Chapter 3 lipids

Classification of Lipids
Characteristics:
• insoluble in water but soluble in common
organic solvents such as benzene, ether and
chloroform
• electron carriers, as substrate carriers in
enzymic reactions, as components of biological
membranes, and as sources and stores of
energy
• proximate analysis of foods they are included in
the ether extract fraction

Plant Lipids
Plant lipids are of two main types:
1. Structural -- structural lipids are present as
constituents of various membranes and protective
surface layers and makeup about 7 per cent of the
leaves of higher plants
 surface lipids are mainly waxes, with relatively
minor contributions from long-chain hydrocarbons,
fatty acids and cutin.
 membrane lipids present in mitochondria, the
endoplasmic reticulum and the plasma membranes,
are mainly glycolipids (40–50 per cent) and
phosphoglycerides.

2. Storage--Plant storage lipids occur in fruits and
seeds and are, predominantly, triacylglycerols.
Over 300 different fatty acids have been
isolated from plant tissues, but only about
seven are of common occurrence. The most
abundant is alpha-linolenic acid; the most
common saturated acid is palmitic acid and the
most common monounsaturated acid is oleic
acid.

Animal Lipids
Animal lipids are the major form of energy storage,
mainly as fat
 may constitute up to 97 per cent of the adipose
tissue of obese animals
The yield of energy from the complete oxidation of
fat is about 39 MJ/kg DM compared with about 17
MJ/kg DM from glycogen, the major carbohydrate
form of stored energy
 stored fat is almost anhydrous, whereas stored
glycogen is highly hydrated
Weight for weight, fat is, therefore, about six times
as effective as glycogen as a stored energy source.

structural lipids of animal tissues
mainly phosphoglycerides
muscle and adipose tissue— 0.5-1%
liver – 2-3%
The most important non-glyceride neutral lipid
fraction of animal tissue is made up of
cholesterol and its esters, which together make
up 0.06–0.09 per cent of muscle and adipose
tissue

FATS AND OILS

Fats and oils are constituents of both plants
and animals and are important sources of
stored energy.
Both have the same general structure but
have different physical and chemical
properties
Oils – liquid at ordinary room temperature;
tend to be more chemically reactive
compared to solid fats

Functions of fats and oils:
1.Major function – energy source
2.Thermal insulator
3.In some warm-blooded animals, it is a source
of heat for maintaining body temperature
This is especially important for animals that
are born hairless, those that hibernate, and
those that are cold-adapted.
• These animals have “brown fat” (special
deposits in which oxidation is uncoupled from
adenosine triphosphate (ATP) production
• all the energy is liberated as heat

4. fat has a vital role in providing individual fatty
acids with specific nutritional roles within the
animal body.

Structures of
Fats

• Fats are esters of fatty acids with the trihydric
alcohol glycerol
• they are also referred to as glycerides or
acylglycerols
• Triacylglycerols differ in type according to the
nature and position of the fatty acid residues.
Those with three residues of the same fatty
acid are termed simple triacylglycerols,

• Triacylglycerols are named according to the
fatty acids they contain
• There is evidence that the configuration of the
constituent triacylglycerols of fats can
influence the extent to which they are
digested.
• The fatty acid composition of the
triacylglycerols determines their physical
nature.
• Those with a high proportion of low-molecularweight (short-chain) and unsaturated acids
have low melting points.

Composition of Fats
--Gas chromatography has made it possible for
analysis of fats for individual fatty acids
--plant and marine oils, especially those of fish,
are more highly unsaturated than those of
mammalian origin. This is because of the
presence of varying amounts of linoleic and
linolenic acids in addition to the
monounsaturated oleic acid, which is
quantitatively the major fatty acid in most
natural fats.

• fish oils have significant concentrations of highly
unsaturated C20 and C22 acids.
• In mammalian depot fat, the proportion of the
more unsaturated acids is lower and there is a
higher proportion of high-molecular-weight
saturated acids such as palmitic and stearic acids,
with smaller but significant contributions from
lauric (dodecanoic) and myristic (tetradecanoic)
acids.
• For this reason, fats such as pig lard, and beef and
mutton tallow are firm and hard, whereas fish and
plant oils are softer and frequently are oils in the
true sense.

• Within individual animals, subcutaneous fats
contain a higher proportion of unsaturated
acids and are thus softer than deep-body fat.
• The physical nature of fat varies between
animals
marine mammals have softer body fat than
land mammals.
The reason in both cases is that animal fat has
to maintain a degree of malleability at the
temperature of the tissue, which is influenced
by ambient temperatures

• This is the reason why the fats of the feet and
ears, which are inclined to be colder than the
interior of the body, tend to be unsaturated.
• In ruminants, milk fats are characterised by
their high content of low-molecularweight fatty
acid.
• As a result they are softer than the depot fats
of the respective animals but not as soft as fats
of vegetable and marine origin, being semi-solid
at ordinary temperatures.
• Milk fats of non-ruminants resemble the depot
fat of the particular animal

• In most commercially important edible plant
oils, the dominant fatty acids are oleic, linoleic
and linolenic acids.
• Coconut oil is an exception in having the
saturated 12:0 lauric acid as its major acid.
Families of plants tend to produce
characteristic oils that frequently contain
unusual fatty acids. Examples are the erucic
acid of rapeseed; ricinoleic acid, the 18-carbon,
monoenoic, hydroxy acid of the castor bean;
and vernolic acid, the 18-carbon, trienoic,
epoxy acid of the Compositae.

Essential Fatty Acids
•
• *Plant and marine oils(fish) are more highly
unsaturated than those of mammalian origin
because of varying amounts of linoleic and
linolenic acids in addition to monounsaturated oleic which the major fatty acid
in most natural fats. In mammalian depot
fat, there is higher proportion of saturated
acids like: palmitic and stearic, lauric and
myristic.

Fats like lard, beef and mutton tallow are firm
and hard whereas fish and plant oils are softer
and frequently are oils in the true sense.

• *Linoleic and ᾳ-linolenic acids are
essential fatty acids, they form part of
various membranes and play a part in lipid
transport and certain lipoprotein enzymes.
They are the source materials for the
synthesis of eicosanoids. They help
regulate functions like blood clotting, blood
pressure, smooth muscle contraction and
immune response. Also involved in
maintaining the fluidity of mammalian cell
membranes.

- Oil seeds are rich sources of linoleic acid
and linseed is a good source of ᾳ-linolenic
acids.
• - Pigs and poultry having oil seed residues
in their diet will have adequate supply of
essential fatty acids unlike ruminants.

Notes: Symptoms associated with EFA
deficiencies
•Growth retardation, increased permeability to
water and increased water consumption,
increased susceptibility to bacterial infections,
sterility, less stable biomembranes, capillary
fragility, kidney damage, haematuria and
hypertension, decreased visual acuity, decreased
myocardial contractility, decreased ATP synthesis
in liver and heart, decreased nitrogen retention

GLYCOLIPIDS
The lipids of grasses and clovers
(forms the major part of dietary fat of
ruminants) are galactolipids. The fatty acids
of the galactosides of grasses and clovers
consist largely of Linoleic and ᾳ-linolenic
acids.
Rumen microorganisms are able
to breakdown galactolipids to give
galactose, fatty acids and glycerol. In
animal tissues, glycolipids are present
mainly in the brain and nerve fibers.

PHOSPHOLIPIDS
- primarily as constituents of the lipoprotein
complexes of biological membranes. Abundant
in the heart, kidneys and nervous tissues.
Myelin of the nerve axons contains 55% of
phospholipids.
- Eggs are one of the best animal sources, soya
beans in plants.
- Contain phosphorus in addition to carbon,
hydrogen and oxygen. Phosphoglycerides are
surface active and play a role as emulsifying
agents in biological systems (duodenum)

WAXES
- are simple non-popular lipids consisting of
long chain fatty acid combined with a
monohydric alcohol of high molecular weight.
Usually solid at ordinary temperature.
- Widely distributed in plants and animals
where they often have protective function.
The hydrophobic nature of the wax coating
reduces water loss caused by transpiration in
plants, and provides wool and feathers with
water proofing in animals. Ex. Lanolin –
obtained from wool and spermaceti, a product
of marine animals.

In plants, waxes are usually included
in the cuticular fraction where they
form a matrix in which cutin and
suberin (surfaces of the
underground part of plants and
healed wound surfaces)are
embedded

STEROIDS
Include such biologically important
compounds : sterols, bile acids,
adrenal and sex hormones
a.Sterols – phytosterols in plants,
mycosterols in fungi ( not absorb in
the gut and not found in animal tissues),

zoosterols in animal origin.

b. Cholesterol – zoosterol present in animal
cells, low solubility in water (0.2mg/100ml).
-major sterol in human and is important as a
constituent of various biological membranes.
- is particularly important in the myelinated
structures of the brain and CNS and may
constitute up to 170 g/kg. It is the precursor
of the steroid hormones and bile acids.
- normal concentrations in the blood
plasma range from 1200-2200mg/liter, 30%
of free state and remainder being bound to
lipoproteins.

-very insoluble and prolonged high

levels in blood result in its deposition
on the walls of the blood vessels.
These deposits eventually harden to
atherosclerotic plaque. This narrows
the blood vessel and serves as a site
for clot formation and may
precipitate myocardial infarction or
heart attack.

Notes:
The risk of coronary heart disease is
directly related to the plasma
concentration of LDL-cholesterol and
inversely related to that of HDLcholesterol, and that the risk is
reduced significantly by lowering
elevated serum cholesterol levels.

b.1. 7-dehydrocholesterol – derived from
cholesterol is important as the precursor of
vitamin D3, which is produced when the
sterol is exposed to ultraviolet light.
b.2. Ergosterol – this phytosterol is widely
distributed in brown algae, bacteria and
higher plants. It is important as the
precursor of ergocalciferol or vit. D2,into
which it is converted by ultraviolet
irradiation.

b.3. bile acids – are synthesized from
cholesterol and constitutes the major end point
of cholesterol metabolism. Under physiological
conditions the acids exist as salts. They are
produced in the liver, stored in the gallbladder
and secreted into the upper small intestines.
*Importance of bile acids are the following:
a. provide the major excretory pathway for
cholesterol which cannot be catabolised to
carbon dioxide and water by mammals. Bile
contains high concentration of free cholesterol,
about 390mg/100ml.

-bile salts assist along with the detergent
action of phospholipids in preventing the
cholesterol in the bile fluid from crystallizing
out of solution.
-they act as emulsifying agents in preparing
dietary triacylglycerols for hydrolysis by
pancreatic lipase in the process of digestion
-may have a role in activating pancreatic lipase
-facilitate the absorption, from the digestive
tract of the fat- soluble vitamins

b.4.steroid hormones- include the female
sex hormones (oestrogens), male sex
hormones (androgens) and
progesterone, as well as cortisol,
aldosterone and corticosterone,
which
are produced in the adrenal cortex.
*The adrenal hormones have an
important role in the control of glucose
and fat metabolism.

TERPENES
- found in plants, have strong characteristic
odours and flavors and are components of
essential oils such lemon or camphor oil.
ex. phytol moiety of chlorophyll, the carotenoid
pigments, plant hormones such as giberellic
acid and vitamins A,E and K.
- In animals, some of the coenzymes, including
those of the coenzymes Q group, are
terpenes.

Chapter 3 lipids

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Chapter 3 lipids