21 Biochemistry of Fruits 489there are polymers made of different hexoses and
pentoses known as hemicelluloses, and based on
their composition, they are categorized as xyloglu-
cans, glucomannans and galactoglucomannans. The
cellulose chains assemble into microfibrils through
hydrogen bonds to form crystalline structures. In a
similar manner, pectin is biosynthesized from UDP-
galacturonic acid (galacturonic acid is derived from
galactose, a six-carbon sugar), as well as other sug-
ars and derivatives and includes galacturonans and
rhamnogalacturonans that form the acidic fraction
of pectin. As the name implies, rhamnogalacturo-
nans are synthesized primarily from galacturonic
acid and rhamnose. The acidic carboxylic groups
complex with calcium, which provide the rigidity to
the cell wall and the fruit. The neutral fraction of the
pectin comprises polymers such as arabinans (poly-
mers of arabinose), galactans (polymers of galac-
tose) or arabinogalactans (containing both arabinose
and galactose). All these polymeric components
form a complex three-dimensional network stabi-
lized by hydrogen bonds, ionic interactions involv-
ing calcium, phenolic components such as diferulic
acid, and hydroxyproline-rich glycoproteins (Fry
1986). It is also important to visualize that these
structures are not static, and the components of cell
wall are constantly being turned over in response to
growth conditions.
LIPIDS ANDBIOMEMBRANES
By structure, lipids can form both structural and stor-
age components. The major forms of lipids include
fatty acids, diacyl- and triacylglycerols, phospho-
lipids, 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,
which 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 biomem-
branes, formed by the assembly of phospholipids
and several neutral lipids that include diacylglyc-
erols and sterols, the major constituents of the bio-
membranes. Virtually all cellular structures include
or are enclosed by biomembranes. The cytoplasm is
surrounded by the plasma membrane; the biosyn-
thetic and transport compartments such as the endo-
plasmic reticulum and the Golgi bodies form an
integral network of membranes within the cell. Pho-
tosynthetic activity, which converts light energy into
chemical energy, and respiration, which further con-
verts 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 composi-
tion and enzyme complexes to perform their desig-
nated functions.
The major phospholipids that constitute the bio-
membranes include phosphatidylcholine, phospha-
tidylethanolamine, phosphatidylglycerol, and phos-
phatidylinositol. Their relative proportion may vary
from tissue to tissue. In addition, metabolic inter-
mediates of phospholipids such as phosphatidic acid,
diacylglycerols, free fatty acids, and so on, are also
present in the membrane in lower amounts. Phos-
pholipids are integral functional components of
hormonal and environmental signal transduction
processes in the cell. Phosphorylated forms of phos-
phatidylinositol such as phosphatidylinositol-4-
phosphate and phosphatidylinositol-4,5- bisphos-
phate are formed during signal transduction events,
though their amounts can be very low. The mem-
brane also contains sterols such as sitosterol, cam-
pesterol, and stigmasterol, as well as their gluco-
sides, and they are extremely important for the
regulation of membrane fluidity and function.
Biomembranes are bilamellar layers of phos-
pholipids. The amphipathic nature of phospholipids,
which have hydrophilic head groups (choline, ethan-
olamine, etc.) and hydrophobic fatty acyl chains,
thermodynamically favors their assembly into bil-
amellar or micellar structures when they are exposed
to an aqueous environment. In a biomembrane, the
hydrophilic head groups are exposed to the external
aqueous environment. The phospholipid composi-
tion between various fruits may differ, and within
the same fruit, the inner and outer lamella of the
membrane may have a different phospholipid com-
position. Such differences may cause changes in
polarity between the outer and inner lamellae of the
membrane and lead to the generation of a voltage