Biochemistry of Fruits - Postharvest Biology and Technology of Fruits, Vegetables, and Flowers - page 23

3.2.2 Lipids and biomembranes
By structure, lipids can form both structural and storage components. The major forms of
lipids include fatty acids, diacyl- and triacylglycerols, phospholipids, sterols, and waxes
that provide an external barrier to the fruits. Fruits, in general, are not rich in lipids with
the exception of avocado and olives that store large amounts of triacylglycerols or oil.
As generally observed in plants, the major fatty acids in fruits include palmitic (16:0),
stearic (18:0), oleic (18:1), linoleic (18:2), and linolenic (18:3) acids. Among these, oleic,
linoleic, and linolenic acids possess an increasing degree of unsaturation. Olive oil is rich
in triacylglycerols containing the monounsaturated oleic acid and is considered as a healthy
ingredient for human consumption.
Compartmentalization of cellular ingredients and ions is an essential characteristic of all
life forms. The compartmentalization is achieved by biomembranes, formed by the assembly
of phospholipids and several neutral lipids that include diacylglycerols and sterols, the major
constituents of the biomembranes. Virtually, all cellular structures include or are enclosed
by biomembranes. The cytoplasm is surrounded by the plasma membrane, the biosynthetic,
and the transport compartments such as the endoplasmic reticulum and golgi bodies form
an integral network of membranes within the cell. Photosynthetic activity, which converts
light energy into chemical energy, and respiration, which further converts chemical energy
into more usable forms, occur on the thylakoid membrane matrix in the chloroplast and
the cristae of mitochondria, respectively. All these membranes have their characteristic
composition and enzyme complexes to perform their designated function.
The major phospholipids that constitute the biomembranes include phosphatidylcholine,
phosphatidylethanolamine, phosphatidylglycerol, and phosphatidylinositol. Their relative
proportion may vary from tissue to tissue. In addition, metabolic intermediates of phos-
pholipids such as phosphatidic acid, diacylglycerols, and free fatty acids are also present in
the membrane in lower amounts. Phospholipids are integral functional components of hor-
monal and environmental signal transduction processes in the cell. Phosphorylated forms
of phosphatidylinositol such as phosphatidylinositol-4-phosphate and phosphatidylinositol-
4,5-bisphosphate are formed during signal transduction events, though their amounts can
be very low. The membrane also contains sterols such as sitosterol, campesterol, and stig-
masterol, as well as their glucosides, and they are extremely important for the regulation of
membrane fluidity and function.
Biomembranes are bilamellar layers of phospholipids. The amphipathic nature of phos-
pholipids having hydrophilic head groups (choline, ethanolamine, etc.) and hydrophobic
fatty acyl chains thermodynamically favor their assembly into bilamellar or micellar struc-
tures when exposed to an aqueous environment. In a biomembrane, the hydrophilic head-
groups are exposed to the external aqueous environment. The phospholipid composition
between various fruits may differ, and within the same fruit, the inner and outer lamella of
the membrane may have a different phospholipid composition. Such differences may cause
changes in polarity between the outer and inner lamellae of the membrane and lead to the
generation of a voltage across the membrane. These differences usually become operational
during signal transduction events.
An essential characteristic of the membrane is its fluidity. The fluid-mosaic model of
the membrane (Singer and Nicholson, 1972) depicts the membrane as a planar matrix
composed of phospholipids and proteins. The proteins are embedded in the membrane