BLBS102-c27 BLBS102-Simpson March 21, 2012 13:25 Trim: 276mm X 219mm Printer Name: Yet to Come
27 Biochemistry of Fruits 535diacyl- 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 gener-
ally 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.
Compartmentalisation of cellular ingredients and ions is an
essential characteristic of all life forms. The compartmentalisa-
tion is achieved by biomembranes, formed by the assembly of
phospholipids and several neutral lipids that include diacylglyc-
erols 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 transport compartments such
as the endoplasmic reticulum and Golgi bodies form an in-
tegral network of membranes within the cell. Photosynthesis,
which converts light energy into chemical energy, occurs on the
thylakoid membrane matrix in the chloroplast, and respiration,
which further converts chemical energy into more usable forms,
occurs on the mitochondrial cristae. All these membranes have
their characteristic composition and enzyme complexes to per-
form their designated function.
The major phospholipids that constitute the biomembranes
include phosphatidylcholine, phosphatidylethanolamine, phos-
phatidylglycerol and phosphatidylinositol. Their relative pro-
portion may vary from tissue to tissue. In addition, metabolic
intermediates of phospholipids such as phosphatidic acid, dia-
cylglycerols, free fatty acids and so on are also present in the
membrane in lower amounts. Phospholipids are integral func-
tional components of hormonal and environmental signal trans-
duction processes in the cell. Phosphorylated forms of phos-
phatidylinositol 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 stigmasterol, as well as their glucosides, and they are ex-
tremely important for the regulation of membrane fluidity and
function (Whitaker 1988, 1991, 1993, 1994).
Biomembranes are bilamellar layers of phospholipids. The
amphipathic nature of phospholipids having hydrophilic head
groups (choline, ethanolamine, etc.) and hydrophobic fatty acyl
chains, thermodynamically favour their assembly into bilamellar
or micellar structures when exposed to an aqueous environment.
In a biomembrane, the hydrophilic headgroups 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 dif-
ferent phospholipids 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 dur-
ing 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 comprising phos-
pholipids and proteins. The proteins are embedded in the mem-
brane bilayer (integral proteins) or are bound to the periphery
(peripheral proteins). The nature of this interaction results from
the structure of the proteins. If the proteins have a much larger
proportion of hydrophobic amino acids, they would tend to be-
come embedded in the membrane bilayer. If the protein contains
more hydrophilic amino acids it may tend to prefer a more aque-
ous environment, and thus remain as a peripheral protein. In
addition, proteins may be covalently attached to phospholipids
such as phosphatidylinositol. Proteins that remain in the cytosol
may also become attached to the membrane in response to an
increase in cytosolic calcium levels. The membrane is a highly
dynamic entity. The semi-fluid nature of the membrane allows
for the movement of phospholipids in the plane of the mem-
brane, and between the bilayers of the membrane. The proteins
are also mobile within the plane of the membrane. However, this
process is not always random and is regulated by the functional
assembly of proteins into metabolons (functional assembly of
enzymes and proteins, e.g., photosynthetic units in thylakoid
membrane, respiratory complexes in the mitochondria, cellu-
lose synthase on plasma membrane, etc.), their interactions with
the underlying cytoskeletal system (network of proteins such as
actin and tubulin), and the fluidity of the membrane.
The maintenance of homeostasis (life processes) requires the
maintenance of the integrity and function of discrete membrane
compartments. This is essential for the compartmentalisation of
ions and metabolites, which may otherwise destroy the cell. For
instance, calcium ions are highly compartmentalised within the
cell. The concentration of calcium is maintained at the millimo-
lar levels within the cell wall compartment (apoplast), endoplas-
mic reticulum and the tonoplast (vacuole). This is achieved by
energy-dependent transport of calcium from the cytoplasm into
these compartments by ATPases. As a result, the cytosolic cal-
cium levels are maintained at low micromolar (< 1 μM) levels.
Maintenance of this concentration gradient across the membrane
is a key requirement for the signal transduction events, as reg-
ulated entry of calcium into the cytosol can be achieved simply
by opening calcium channels. Calcium can then activate several
cellular biochemical reactions that mediate the response to the
signal. Calcium is pumped back into the storage compartments
when the signal diminishes in intensity. In a similar manner,
cytosolic pH is highly regulated by the activity of proton AT-
Pases. The pH of the apoplast and the vacuole is maintained near
four, whereas the pH of the cytosol is maintained in the range
of 6–6.5. The pH gradient across the membrane is a key fea-
ture that regulates the absorption, or extrusion of other ions and
metabolites such as sugars. The cell could undergo senescence
if this compartmentalisation is lost.
There are several factors that affect the fluidity of the mem-
brane. The major factor that affects the fluidity is the type and
proportion of acyl chain fatty acids of the phospholipids. At a
given temperature, a higher proportion of unsaturated fatty acyl
chains (oleic, linoleic, linolenic) in the phospholipids can in-
crease the fluidity of the membrane. An increase in saturated