1. Lipids
The term lipids derived from a (Greek, 'lipos'- fat) was proposed by a biochemist
named Bloor to describe natural occurring heterogeneous group of organic
compounds such as fatty acids, acylglycerol, phosphoglycerides, steroids and
terpenes which are soluble in non polar organic solvent such as ether, benzene,
chloroform and hexane but insoluble or sparingly soluble in water. They are the
components of living system bacteria, plants and animals consisting of basically
carbon, hydrogen and oxygen in addition some have nitrogen and phosphorus.
Occurrence
Fats and oils are widely distributed in nature in both animal and plant tissues.
They occur in relatively higher concentration in the seeds of certain plants (oil
seeds) where they function to supply food for the use of growing seedlings.
Animals stored deposit of fats In their adipose tissues, these
Stored fats constitute a reserve, which can be used as a source of energy.
Physiological Role and Biological importance
The general and important function of some classes of lipids in Biological system
are as follows:-
● Major source of metabolic energy IN. Animals, birds, insects and high lipids
contain seed.
● Basic structural component of cell membrane.
● As a protective waterproof coating on the surface of cuticles of leaves, or
fruits of plants and feathers of birds.
● Fat stored subcutaneously in warm blooded animals serve as insulation
against an unfavourable environment and also fatty tissue around vital
organs give protection against mechanical injury and as activators of
enzymes.
General properties
● Soluble in non polar solvent but only sparingly soluble in water.
● Greasy or fat like in nature and show translucent properties.
● Polar lipids are amphipathic greek word amphi means double:- 1 tail and
1 head.
● One end of the lipid molecule is the head which is polar and called
hydrophilic.
● The other end is called tail and is nonpolar and hydrophobic.

2. ● Most lipids contain fatty acids. The gyceride ester of saturated fatty acids
are usually liquid at room temperature and fats and oils contain
unsaturated fatty acids slowly become rancid(‫)بدبودار‬ when exposed to light,
heat, moisture and air.

3. Fatty acids
Most lipids contain fatty acids, which may be obtained from complete hydrolysis
of simple and complex lipids.
Fatty acids contain an even number (2-30) of Carbon atom in a straight chain
with attached hydrogen and having an acidic group (carboxylic acid).
General formula
R-COOH-> Carboxyl group
|
Alkyl group
Most naturally occurring fatty acids, whether saturated or unsaturated are
straight chain compounds with even no.of cabin atoms although the range of
chain length is greater than 2 or more than 8 carbon.
The lower number chain length C4 to C10 (occurs mainly in milk fat).
Intermediate chain length C10 to C14 and higher number chain length C16 to
C-20 are found in most animals and vegetable fats.

4. Saturated fatty acids
They contain no double bond. In which hydroxyl carbon tail is flexible and can
exist in large numbers of confirmation. Because each single bond in the
backbone has complete freedom of rotation. They are solid at room temperature.
They obtained from animals.e.g:- palmitic acid
Unsaturated fatty acids
On the other hand one or more rigid kinks contribute by non-rotating double
bonds. They contain double bonds. They are liquid at room temperature. They
obtained from plants. e.g:- oleic acid.
How the solubility and melting points of fatty acids are affected?
Solubility and melting points of fatty acids increases with increasing number of
carbon in hydrocarbon chain.
Solubility:- Because of the essentially hydrocarbon nature of the common fatty
acids, they exhibit low solubility in water. The low molecular weight fatty acids
(acetic acid, butyric acid) are miscible with water, whereas fatty acids with more
than six carbon (caproic acid) are essentially insoluble in water but soluble in
nonpolar solvents.
Melting points:- A decrease in chain length and an increase in degree of
unsaturation of the chain will lower the melting points of fatty acids.

5. Classification of lipids
Although there are different ways of classification a useful classification of lipids
by Bloor is as follows:-
● Simple lipids:- These are the esters of fatty acids with alcohol;
saponifiable, include the most abundant of all lipids fats and oils or
triglycerides and the less abundant waxes.
● Compound lipids:- These are the esters of fatty acids containing other
groups in addition to alcohol and fart acids; saponifiable include
phosphoglycerides and sphingolipids.
● Derived lipids:- These are the derived from hydrolysis of simple and
compound lipids. Non saponifiable,(except fatty acids) include Fatty acids
sterol, terpenes and fat soluble vitamins.
Classification of lipids
/
Saponifiable Non-saponifiable
/ . |
Simple Complex Derived
/
Triglycerides phospholipids
& Waxes & sphingolipids
Glycerol:-
Glycerol is trihydric alcohol and is the constituent of all fats and oils. It has the
ability to react with three molecules of fatty acids to form a triple ester called
triglycerides.
O O
|| ||
CH2-OH OH-C- OH CH2-O-C-OH
O | O
|| ||
CH-OH OH-C-OH CH-O-C-OH
O O
|| | ||
CH2-OH OH-C-OH CH2-O-C-OH
Glycerol F.A triglycerides

6. Triglycerides
These are the most abundant group of naturally occurring lipids. Chemically they
are the esters of fatty acids with the trihydric alcohol, glycerol. One, two or all
the three hydroxyl group of glycerol can be esterified to give rose mono,di and
triglycerides. Mono and diglyceride are important intermediate in metabolism of
lipids, most of the natural lipids are triglycerides. In triglycerides, fatty acids are
linked with glycerol by the ester bond between hydroxyl group of glycerol and
carboxyl group of fatty acids.

7. Classification of Triglycerides
1.Simple Triglycerides:- Contain single kind of fatty acids. E.g:- tristearin and
tripalmatin
2. Mixed Triglycerides:- contain two or more kinds of fatty acids. E.g:- 1-stereo,
2-oleo, 3-palmitin.
● All the fats and oils obtained from both animals and plants origin are
triglycerides.
● Those obtained from animal fat contain a higher percentage of saturated
acids.
● Those of plant origin are rich in unsaturated fatty acids.
General properties of Triglycerides
● Main storage form of energy in higher animals, migratory birds and oil
seeds.
● They have a high caloric value( 9kcal/g) in contrast to carbohydrates and
proteins (4kcal/g).
● Fats serve as thermal insulation showing the loss of heat through the skin,
regulate the body temperature and afford protection to vital organs.
Waxes
Waxes are a mixture of long chain alkanes, ketones and esters of long chain fatty
acids. They perform different functions in plants and animals:-
In plants:- They form protective coating on fruits and leaves. Waxes protect
plants from water loss and abrasive damage.
In animals:- Some insects secrete waxes. They also provide a water barrier in
insects, birds and sheep.
Phospholipids
These are the second largest class of complex lipids occurring both in plants and
animals tissues, especially most active tissue in the animal body (brain, liver) and
very small amounts elsewhere in cells.
They consist of phosphorus and nitrogen in addition to fatty acids and polar head
group.
Phospholipids are derivatives of phosphatidic acid. Phosphatidic acid is
composed of 1 glycerol, 2 fatty acids and phosphoric acid. Nitrogen bases like

8. choline, ethanolamine and serine are the most important components of
phospholipids.
Phosphatidylcholine is the most common phospholipid.
Terpenoids
Oxygen containing terpenes. Terpenes are a derived class of lipids, they are non-
saponifiable in nature. They are constructed of multiples of five carbon
hydrocarbons, isoprene. Terpenes containing two, three, four, six, eight isoprene
unit called mono, di, tri and tetraterpenes, respectively, They may be either linear
or cyclic.The basic molecular formula of terpenes multiples of that (C5H8)n, n is
the number of linked isoprene.
CH3
|
CH2=C-C=CH2
|
H
Isoprenoid unit