning calorimetry (DSC). With
DSC, both enthalpy and cooperativity of the transition may be determined, enthalpy
being the energy required to melt the acyl chains and cooperativity reflecting the
number of molecules that undergo a transition simultaneously. However, difficulties
in determining membrane transitions have been attributed to entropy/enthalpy com-
Figure 3 The phospholipid gel–liquid crystalline phase transition.
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.