Lipids: Definition, Classification, Functions

Lipids

Definition

Lipids are the heterogenous group of compounds, actually or potentially related to fatty acids. Chemically, they can be defined as esters of fatty acids with alcohol.

Lipids are relatively insoluble in water and soluble in solvents like ether, chloroform and benzene.

Classification : Lipids are classified into :-

1. Simple lipids

These are esters of fatty acids with various alcohols. They are further classified into, Fats : Esters of fatty acids with glycerol. A fat in the liquid state is known as oil. Simple lipids in animals are called as fats and in plants as oils.

(eg) Triacyl glycerol.

Waxes : Esters of fatty acids with high molecular weight monohydric alcohols.

(eg) : Cholesterol ester, myricyl palmitate and cetyl palmitate.

2. Compound lipids

These are esters of fatty acids with alcohol, but it contains extra groups in addition to alcohol and a fatty acid. They are subdivided into the followings :

A.Phospholipids (Phosphatides)

They are esters of fatty acids with glycerol containing an esterified phosphoric acid and a nitrogen base. These lipids are present in large amounts in nerve tissue, brain, liver, kidney, pancreas and heart. Phospholipids are further classified into 3 types based on the type of alcohol present in the phospholipid

(i) Glycerophosphatides - In this, glycerol is the alcohol group.

(eg) Lecithin - Choline as base

Cephalin - Ethanolamine as base

Phosphatidyl serine - Serine as base

Plasmalogens - either Choline or Ethanolamine as base.

(ii) Phosphoinositides

In this phospholipid, inositol - an hexahydric alcohol is present along with the back bone glycerol (eg) Phosphatidyl inositol (lipositol)

(iii) Phosphosphingoside - in this phospholipids, sphingosine is the alcohol (amino alcohol)

(eg) Sphingomyelin.

B. Glycolipids - These lipids contain a carbohydrate moiety along with an amino alcohol. These are further classified into Cerebrosides : Contain galactose, a high molecular weight fatty acid and sphingosine.

Gangliosides - contain ceramide (sphingosine + fatty acid) glucose, galactose, N-acetyl galactosamine and sialic acid.

C. Lipoproteins - lipids like triacyl glycerol, phospholipids, cholesterol and cholesteryl esters and free fatty acids combine with protein in particular proportions and form a hydrophilic lipoprotein complex (eg) chylomicrons, very low density lipoprotein (VLDL), low density lipoprotein (LDL) and high density lipoprotein (HDL). The protein moiety in the lipoprotein is known as apoprotein.

3. Derived lipids :

These are lipids which are derived from the hydrolysis of simple and compound lipids. (eg) diacyl glycerol, fatty acids, glycerol and cholesterol.

Functions

·           Fats stored in adipose tissue serve as an efficient source of energy

·           Fats serve as a thermal insulator in the subcutaneous tissue and around the internal organs.

·           They also act as electrical insulator against nerve impulse transmission.

·           Cholesterol and phospolipids are important constituents of cell membranes.

·           Lipoproteins and glycolipids maintain cellular integrity and permeability.

·           Fats serve as the important source of fat soluble vitamins.

·           The phosphatides of blood platelets are involved in blood clotting mechanism.