Food Lipids: Chemistry, Nutrition, and Biotechnology

variant [1]. Phosphonolipids are major constituents in three phyla and are synthesized
by phytoplankton, the base of the food chains of the ocean.
III. PHOSPHOLIPID MESOPHASES
Phospholipids are characterized by the presence of a polar or hydrophilic head group
and a nonpolar or hydrophobic fatty acid region. It is this amphipathic character that
drives the macroassembly of phospholipids in the presence of water to a bilayer
organization in which the polar regions tend to orient toward the aqueous phase and
the hydrophobic regions are sequestered from water (Fig. 2A). Another macromolecular
structure commonly adopted by phospholipids and compatible with their amphipathic
constraints is the hexagonal (HII) phase (Fig. 2B). This phase consists of
a hydrocarbon matrix penetrated by hexagonally packed aqueous cylinders with diameters
of about 20 A˚ . Table 1 lists less common macromolecular structures that
may be adopted by phospholipids in a solid or liquid state. The ability of phospholipids
to adopt these different structures is referred to as lipid polymorphism. Additional
information on structure and properties of these mesophases of phospholipids
may be found in the review of Seddon and Cevc [2].
IV. BIOLOGICAL MEMBRANES
Phospholipids, along with proteins, are major components of biological membranes,
which in turn are an integral part of prokaryotes (bacteria) and eukaryotes (plants
and animals). The predominant structures assumed by phospholipids in membranes
are the bilayer and HII structure, which is dictated by the phase preference of the
individual phospholipids (Table 2). It is immediately apparent that a significant proportion
of membrane lipids adopt or promote HII phase structure under appropriate
conditions. The most striking example is phosphatidylethanolamine (PE), which may
compose up to 30% of membrane phospholipids. Under such conditions, portions of
the membrane that adopt an HII phase would be expected to be incompatible with
maintenance of a permeability barrier between external and internal compartments
at those areas. Consequently, alternative roles for those structures must exist.
A. Membrane Permeability
The ability of lipids to provide a bilayer permeability barrier between external and
internal environments constitutes one of their most important functions in a biological
Figure 2 Mesomorphic structures of phospholipids: (A) lamellar and (B) hexagonal II.
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.