- Lipids are heterogeneous compounds related to fatty acids.
- They are insoluble in water but soluble in nonpolar solvents such
as ether, acetone and chloroform.
Biological importance of lipids
- Lipids act as a source of energy. They
are superior to carbohydrate and protein since they yield twice
the energy produced by the same weight of carbohydrates or proteins.
- They are the natural solvent for fat-soluble
- They contain essential fatty acids.
- Lipids in adipose tissue serve as energy
- Lipids have a role in protection and
fixation of internal organs as kidneys.
- Lipids in myelin sheath of nerve fibers
serve as electrical insulator.
- Lipids under the skin serve as thermal
- Lipoproteins are essential components
in the structure of cell membrane and mitochondria. Also, they
are important for lipid transport in the blood.
- Acetyl CoA derived from fatty acids oxidation
is used for biosynthesis of may important compound e.g. steroids.
Classification of lipids
Lipids are classified into simple, compound (conjugated) and
I- Simple lipids
They are formed of fatty acids and alcohol. They are further
classified according to the type of alcohol present into:
1- Fats and oils
II- Compound lipids
They are formed of simple lipids and other non-lipid part e.g.:
III- Derived lipids
These are substances derived from simple lipids and compound
lipids by hydrolysis. They also, include substances related
to lipids. Derived lipids include:
- Fatty acids
- prostglandins and leukotriens derived
from arachidonic acid.
Simple lipids are esters of fatty acids with various alcohols.
The alcohol may be glycerol or other long chain alcohol.
Simple lipids are classified into fats, oils and waxes, according
to the type of alcohol they contain.
1- Fats and oils
They are esters of fatty acids with glycerol. They are similar
chemically, but they differ in physical properties, as oils are
liquid while fats are solid at room temperature.
They are called triglycerides because they are formed of glycerol
and 3 fatty acids.
The 3 fatty acids may be the same e.g. palmetic or stearic acids
forming tripalmetine or tristearine respectively, or the may be
different forming triglycerides with 3 different fatty acids.
They are esters of acids with long chain monohydric alcohol.
They are solid at room temperature
The most important waxes in human body are cholesterol esters,
which are present in blood and other tissues.
The following table shows the differences between fats and waxes
|Consistency at room temperature
||Fats are solid
Oils are liquid
||Glycerol and 3 fatty acids
||Long chain alcohol and fatty acid
|Long chain alcohol
||They can undergo rancidity
||Do not undergo rancidity
||Digested by lipase
|Utilization by human body
||Can be utilized
||Can not be utilized
It is a trihydric alcohol.
It is colourless and viscid fluid with sweat taste.
It is miscible with water in all proportions.
With strong dehydrating agents, as concentrated sulphuric acid,
glycerol can be converted to acrolein that has very irritating
odour. This is called acrolein test.
Importance of glycerol:-
- It is used in pharmaceutical and cosmetic
- It is used as explosive in the form
- It is used in medicine as a vasodilator
agent in coronary heart diseases in the form of nitroglycerine.
These are organic acids, which usually contain an even number
of carbon atoms. They are further classified into saturated and
unsaturated fatty acids according to absence or presence of double
A- Saturated fatty acids
- They have no double bonds. They have the general formula
- They are further classified, according to the number of carbon
atoms, into short chain and long chain fatty acids.
The following table shows the differences between short chain
and long chain fatty acids.
chain fatty acids
|| Long chain
|Number of carbon atoms
||Less than 10 carbons
||More than 10 carbons
|Consistency at room temperature
|Solubility in water
||Acetic contains 2 carbons
Butyric contains 4 carbons
|Palmetic contains 16 carbons
Stearic contains 18 carbons
The following table shows the formulae of the most common saturated
of carbon atoms
B-Unsaturated fatty acids
They have one or more double bonds.
- Oleic acid that contains 18 carbon atoms
and one double bond
- Linoleic acid that contains 18 carbon
atoms and 2 double bonds
- Linolenic acid that contains 18 carbon
atoms and 3 double bonds
- Arachidonic acid that contains 20 carbon
atoms and 4 double bonds
Essential fatty acids
- They are polyunsaturated fatty acids
i.e. fatty acids, which contain more than one double bond.
- They include linoleic, linolenic and
- They are essential for growth.
- They must be taken in diet because the
body cannot synthesize them, as the enzymes that are needed
for their synthesis are absent in humans.
- Arachidonic acid, which is one of the
essential fatty acids, is important for biosynthesis of prostaglandins.
General properties of fatty acids
- They usually contain an even number
of carbon atoms.
- They have straight chains.
- They may be saturated or unsaturated.
Unsaturated fatty acids are more reactive than saturated fatty
- Some fatty acids are hydroxylated e.g.
- Lower fatty acids are soluble in water
and this solubility decrease with increasing chain length.
- Palmitic, stearic and oleic acids make
up the bulk of animal depot fat.
Physical properties of fatty acids
- They are colourless, odourless and
- Solubility in water
a)- Short chain fatty acids are soluble in water. The solubility
decreases with the increase in chain length.
b)- Long chain fatty acids are insoluble in water but soluble
in nonpolar solvents.
- Melting point
It depends on the length of the chain of fatty acid and the
degree of unsaturation.
a)- Short chain and unsaturated fatty acids have lower melting
point. They are liquid at room temperature.
b)- Long chain saturated fatty acids have higher melting point.
They are solid at room temperature.
- Optical activity
Fatty acids that contain double bond can be present in cis and
trans stereoisomeric forms.
Cis configuration means that the groups around the double bond
are on the same side of the bond
Trans configuration means that the groups around the double
bond are on the opposite sides of the bond.
For example, oleic acid, which is a cis form, and its
isomer eliadic acid, which is a trans form
Chemical properties of
fatty acids :-
Salt formation (Reaction
Being acids, fatty acids react with alkalie to form salts
Salts of fatty acids are called soaps. Sodium and potassium
soaps are soluble in water so they are called soft soaps.
Calcium and magnesium soaps are insoluble in water so they
are called hard soaps.
Ester formation (Reaction
Fatty acids react with alcohol to form esters. For example,
fatty acids react with glycerol to form mono, di and triglycerides.
Also, fatty acids react with long chain monohydric alcohols
to give waxes. The most common wax in the body is cholesterol
Addition of hydrogen to unsaturated fatty acids at the double
bond changes it to the corresponding saturated fatty acids
Halogens, as chlorine (Cl), fluorine (F) and iodine (I), can
be added to unsaturated fatty acids at the double bond forming
halogenated fatty acid.
Fatty acids can be reduced to fatty aldehydes, fatty alcohols,
Unsaturated fatty acids are oxidized by oxygen when exposed
to air forming lipid peroxides, fatty aldehydes, ketones and
short chain fatty acids
Oxidation of unsaturated fatty acid with dilute solution of
potassium permanganate gives dihydroxy fatty acid and then cleaves
the fatty acid at the double bond.
Physical properties of fats and oils
are colourless, odourless and tasteless.
The presence of any colour, odour or taste in fat is due to addition
of foreign substances.
- Specific gravity
Specific gravity of fats and oils is less than that of water,
so they float on the surface of water
Fats and oils are insoluble in water. They are soluble in fat
solvents (nonpolar solvents) as benzene, ether, alcohol and chloroform.
- Melting point
- Oils have a low melting point while fats have a higher melting
- Oils are liquid at room temperature because they contain high
proportions of unsaturated fatty acids, while fats are solid at
room temperature, as they do not contain unsaturated fatty acids.
- The consistency of fat at room temperature gives an idea of
its saturation. If it contains high amount of unsaturated fatty
acids, it is liquid at room temperature, but if it does not contain
unsaturated fatty acids, it is solid at room temperature.
Chemical Properties of
fats and oils
- Acrolein test
Fats and oils contain glycerol so; when they are dehydrated by
concentrated sulphuric acid they give very irritant and pungent
odour due to formation of acrolein.
Hydrolysis means breakdown of substance by addition of water.
Fats and oils can be hydrolyzed by superheated steam, or by lipase
enzyme to glycerol and 3 fatty acids.
- Saponification (Action of alkalie)
Alkalie react with fats or oils giving glycerol and soap (salts
of fatty acids). Sodium and potassium soaps are soluble in water
so they are called soft soaps, while calcium and magnesium soaps
are insoluble in water so they are called hard soaps
Addition of hydrogen to fats or oils depends on the presence of
unsaturated fatty acids changing them to saturated fatty acids.
Hydrogen is usually added at high temperature in the presence
of nickel as a catalyst.
This reaction changes oil, which is liquid at room temperature,
to fat, which is solid at room temperature. This is the basis
of margarine preparation from oils.
Halogenation means addition of halogen as iodine (I) fluorine
(F) and chlorine (Cl) to fat.
It depends on the presence of unsaturated fatty acids. The halogen
is added at the double bonds that are present in the unsaturated
The unsaturated fatty acids present in fats and oils react with
oxygen when exposed to air forming lipid peroxides, fatty aldehydes,
ketones and short chain fatty acids.
Rancidity is a condition in which fat attains a bad taste and
Types of rancidity
There are 2 types of rancidity:
Fats are hydrolyzed in presence of moisture and warm temperature
and also by bacterial enzymes into glycerol and fatty acids.
It occurs by oxidation of unsaturated fatty acids present
in fats and oils forming lipid peroxides, fatty aldehydes,
ketones and short chain fatty acids
Predisposing factors of rancidity
Rancidity is predisposed
Fats and oils attain
Fats and oils attain
Production of toxic
compounds as lipid peroxides, aldehydes and ketones.
Rancidity can be prevented by:
Addition of antioxidants
to fats and oils specialty the natural antioxidant tochopherol
Also, avoid exposure
of fats to light, moisture and high temperature.