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Introduction

LIPIDS

The word lipid has been derived from the Greek word liposand they are the esters of fatty acids with alcohols. Generally, they are triglycerides, containing three fatty acids linked to the alcohol, glycerol. Lipids are hydrophobic and may contain some hydrophilic groups. They are made up of C, Hand O, but sometimes P, N, S are also present. They are insoluble in water (polar solvent) but, soluble in organic solvents (non-polar solvents), e.g. benzene, petroleum ether. acetone, chloroform, hot alcohol, etc.

Lipids are of three types:

 

  1. Simple Lipids: They are esters of fatty acids with alcohols, e.g. fats (solid lipids), oils (liquid lipids), waxes, cutin, suberin, etc. Fats(having higher m.p. and generally, solid at room temperature) and oils(having lower m.p. and liquid at room temp.) are the esters of fatty acids with alcohols, whereas, waxes are esters of high molecular weight fatty acids with alcohols other than glycerol.

(i) Oils and Fats: They are generally triglycerides, where three fatty acids (that may be similar or dissimilar) are esterified with the three hydroxyl groups of the trihydroxy alcohol, glyceroland are the main energy storing compounds in animals. In monoglycerides

and diglycerides, only one or two hydroxyls of the glycerol are esterified with one or two fatty acids, respectively. During each ester group formation between the -COOH of fatty acids and the -OH of alcohol a molecule of H20 is given out.

The C chain length in fatty acids ranges from 4-24 and these fatty acids can be saturated (without any double or triple bonds) or unsaturated (with one or more double or triple bonds), e.g., Saturated fatty acids (have high m.p.):

(i) Butyric acid (4C chain)

(ii) Palmitic acid (16 C chain)

(iii) Stearic acid (18 C chain)
(iv) Lignoceric acid (24 C chain)

Unsaturated fatty acids (have low m.p.):

(i) Oleic acid (18 C chain with 1 double’ bond, C9= ClO)
(ii) Linoleic acid (18 C chain with 2 double bonds)

(Iii) Linolenic acid (18 C chain with 3 double bonds)

Oils rich in unsaturated fatty acids are found ingroundnut, mustard, sesame (til), safflower, etc. seeds, that provide nourishment to young plant during their germination

As the number of double bonds in fatty acids increases, their m.p. decreases. Fatty acids 16 C and 18 C chain length are most common, particularly the unsaturated ones, in living systemHydrocarbon chains of the fatty acids are non-polar (hydrophobic).

Upon hydrolysis, these lipids by alkali of (saponification) form soaps (salts of fatty and alcohol (glycerol)

Similarly, hydrolysis of these lipids by the enzyme lipase yields fatty acids and alcohol.

(ii) Waxes: They are esters of high molecular weight saturated fatty acids with long chain alcohols other than glycerol.

They are found as water barrier in exoskeleton of insects, in honey beehives, feathers of birds, furs of animals, as protective coating over human skin, fruits and leaves, etc. In many bacteria the wax present over the cell walls imparts pathogenicity as they are not engulfed by WBC of blood, e.g. Mycobacterium tuberculosis causing tuberculosis, leprosy bacterium etc.

2. Compound Lipids: They have additional groups in addition to fatty acids and alcohols, e.g. phospholipids, glycolipids, etc. (Fig4.5). In phospholipids, the phosphoric acid and two fatty acids are esterified to glycerol. They are amphipathic having both polar and non-polar groups, and in aqueous media they arrange in the form of a lipid bilayer with fatty acid tails towards the inside and phosphate groups towards the outside in contact with aqueous medium. They are found in cell membrane, egg yolk, etc.

In glycolipids, fatty acids and one or more sugars (usually galactose) are esterified to glycerol

3. Derived Lipids: Though, they have similar physical properties like lipids, but, chemically they have fused hydrocarbon four-ring structure (sterols) with hydrocarbon side chains. Thus, they are not esters of fatty acids with alcohols and, thus, are not true lipids, e.g. vitaminsA, D, E and K, cholesterol (Fig. 4.6) found in cell membrane, blood, bile, hormones like testosterone produced by
testes, estrogen and progesterone by ovary, cortisol by adrenal cortex that regulates metabolism of fats, carbohydrates and proteins, etc.

QUESTIONS

  1. What are lipids and discuss their role in living system.
  2. Write short notes on

(i)                 Simple lipids

(ii)               Compound lipids

(iii)             Honey bee wax

(iv)             Oils and fats