Chemically, Lipids can be defined as esters of fatty acids with alcohol. They are insoluble in water and soluble in organic solvents such as alcohol, ether, benzene and chloroform.
Based on the chemical nature, lipids are classified as
These are esters of fatty acids with glycerol or long chain alcohols. They are further classified as follows.
(a) Fats: Fats are glyceric esters of fatty acids. Lipids in animals are called as fats while the lipids in plants are called as oils. Example: Triacylglycerol.
(b) Waxes: Waxes are esters of fatty acids with long chain monohydric alcohols.
Examples: cerylmyristate (bees wax).
Compound lipids are esters of fatty acids with alcohol, and they contain extra groups. Depending upon the extra group present they are subdivided as follows:
A glyceric ester containing phosphate and nitrogen base or an alcohol are called phospholipids. These lipids are present in large amounts in nerve tissue, brain, liver, kidney, pancreas and heart. Phospholipids are further classified into three types based on the type of group connected to phosphatidyl group.
In these phospholipids, a nitrogen base is connected with phosphatidyl group.
Examples: Lecithin, Cephalin
In these phospholipids, inositol is connected with phosphatidyl group.
Example: Phosphatidylinositol (lipositol)
A group of phospholipids containing 1-phosphocholine combined with a ceramide (sphingosine + fatty acid). Example: Sphingomyelin
The lipids which contain a carbohydrate moiety linked with ceramide is called a glycol lipid.
Examples : Cerebroside and gangliosides.
A complex of proteins, lipid and cholesterol is called as a lipoprotein. The protein moiety in the lipoprotein is known as apoprotein.
· Very low density lipoprotein (VLDL)
· Low density lipoprotein (LDL)
· High density lipoprotein (HDL)
Lipids that are derived from the hydrolysis of simple and compound lipids are called derived lipids.
Examples: diacylglycerol, fatty acids, glycerol and cholesterol.
Lipids perform several biological functions such as,
· Triglycerides serve as energy reserve of the body.
· Lipids are important components of cell membranes which regulates membrane permeability.
· Phospholipids, provide fluidity and flexibility to the cell membranes.
· Lipids act as signalling molecules.
· Fat layer provides insulation from cold.
· Lipoproteins transports lipids throughout the body.