The lipids on the basis of their occurrence and constitution are classified into three groups:
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1. Simple lipids:
The simple lipids are the esters of fatty acids with trihydric alcohol, glycerol. Ester is a combination of an alcohol with an acid. Simple lipids include the true fats, oils and waxes.
(i) Fats:
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Fats are the most important lipids which also include the oils occurring in liquid state. They are used as fuel in the biological oxidation.
The chief sources of fats are butter, ghee, liver oils, groundnut and almonds. There are certain seeds, which contain oil, these are castor oil seeds, soyabeans, linseeds (flax seeds), cotton seeds and palm kernels.
They occur as storage forms of energy in the vertebrates, insects and some other animals.
The true fats, sometimes called neutral fats, are the simple lipids. They are known as the triglycerides of the fatty acids because the fat molecules are made up of one molecule of glycerol (g) ‘cerine) and three molecules of fatty acids.
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Glycerol molecules contain a chain of three carbon atoms and have the formula C,H6. Fatty acids vary in their chemical makeup but those found most often in common fats contain sixteen to eighteen carbon atoms. The structural formula of a fat may be represented as follows:
H2=—C—O—R
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H—C=0—R
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H—C—O—R
During the formation of fats, each of the three fatty acid molecules is joined to the glycerol molecule by a dehydration synthesis reaction, in which a molecule of water is released for each fatty acid molecule joined.
In fat digestion water is combined with fat molecules in a process known as hydrolysis that results in the breaking of the fat molecule into three fatty acids and glycerol again.
In the body of the animals fats are found mostly as saturated fats in the several depots of adipose tissues, over the kidneys, in the great omentum, etc.
(a) Fatty acids:
Fatty acids are the compounds of straight chain of carbon atoms with one carboxyl group at the end.
Although in some cases these chains are very long and may have side chains also. The fatty acids, on which the consistency of fat depends and which take part in the formation of fats and oils, are of two types:
i. Saturated fatty acids,
ii. Unsaturated fatty acids.
(i) Saturated fatty acids:
Myristic, palmitic and stearic acids are said to be the most common saturated fatty acids as all the bonds of carbon atom are saturated. These are found in abundance in all naturally occurring fats.
(ii) Unsaturated fatty acids:
Oleic, linoleic, linolenic and arachidonic acids are the most common unsaturated fatty acids as their double bonds are not satisfied.
These fatty acid have special physiological value and are easily oxidised or may even ^oxidize spontaneously on exposing to sunlight or ultraviolet rays.
They may change into saturated fatty acjds on the addition of reducing agents. It is all due to the presence of unsatisfied double bonds.
Properties of fats:
The variable but important properties of fats can be studied under the following heads :
1. Solubility:
Fats are tasteless, colourless, odourless and semisolid substances which are insoluble in water but readily soluble in non-polar solvents like ether, alcohol and chloroform.
2. Consistency:
The fats occur either in the form of semisolid like mutton, soft solid like butter or oily like olive.
This varied consistency of fats is determined by the relative proportion of saturated and unsaturated fatty acid groups in their composition.
Fats in which palmitates and stearates predominant are comparatively hard at room temperature, while those composed chiefly of olein are oils.
3. Melting point:
Fats vary in their melting points. It is due to the relative proportions of the fatty acid present in the fat.
4. Desaturation:
Fats which occur in nature are generally of two types, saturated with saturated fatty acids and unsaturated with unsaturated fatty acids like olein.
The saturated fats do not absorb iodine where as the unsaturated fats do so. During oxidation decarboxylation or loss of carbon atoms takes place, after the rupture of the fat at the point of unsaturation.
Desaturation thus, means breaking of longer chains into shorter ones, needed for easy transport, esterisation and combustion.
5. Saponification:
Fats, when boiled with an aqueous or alcoholic solutions of alkalis (NaOH and KOH), undergo saponification forming soaps.
From a dilute solution the soap could be thrown down after adding the neutral salts.
C3Hs (COOC17H3S)3+3NaOH = 3C17H35COONa + C3H5 (OH)3 Tristearin Caustic soda Sodium steorate Glycerol.
6. Emulsification:
When a fat is shaken with water, it breaks down and separates out into definite layers of oil on water to form an emulsion. Mucilage, alkalis and bile salts are known more emulsifying agents than water.
7. Hydrolysis:
Natural fats are hydrolysed under prolong heating and readily break up into their components, fatty acids and glycerol.
(ii) Waxes:
Waxes are esters of fatty acids and complex monohydric alcohol instead of glycerol of fats. They are fairly hard and their melting points are sufficiently high.
They form protective and water resistant layers. They do not occur in human body. The fat spliting enzyme, lipase has no action on them and as such they are not suitable as food.
2. Complex lipids:
Lipids with additional radicals are called complex lipids. Some of the important complex lipids are as follows:
(i) Phospholipids or phosphatides:
Such type of lipids contain fatty acids, glycerol, phosphoric acid and an organic nitrogenous base in their molecules. Lecithins, cephalins and sphingomyelin are the most important phospholipids.
These lipids probably occur in every living cell and play an important part in the structure of the cell particularly the cell membrane.
These lipids can also react with protein to form lipoproteins. They are found in abundance in the brain and nervous tissues. They possess both hydrophilic and hydrophobic proporties.
(ii) Glycolipids or glucolipids:
These lipids contain fatty acids, carbohydrate radicals and complex amino alcohols like sphinogosine in their molecules.
These were long known as the crebrosides, according to Thudichum as they are found in abundance m nervous tissue.
The two cerebrosides which have been distinguished so far, are phrenosin and cerasin. These on hydrolysis yield fatty acids, sphinogosine and galactose.
(iii) Chromolipids:
These lipids, besides fatty acids and glycerol contain carotenoids and other related pigments in their composition, example—carotene.
(iv) Aminolipids and sulpholipids:
These lipids, in addition to fatty acid and glycerol, contain amino acids and sulphur compounds in their composition respectively.
3. Derived lipids:
These lipids are obtained by the hydrolysis of simple and compound lipids. Although these are the products of hydrolysis of lipids, but even then they have some properties of lipid. Examples—cholesterol and ergosterol.