Bloor in 1943 proposed the term lipid for those naturally occurring compounds which are insoluble in water but soluble in organic solvent. These are also the compounds containing carbon, hydrogen and oxygen like carbohydrate but contain much lesser ration of oxygen than carbohydrates.
Lipids are important diverse group of biological molecules widely distributed among plants and animals.
(i) Acyl glycerol (fats and oil)
(ii) Waxes
(iii) Phospholipids
(iv) Terpenoids
(i) ACYL GLYCEROL
These are found in plants and provide energy for different metabolic activity and are very rich in chemical energy. When compared an equal amount of acyl glycerol contains over twice the energy content than carbohydrate. It consists of glycerol molecule linked to three fatty acids. This condensed molecule is also called a Tetracyl glycerol (Trigly ceride). They are two types
(a) Saturated acyl glycerol: They contain saturated fatty acids i.e. they do not contain any double bond between carbon atom. They solid at ordinary temperature, mostly found in animals e.g. stearin.
(b) Unsaturated acyl glycerol: They are liquid at ordinary temperature. They are found in plant also called oil e.g. Linolin found in cotton seed contains Linoleic acid.
(ii) WAXES
Waxes are simple lipids with one molecule of fatty acids forming bond with one molecule of long chain alcohol e.g. Bee’s wax.
Waxes are found as protective coating on stems, stalks, leaves, petals, fruit waxes are water repellent and non reactive.
(iii) PHOSPHOLIPIDS
Phospholipids is most important class of lipids from biological point of view. Phospholipid is similar to Triacyl glycerol or an oil except that one fatty acid is replaced by phosphate group. Phospholipid molecule consists of two end which are called hydrophilic (water loving) end (heat) and hydrophobic (water fearing) end (tail).
Phospholipids are present in all living cells frequently associated with membranes and are related to vital functions such as regulations of cell permeability and transport processes. Proteins of cell membrane depend on their phospholipids component.
(iv) TERPENOIDS
Terpenoid is a large and important class of lipids built up of isoprenoid (C5H8) units. Steroids, carotenoids and terpens are important classes of it. They help in oxidation reduction processes as terpens, some are components of essential oils of plants e.g. Menthal, camphor, mint etc. Plant pigments like carotene, xanthophylls are also form of terpenoids.
(a) Terpenses: They are volatile in nature produce special fragrance. Some of them are used in perfumes e.g. Myrcene from oil of bay Geraniol from rose, limonene from lemon oil and Menthol from peppermint oil.
(b) Carotenoids: They consist of fatty acids like carbon chain which are conjugated by double bonds and carrying 6 membrane carbon ring at each end. These compounds are pigments producing red, orange, yellow, cream and brown colours in plants.
Another group of pigmented compounds are Tetra phyrrol which are present as an important part of familiar chlorophyll and cytochromes pigments.
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