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Lipids:
The lipids are a large and diverse group of naturally occurring organic compounds. They are soluble
in non polar organic solvents like ether, chloroform, acetone & benzene, and generally insoluble in
water. Though there is great structural variety among the lipids, the property of their
solubilities put them in one group of compounds. Lipids are one of the very important group of
compounds that are involved in the life of an organism. The lipids are divided in to sub groups
depending on their functional characteristics.
They are, Fatty acids, Fats and oils ( saturated or unsaturated), Soaps and detergents, waxes,
Phospholipids, eicosonoids, terpenes, steroids, lipid soluble vitamins and bio synthetic
pathways. Each lipid has a different structure. All the lipids have a large number of Carbon
hydrogen bonds which will make them highly energy rich.
Fats and oils predominantly are tri esters of glycerol and aliphatic fatty acids containing up to 22
carbon atoms. Waxes are esters of long chain fatty acids usually containing 24-28 carbon atoms,
with long chain primary alcohols or with alcohols of the steroid group.
The general classes of lipids are neutral lipids, glycolipids and phospholipids occur in most species.
Glycolipids are notably absent from many Mycoplasma species. Generally the percentage by
weight of phospholipids equals the combined percentages of neutral and glycolipids.
Examples of Lipids
Lipids are one of the most predominant component of life. Though it cannot be synthesized by body it
is present in various sources and can be stored in the body. Some of the sources of lipids are listed
below.
1. Edible Oils: Oils extracted from peanuts, rice bran, corn, olive, sun flower kernels, cotton
seed, and vegetable oils etc., are high on lipids. Oils from most of the sources contain mono
unsaturated fats that are difficult to digest and are bad for health. Cod liver oil and whale oil
are also lipid examples which are not good for health and should not be consumed. Cheese,
cottage cheese, margarine, butter, cream like milk products are high on fats.
2. Nuts: Peanuts, cashew nuts, almonds, walnuts etc. contain lipids which are not harmful and
they help in cell growth. They contain saturated fats that are easy to digest.
Many vegetables, grains, and fruits also contain lipids.
Classification of Lipids:
Lipids are also organic compounds containing carbon, hydrogen and oxygen. However, hydrogen and
oxygen occur in a ratio much higher than in water. Lipids form about 3.5% of the total chemical
composition of a cell. Lipids are generally insoluble in water but soluble in organic solvents like ether
and chloroform.
The biologically important lipids can be classified into 3 types namely simple lipids, compound lipids
and derived lipids.
1. Simple Lipids
2. Compound Lipids
3. Phospholipids
4. Glycolipids
5. Lipoproteins
6. Sterols
Simple Lipids
These are esters of fatty acids with various types of alcohol. They are distinguished into fats and oils.
Fats are esters of fatty acids and glycerol. A fatty acid is an organic acid with a hydrocarbon chain
ending in a carboxyl (COOH) group. Most fatty acids have an even number of carbon atoms ranging
between 14 to 22 (most commonly 16 or 18). The carbon and hydrogen atoms form a long
hydrocarbon tail that is hydrophobic (having no affinity for water).
fig. 13.9 - Formation of a Lipid
Glycerol is a type of alcohol having three hydroxyl (-OH) groups.
A fatty acid is described as saturated if there are no double bonds between carbons of the molecular
chain. For example, Palmitic acid (16 carbon) and stearic acid (18 carbon).
fig. 13.10 - Common Fatty Acids
A fatty acid is described as unsaturated if one (mono) or more (poly) double bonds occur between the
carbon atoms of the chain. The 18 carbon unsaturated fatty acids are oleic acid, linoleic acid and
linolenic acid with one, two and three double bond respectively.
Fats that are generally liquids at room temperature are called oils. They are rich in unsaturated fatty
acids. For example, Groundnut oil, sunflower oil, safflower oil and so on.
Waxes are esters of long chain fatty acids with long chain alcohols in place of glycerol. Waxes are of
common occurrence in both plants and animals. They form a waterproof protective coating on animal
furs and plant leaves and stem. Cutin in the cuticle of leaves, suberin in the endodermis of root,
sebum in the hairs of mammals, cerumin in the wax glands of ear, bee wax produced by bees, are
some common examples.
Compound Lipids
These are lipids, which contain an inorganic or organic group in addition to fatty acids and glycerol.
They are of the following 3 types:
Phospholipids:These are lipids containing a phosphate group. A phospholipid molecule has a
strongly nonpolar and hydrophobic (water insoluble) tail region represented by fatty acid chains and a
strongly polar or hydrophilic (water soluble) head region represented by the phosphate group. Due to
this property, a phospho-lipid placed in water forms a lipid bilayer in a characteristic manner. Such an
arrangement forms a basic component of plasma membranes.
fig. 13.11- A Lipid Bilayer in an Aqueous Medium
Glycolipids:These are lipids containing a carbohydrate group, usually galactose. They are found
in the nerve cell membranes especially in the myelin sheath.
Lipoproteins:These are lipids, usually phospho-lipids which contain a protein molecule. They
occur in the cell membrane. They are also found in milk and egg yolk.
Sterols:These are lipids that do not have a straight chain. They are composed of four fused
carbon rings and a long hydrocarbon side chain. The most familiar example is cholesterol, which is
abundant in animal cells.
Cholesterol is an essential component of animal cell membrane. It is a precursor of several steroid
hormones produced in animals. It is also involved in the formation of bile salts and vitamin-D.
fig. 13.12 - Structures of Steroids
Ergosterol is a plant sterol, which can get converted into vitamin-D under the influence of ultraviolet
rays.
Some polyunsaturated fatty acids like archidonic acid form compounds called prostaglandins, which
are involved in influencing a variety of body functions.
GENERAL CLASSIFICATION OF LIPIDS
simple lipids (neutral lipids)
compound lipids
derived lipids
Simple lipids: esters of fatty acids with alcohols.
fats waxes
Compound lipids: compounds containing other groups in addition to an ester of a fatty acid
with an alcohol.
phospholipidscerebrosides
other compound lipids (spingolipids)
Derived lipids: substances derived from neutral lipids or compound lipids and having general
properties of lipids
fatty acids alcohols
hydrocarbonsmonoglycerides