Download Chemistry of lipids Lipids are defined as a group of naturally

Chemistry of lipids
Lipids are defined as a group of naturally occurring
substances consisting of higher fatty acids, their naturally
occurring compounds and substances found normally in
association with them. They are insoluble in water but
soluble in organic solvents like ether, chloroform, benzene
and acetone. They include fatty acids, triacylglycerols,
ketone bodies, cholesterol, phospholipids and spingolipids.
The terms “fats“and “oils“are commonly used to denote
crude lipid mixtures which are obtained from natural
sources. Fats are solids and oils are liquids at room
temperature.
Occurrence
Fates are widely distributed in plants and animals. In
plants they are present in nuts, seeds and oils. The nervous
system of animals is rich in lipids like cholesterol,
phospholipids and glycolipids. Blood contains lipoproteins.
The fat depots such as subcutaneous tissues, mesenteric
tissues, fatty tissues around the kidney and yellow bone
marrow contain large amounts of fat. Food sources rich in
fat are milk, egg, meat, liver, fish oils, nuts, seeds and oils.
Biological Significance of Fats
1. Lipids form one of the three main types of foodstuffs
and act as fuel in the body. It yields more heat and energy
than carbohydrates. Their caloric value is 9 Kcals / gm.
2. Deposits of fat underneath the skin exert insulating
effect to the body. They protect the body from excessive
heat or cold. Fate people can withstand heat or cold better
than thin people.
3. The mesenteric fat around organs like kidney provides
padding and protect the internal organs.
4. Building materials. Breakdown products of fats can be
utilized for building biologically active materials like
cholesterol, which in turn can be utilized for synthesis of
certain hormones.
5. Lipids supply the essential fatty acids which cannot be
synthesized in the body.
6. The nervous system is particularly rich in lipids.
7. Some vitamins like A, D, E and K are fat soluble, hence
lipid is needed for absorbing these vitamins.
8. Lipoproteins and phospholipids are important
constituents of many natural membranes like cell walls
and mitochondrion.
Many natural membranes like cell walls and
mitochondrion. An adult ingests almost 60-150 g of lipids
per day of which more than 90% in triacylglycerol (TAG).
Balance is cholesterol, cholestryl esters, phospholipids and
free fatty acids (FFA).
Classification of Lipids
1. Simple Lipids
Simple lipids are esters of fatty acids with various
alcohols. Neutral fats are trimesters of fatty acids with
glycerol. The alcohol in fats is glycerol and the alcohol in
waxes is anything other than glycerol.
2. Compound Lipids
Compound lipids contain some other chemical groups in
addition to alcohol and fatty acids. There are 4 sub
divisions under this group.
3. Phospholipids
They contain fatty acids, glycerol, phosphoric acid and a
nitrogenous compound.
1. Lecithin 2. Cephalin
3. Sphingomyelin
Glycolipids
They are lipids containing carbohydrate and nitrogen but
no phosphoric acid and glycerol (also called cerebrosides).
Sulpholipids
Lipids containing sulphate groups.
Lipoprotein
They are attached to proteins. They are present in plasma
and tissues.
3. Derived Lipids
These are substances derived from groups mentioned
above by hydrolysis. They are: (a) Fatty acids, (b)
Alcohols other than glycerol, (c) Glycerides, and (d) Bases.
Bases include: (i) Choline, (ii) Sphingosine, (iii) Glycerides,
and (iv) Serine.
Substances Associated with Lipids
a. Carotenoids,
b. Tocopherols,
c. Vitamins A, D, E and K,
D. Steroids (Cholesterol).
Chemical Composition of Fats
Animals and vegetable fats are complex mixtures of
glycerides, that is they are esters of glycerol and fatty
acids. Triglycerides or TAG neutral fats are composed of 3
molecules of fatty acids, esterified to glycerol. A
triglyceride is formed by the condensation of one molecule
of glycerol with three molecules of fatty acids;
CH2 OH+H
O
║
O- C- R1
CH OH+H
O
║
O-C-R1
CH2 OH+H
Glycerol
O
║
O-C-R1
Fatty Acid
O
║
CH2-O-C-R1
O
║
CH-O-C-R1 + H2O
O
║
CH-O-C- R1
Simple Glyceride
Common fatty acids present in natural fats are:
Palmitic acid
CH3 (CH2) 14COOH
Stearic acid
CH3 (CH2)16 COOH
Oleic acid
CH3 (CH2) CH=CH (CH2)7 COOH
Properties of Fats
Physical Properties
1. They are greasy to touch and leave an oily impression
on paper
2. They are insoluble in water but soluble in organic
solvents.
3. They have less specific grvity than water. Specific
gravity of solid fat (0.86) is less than liquid fat (0.95)
4. Pure glycerides are tasteless, odourless, colourless and
neutral in reaction. But after exposure to air for some
time, they become acidic and develop a yellow colour due
to partial hydrolysis and oxidation of unsaturated fatty
acids in them.
5. The flavour of butter is due to the presence of bacterial
flora, which is carefully controlled to impart special
flavour to butter. The colour of butter, human fat and egg
yolk are due to the presence of carotene and xanthophil
contained in them.
6. The hardness or consistency depends upon the relative
amounts of saturated and unsaturated fatty acids present.
Fats containing saturated fatty acids are solid at room
temperature. Fats containing unsaturated fatty acids are
liquid at room temperature and these are oils.
7. Fate have definite melting points. The melting point of a
fat is always higher than the temperature at which it
solidifies.
8. When a liquid fat is placed on water, it spreads
uniformly over the surface of water and if the quantity is
sufficiently small, it will form a layer of 1 molecule
thickness. The effect is to lower the surface tension and
help the transport of fat.
9. Though fats are insoluble in water, they can be broken
down into minute droplets and dispersed in water. This is
emulsification
Essential fatty Acids (polyunsaturated Fatty Acids)
Polyunsaturated fatty acids like linoleic acid ( C-18,2
double bond ) , linolenic noleic acid ( C-18,2 double bond )
, linolenic acid ( C-18 , 3 double bond ) and arachidonic
acid ( C-20, 4 double bonds ) are not synthesized by the
body and hence should be taken in diet. Inseed, cotton
seeds, peanut and corn oils are good sources. Essential
Fatty acids reduce blood cholesterol levels.
Chemical Properties
Acrolein formation when glycerol is heated in the presence
of a dehydrating agent such as potassium bisulphate,
acrolein is produced.
CH2 OH
CHO
KHSO4
CH-OH
-2H2O
CH2 OH
CH
║
CH2
Glycerol
Acrolein
Acrolein has a characteristic unpleasant odour and is
easily identified on the basis of this smell. This reaction
occurs whether glycerol is in free or esterified form as in
the triglycerides.
Hydrogenation Unsaturated fats can be hydrogenated by
the addition of hydrogen across the double bonds of the
fatty acids in the presence of nickel as catalyst , to give
saturated fats .This process is called “hardening” of oil
whereby vegetable oils are hydrogenated to produce
commercial cooking fats ( Dalda ) .
Saponification
Hydrolysis of a fat by alkali is called
saponification. The products of hydrolysis are glycerol and
alkali salts of fatty acids called soaps. Soaps are cleansing
agents. Since the common fats contain palmitic, stearic
and oleic acids predominantly, soaps used for washing
consist largely of sodium their sodium and potassium salts
are soluble in water.
CH 2COR
CH2OH
Hydrolysis
CH-OCOR+ 3NaOH
CH-OH+3RCOONa
CH2OCOR
CH2-OH
Fat
Glycerol
Rancidity Rancidity is a chemical change resulting in
unpleasant odour and taste on storage when fats are
exposed to light, heat, air and moisture. Rancidity may be
due to hydrolytic or oxidation change taking place at the
double bonds of the unsaturated fatty acids resulting in
short chain aldehydes or ketones which have unpleasant
odour.
Rancidity is more rapid at high temperature. Substance
like ascorbic acid (vitamin C) and vitamin E prevent and
are called antioxidants. Antioxidants are added to food
fats improve their storage qualities.