Lipids are structurally the most diverse biological molecule which is insoluble in polar solvents but soluble in non-polar solvents. Lipids are hydrophobic in nature, so it is insoluble in water. They contain predominately carbon and hydrogen atom in addition of oxygen, nitrogen and phosphorous atom. Lipids are insoluble biomolecule, defined by an overall lack of polarity necessary for solubility in water based solutions.

Types of Lipids:

In 1943 Bloor proposed this classification of lipid. According to him lipids can be categorized in three categories:

1.       Simple lipid

2.       Compound lipid

3.       Derived lipid


1.       Simple lipid: They are the esters fatty acid. Simple lipids include :

a.       Fats

b.      Waxes


a)      Fats: They are the esters of fatty acids with trihydroxy alcohol glycerol. Fatty acid consists of carbon, hydrogen, oxygen and carboxyl group at the end of linear hydrocarbon chain which contain atleast four carbon atom. Hydrocarbon chains are non-polar so they are mainly hydrophobic despite having one polar functional group.

Fatty acids are basically of two types on the basis of type of covalent bond present in hydrocarbon chain of fatty acid:                                                 

  i.   Saturated Fattyacid:  If all C-C bond in hydrocarbon chain are single covalent bond with as many hydrogen atoms are possible. Saturated fatty acid found in dietary animal product such as egg, milk and  meat.

   ii.  Unsaturated Fatty acid: When one or more C-C double bonds are present, the fatty acid becomes unsaturated with hydrogen atom. They are found in plants products such as vegetables, fruits and grains. Greater number of double bond higher degree of unsaturation.

Unsaturated fatty acids can be of two kinds on the basis of degree of unsaturation:

Monounsaturated fatty acid: It contains just a single double bond.

Polyunsaturated fatty acid: It contains two or more double bond.

a)      Waxes: They are the esters of long chain fatty acids with long chain of monohydroxy alcohol. Waxes have two monomers, one fatty acid is bonded through an ester linkage to one alcohol. The hydrocarbon chain in alcohol monomer of waxes varies from short linear chain to complex ring structure. Waxes provide protective barrier to prevent water loss and protect cell from external environment.

On the basis of number of glycerol attached to fatty acid simple lipids categorized in three categories:

  i. Monoglyceride: When single glycerol attached to fatty acid group then it is termed as monoglyceride.

  ii. Diglyceride: In diglyceride two glycerol attached with fatty acid.

  iii. Triglyceride: Three glycerol attached with fatty acid.

·         Fatty acids are triglycerides that have no double bond in carboxylic acid chain.

·         Trans fat: They are synthetic kind of triglyceride. They have trans double bond in their carboxylic acid chain. Trans means carbon on either side of double bond are on opposite side of double bond.

Fatty acids are also categorized in two types:

a)      Essential fatty acid: Such lipids cannot be constructed through any chemical pathways. In human they must be obtained from diet.

                             Eg: Linoleic acid and linolenic acid

b)      Non essential fatty acid: They are not necessarily to be taken through diet. They must be synthesized through chemical pathways.


2.       Compound lipid:

They are the esters of fatty acid and alcohol contain other group also. Compound lipids are categorized in three categories:

a.       Phospholipid or phosphatids:

Phospholipids are compound containing fatty acid and glycerol in addition to phosphoric acid, nitrogen bases and other substituents. They usually possess one hydrophilic head and two non polar tails.

Phospholipids are basically of three types;

§  Phosphoglycerides: They are found in membrane. They contain fatty acid molecule which are esterified to hydroxyl group and glycerol. The glycerol group also form an ester linkage with phosphoric acid.

Eg: Lecithin, Cephalin

§  Phosphoinositides: They are found in brain tissue and soyabean. They also play an important role in transport process of cell.

§  Phosphosphingosides: They are found in nerve tissue.

 Eg: Sphingomylein

b.      Glycolipid: In this type carbohydrate is attached to fatty acid group. It contain nitrogen but no phosphoric acid. Glycolipids also include certain structurally related compounds comprising the groups like ganglioside, sulpholids and sulphatids.


3.       Derived lipid: It is composed of hydrocarbon rings and a long hydrocarbon side chain. Derived lipids are composed of the substance obtained by the hydrolysis of simple and compound lipid included fatty acid, alcohol, monoglycerides,diglycerides, steroids, terpenes, carotenoids. Most common are:

·         Steroids: Steroids are mass of lipid containing four fused carbon rings. Steroid contains one or few small functional group including hydroxyl, carbonyl and carboxyl. Cholesterol and other steroids containing hydroxyl group are called sterols.

·         Terpenes: It is found in plants. Eg: Natural rubber

·         Carotenoids: They are tetraterpenes. Carotenoids are widely distributed in both plants and animals. They are exclusively of plant origin. Due to presence of many conjugated double bonds they are coloured red or yellow.

Eg: Lycoprene, carotenes,xanthophylls



Function of lipid:

1.       They are storage compounds. Triglycerides serve as a energy reserve of body.

2.       They are important component of cell membrane structure in eukaryotic cell.

3.       Regulate membrane permeability.

4.       Serve as source of fat soluble vitamins like A, D, E, K.

5.       Act as electrical insulators to the nerve fibres, where myelin sheath contain lipid.

6.       They are the components of some enzyme system.

7.       Prostaglandins and steroid hormones act as cellular metabolic regulators.

8.       Cholesterol found in cell membrane, blood and bile of many organisms.

9.       Act as signaling molecule.

10.   A layer of fats in the subcutaneous layer provides insulation and protection from cold.

11.   Cholesterol is the precursor of bile acids, vitamin D and steroids.

12.   Eicosanoids, group of lipid related molecule act as important regulators of various biological function. The major eicosanoids are prostaglandins, thromboxanes and leukotrienes.

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