(1) Term lipid was coined by Bloor.

(2) These are esters of fatty acids and alcohol.

(3) They are hydrophobic insoluble in water but soluble in benzene, ether and chloroform.

(4) Lipids are classified into three groups:–

(A) Simple lipids: These are the esters of fatty acids and glycerol. Again they are typed as:–

(a) Fats and Oils: (Natural lipids or true fats). These are triglycerides of fatty acid and glycerol. Fats which are liquid at room temperature are called oils. Oils with polyunsaturated fatty acids are called polyunsaturated e.g. sunflower oil, lower blood cholesterol.

(b) Fatty acids: Obtained by hydrolysis of fats. Formic acid is simplest fatty acid (HCOOH). These are of 2 types:–

(i) Saturated fatty acids: The fatty acids which do not have double bond in between carbon atoms.e.g. butyric acid, palmitic acid,hexanoic acid, etc. They have high melting points, solid at room temperature and increase blood cholesterol.

(ii) Unsaturated fatty acids: The fatty acids which have double bonds in carbon atoms. e.g. 8 hexadecanoic acid, 9 octadecanoic acid etc. They have lower melting points mostly found in plant fats, liquid at room temperature and lower the blood cholesterol.

(c) Waxes: These are simple lipids composed of one molecule of long chain fatty acid and long chain monohydric alcohol. Waxes have high melting point, insoluble in water, resistant to atmospheric oxidation, chemically inert and not digested by enzymes. They reduce rate of transpiration by making plant tissue water proof and work as excellent lubricant.

(B) Compound lipids: They contain some additional or element. Group with fatty acid and alcohol on the basis of group they may be of following types:

(a) Phospholipids: These contain phosphoric acid. It helps in transport, metabolism, blood clotting and permeability of cell membrane. It is a bipolar molecule i.e. phosphate containing end is hydrophilic whereas fatty acid molecules represent hydrophobic (non-polar tail).

(b) Glycolipids: These contain nitrogen and carbohydrate beside fatty acids. Generally found in white matter of nervous system. e.g. sesocine frenocin.

(c) Chromolipids : It includes pigmented lipids e.g. carotene.

(d) Aminolipids : Also known as sulpholipids. It contains sulphur and amino acids with fatty acid and glycerol. Cutin and suberin are also compound lipids resistant to water and also provide mechanical support in plants.

(iii) Derived lipids: These are obtained by hydrolysis of simple and compound lipids.

(5) Functions of lipids

(a) Oxidation of lipids yields comparatively more energy in the cell than protein and carbohydrates. 1gm of lipids accounts for 39.1 KJ.

(b) The oil seeds such as groundnut, mustard, coconut store fats to provide nourishment to embryo during germination.

(c) They function as structural constituent i.e. all the membrane systems of the cell are made up of lipoproteins.

(d) Amphipathic lipids are emulsifier.

(e) It works as heat insulator.

(f) Used in synthesis of hormones.

(g) Fats provide solubility to vitamins A, D, E, and K.