Lipids are the group of fats and fat like substances. They are the compounds of carbon, hydrogen and oxygen but they differ from carbohydrates in that the hydrogen and oxygen are not in the same proportions as in water. They are insoluble in water but soluble in organic solvents like acetone, ether, alcohol, etc.They needs a large amount of oxygen for their oxidation to release energy. True lipids are esters of fatty acids and an alcohol.
Sources of fats – the principal sources of fats in our diet are cooking oil, butter, ghee, oil seeds, milk, mutton, cheese, eggs, etc.
Classification of lipids – lipids are grouped into three categories:
(a) Simple lipids
(b) Complex lipids
(c) Derived lipids
(a) Simple lipids – these are various types of fats, oils or waxes.
(i) Fats: These are esters of fatty acids and glycerols. These are also called as triglycerides. Each molecule of glycerol can react with three molecules of fatty acids. Depending upon number of fatty acids attached they are called mono, di or triglycerides.
· Monoglyceride- one molecule of glycerol + one molecule of fatty acid.
· Diglyceride- one molecule of glycerol + two molecules of fatty acids.
· Triglycerides- one molecule of glycerol + three molecules of fatty acids.
(ii)Oils: These are fats, which are rich in unsaturated fatty acids (fatty acids with more than one double bond)
Importance of oils – Because of the presence of unsaturated fatty acids they are recommended to people who are suffer from high blood cholesterol of cardio- vascular diseases, they tend to lower the high blood cholesterol without raising the fat. The cholesterol leads to thickening of arteries, which became a cause of high blood pressure.
(iii) Waxes – These are esters of long chains fatty acids and long chains alcohol other than glycerol. They are mainly protective in functions and form water soluble coating on hair and skin in animals and on stems, leaves and fruits of plants.
(b) Complex lipids – These lipids are complex as they are formed in association of either of carbohydrates or phosphates or proteins.
(i) Glycolipids – These are formed by association of lipids with carbohydrate molecules. These are found in membranes.
(Glycolipids = lipids +carbohydrate)
(ii) phospholipids- These are lipids containing a phosphate group.(phospholipids= lipids + phosphate)
(iii) Lipoprotein – These are lipids associated with proteins molecules.(lipoproteins = lipids + proteins molecules)
c. Derived lipids – These include hydrolyzed products of simple and complex lipids.e.g. – Steroids (cholesterol).
Steroids - Steroids are composed of fused hydro- carbon rings and a long hydrocarbon chain.e.g. Cholesterol, Diosgenin.
Importance of cholesterol - It is a steroid lipid. It is said to create abnormal thickening of walls of arteries. It can raise blood pressure and hence may lead to circulatory problems. It is said that is not bad but not useful as well.
Importance of steroids –
(i) Cholesterol is a precursor molecule of many sex hormones like progesterone, testosterone and adrenocortico- tropic hormones.
(ii) Cholesterol on irradiation by ultraviolet rays forms vitamin D necessary for growth of bones.
(iii) Diosgenin is used in the manufacture of anti- fertility pills.
Function of lipids
· Lipids as rich source of energy – Lipids provide the energy fuel. The caloric value of 1gm of fats is 9.45 k cal while the physiological fuel value of 1 gm of fats is 9.0 k cal.
· As heat insulator – Fats are deposited in the subcutaneous tissues. These acts as insulator.
· As food reserve – Triglycerides stored in adipose tissue as principal food reserve.
· As solvent – Lipids acts as a solvent for fat- soluble vitamins like- A, D, E and k.
· Acts as cushions – They absorb mechanical impact around organs like eyeball.
· Phospholipids- They form constituents of membranes of various organs.
· Glycolipids- They form components of cell membranes and myelin of nerve cells.
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