Biology_Today_-_October_2016

Types of Fatty Acids

Possess one or more double bonds in their carbon chains. The general formula

is CnH2n–2xO 2. Unsaturated fatty acids have lower melting point than the

saturated fatty acids. Most plant lipids have unsaturated fatty acids.

e.g., CH 3 (CH 2 ) 7 CH = CH (CH 2 ) 7 COOH

Oleic acid

Unsaturated Fatty Acids

Do not possess double bonds in their carbon chains. They have a

general formula of CnH2nO 2 , e.g., C 16 H 32 O 2 (palmitic acid), C 18 H 36 O 2

(stearic acid). Most animal lipids have saturated fatty acid.

e.g., CH 3 (CH 2 ) 14 COOH

Palmitic acid

Saturated Fatty Acids

Classification of Lipids

Lipids may be broadly classified into simple, complex and derived lipids.

Simple lipids

These are esters of fatty acids with alcohols. They are of two
types:

(i) Neutral or true fats: The neutral (without charge) or true fats are
composed of carbon, hydrogen and oxygen but have far fewer oxygen
atoms than carbon atoms. They are also called as triacylglycerols. A fat molecule consists of two components:
one molecule of alcohol called glycerol esterified to three molecules of the same or different long-chain fatty acids. A glycerol
molecule has 3 carbons, each bearing a hydroxyl (–OH) group.

Pure fats - Fats with three similar fatty acids, e.g., tripalmitin, tristearin.

Mixed fats - Fats having different fatty acids, e.g., butter

Oils - Fats which are liquid at room temperature. They are rich in unsaturated fatty acids, have low melting point and forms hard
fats on hydrogenation, e.g., groundnut oil, sesame oil, mustard oil.

Hard fats - Fats solid at room temperature of about 20°C. They contain long chain saturated fatty acids, e.g., animal fat.

(ii) Waxes - The waxes are lipids which are composed of long-chain saturated fatty acids and a long-chain saturated alcohol
of high molecular weight instead of glycerol.

Beeswax

It is a secretion of the abdominal glands of worker honeybees. It is

a thick, tenacious material plastic at room temperature.

Cerumen

It is a soft, brownish, waxy secretion of the ceruminous glands of the

external auditory canal. It is commonly called earwax.

Paraffin wax

A translucent, waxy substance obtained from

petroleum.

Sebum

It is a fatty secretion of the sebaceous glands

of the skin.

Examples of Waxes

Complex lipids

These are derivatives of simple lipids containing additional groups such as phosphate, nitrogenous base, protein etc.
They are further divided into :

(i) Phospholipids : These are composed of a molecule of glycerol or other alcohol having a phosphate group joined
to one of its outer –OH groups, two fatty acid molecules linked to the other two –OH groups, and a nitrogen-containing
base, bound to the phosphate group. A phospholipid molecule has a strongly nonpolar and hydrophobic “tail” consisting
of fatty acid chains and a polar and hydrophilic “head” comprising a negatively charged phosphate group and a positively
charged base.

(ii) Lipoproteins: Lipoproteins contain lipids (mainly phospholipids) and proteins in their molecules. They are main constituent
of membranes. Lipoproteins occur in the milk and egg yolk. Lipids are transported in the blood plasma and lymph as lipoproteins.
They are the transport vehicles for lipids in the circulation. There are five-types of lipoproteins, namely chylomicrons, very low
density lipoproteins (VLDL), low density lipoproteins (LDL), high density lipoproteins (HDL) and free fatty acid-
albumin complex.