Question

Describe the different types of lipids and their special characteristics. Give examples where appropriate.

Answer 1

Lipids are organic compounds that contain hydrogen, carbon, and oxygen atoms which helps as a structural and functional backbone of living cells.

Simple Lipid :-

Esters of fatty acids with various alcohols.

Fats: Esters of fatty acids with glycerol. Oils are fats in the liquid state.

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

Complex Lipids

Esters of fatty acids containing groups in addition to alcohol and a fatty acid.

Phospholipids: Lipids containing, in addition to fatty acids and alcohol, a phosphoric acid residue. They frequently have nitrogen-containing bases and other substituents, eg, in glycerophospholipids the alcohol is glycerol and in sphingophospholipids the alcohol is sphingosine.

Glycolipids (glycosphingolipids): Lipids containing a fatty acid, sphingosine, and carbohydrate.

Classification of Lipids :-
Lipids that are ester or amides of fatty acids :-

waxes – are carboxylic acid esters where both R groups are long straight hydrocarbon chain . Performs external protective functions. Example: Beeswax

Beeswax is an ester formed by combining palmitic acid with a 30-carbon alcohol.

Fats:

(a) They are esters of fatty acids with glyc­erol.

(b) They are found in nature in large quanti­ties.

(c) They are the best reserve of food material in the human body.

(d) They act as insulator for the loss of body heat.

(e) They act as a padding material for pro­tecting internal organs.

Physical Properties of Fats:

(a) The fats are insoluble in water, but readily soluble in ether, chloroform, benzene, car­bon tetrachloride.

(b) They are readily soluble in hot alcohol but slightly soluble in cold.

(c) They are themselves good solvents for other fats, fatty acids, etc.

(d) They are tasteless, odorless, colourless and neutral in reaction.

(e) Several neutral fats are readily crystallized, e.g., beef, mutton.

(f) Their melting points are low.

Complex lipids:-

Phospholipids (phosphatides):

(i) They are esters of fatty acids with glyc­erol containing an esterified phosphoric acid and a nitrogen base.

(ii) They are present in large amounts in nerve tissue, brain, liver, kidney, pancreas and heart.

Biological functions of phospholipids:

(i) They increase the rate of fatty acid oxida­tion.

(ii) They act as carriers of inorganic ions across the membranes.

(iii) They help blood-clotting.

(iv) They act as prosthetic group to certain en­zymes.

Classification:

It is based on the type of alcohol present in the phospholipid.

There are three types:

1. Glycerophosphatides — In this, glycerol is the alcohol group.

Example:

(i) Phosphatidyl ethanolamine (cephalin).

(ii) Phosphatidyl choline (Lecithin).

(iii) Phosphatidyl serine.

2. Phosphoinositides — In this, inositol is the alcohol.

Example:

Phosphatidyl inositol (Lipositol).

3. Phosphosphingosides — In this, sphingosine is an amino alcohol.

Example:

Sphingomyelin.

Glycolipids:

These contain an amino alcohol (sphingosine or iso-sphingosine) attached with an amide linkage to a fatty acid and glycosidically to a carbohydrate moiety (sugars, amino sugar, sialic acid).

These are further classified into:

(i) Cerebrosides.

(ii) Gangliosides.

(i) Cerebrosides:

(a) Cerebrosides contain galactose, a high molecular weight fatty acid and sphingosine. Therefore, they may also be classified as sphingolipids.

(b) They are the chief constituent of my­elin sheath.

(c) They may be differentiated by the type of fatty acid in the molecule.

(ii) Gangliosides:

(a) These are glycolipids occurring in the brain.

(b) Gangliosides contain ceramide (sphingosine + fatty acids), glucose, galactose, N-acetylgalactosamine and sialic acid.

Sphingolipid, any member of a class of lipids (fat-soluble constituents of living cells) containing the organic aliphatic amino alcohol sphingosine or a substance structurally similar to it. Among the most simple sphingolipids are the ceramides (sphingosine plus a fatty acid), widely distributed in small amounts in plant and animal tissues. The other sphingolipids are derivatives of ceramides.

Steroids:

The steroids are often found in association with fat. They have a similar cyclic nucleus resembling phenanthrene (rings A, B, C) to which a cyclopentane ring (D) is attached. The parent substance is better designated as cyclopentano perhydrophenanthrene. The position on the steroid nucleus are numbered as shown in below.

Methyl side chains occur typically at posi­tions 10 and 13 (constituting C atoms 19 and 18). A side chain at position 17 is usual (as in choles­terol). If the compound has one or more hydroxyl groups and no carbonyl or carboxyl groups, it is a sterol, and the name terminates in -OL.

Prostaglandins (PG):

(a) They virtually exist in every mammalian tissue and act as local hormones.

(b) They have important physiologic and pharmacologic activities.

(c) They are synthesized in vivo by cyclization of the center of the carbon chain of 20-C polyunsaturated fatty acids (e.g., arachidonic acid) to form a cyclopentane ring.

(d) Three different eicosanoic fatty acids give rise to three groups of eicosanoids charac­terized by the number of double bonds in the side chains, e.g., PG1, PG2, PG3. Varia­tions in the substituent groups attached to the rings give rise to different types in each series of prostaglandins, as for exam­ple, “E” type of Prostaglandin has a keto group in position 9, whereas the “F” type has a hydroxyl group in this position.

Prostacyclin’s (PGI):

(a) They are formed in vascular endothelium and continually formed in heart. They are also formed in kidneys.

(b) They are formed from cyclic endo-peroxide PGH2 by the action of microsomal Prostacyclin synthetase.

(c) They inhibit platelet aggregation and gas­tric secretion from the pyloric mucosa.

(d) They decrease blood pressure and protect coronary arteries.

(e) They increase renal blood flow and stimu­late renin production.

(f) They are inhibited by hyperlipemia, vit. E deficiency and radiation.