Lipids
Lipids are esters of fatty acids and polyhydric alcohol. The term ‘lipid’ was first used by Bloor (1943). These are the compounds of C, H, O but the ratio of H and O is more than 2:1 (i.e., the ratio of oxygen is lesses as compared to carbohydrates). These are water-insoluble organic substances which can be extracted from the cells by organic solvents such as ether, chloroform and benzene. Their general form ula is C nH2nO2. Some lipids
have P, N and S also.
1. Simple lipids
Esters of fatty acids with alcohol. Simplest alcohol in fats is glycerol( a trihydric alcohol) e.g., fats, oils and waxes. Triglycerides are common in nature.
2. Compound lipids
These lipids contain an additional group alongwith fatty acids and alcohols, e.g., phospholipids, glycolipids and lipoprotein.
3. Derived lipids
These are isoprenoid structures e.g., steroids, terpenes, carotenoids.
• Fatty acids are carboxylic acid with a chain of more than four carbon atoms ending with COOH group. Plants can synthesize all fatty acids. Animals can not synthesize linoleic, linolenic and arachidonic acid. These are called essential fatty acids. Their deficiency causes sterility, kidney failure and stunted growth.
• Saturated fatty acids have no double bond. Their melting point is high. Palmitic acid,
stearic acid are saturated fatty acids.
• Unsaturated fatty acids are commonly present in vegetable oils, cod/shark oil. Their melting points are low. Oleic acid has one double bond, linoleic acid has two, linolenic acid has three and arachiodonic acid has 4 double bonds. Fatty acids with more than one double bond are called polyunsaturated fatty acid (PUFA).
• Drying oils are unsaturated fatty acids which can be converted in hard fats on
being exposed.
• Edible oils can be converted into hard fats through hydrogenation.
• Waxes are esters of long chain monohydric alcohols like cetyl, ceryl or mericyl.
• Lanolin forms a protective, water insoluble coating on animal fur.
• Bees wax secreted from abdominal glands of honey bees has palmitic acid
and mericyl alcohol.
• Paraffin wax is a petroleum product.
• Cutin is formed by cross esterification and polymerization of hydroxyl fatty
acids and other fatty acids without esterification by alcohols other than glycerol. Cuticle has 50-90% cutin.
• Suberin is condensation product of glycerol and phellonic acid. It makes the
cell wall impermeable to water.
• Phospholipids are triglycerides in which one fatty acid is replaced by phosphoric
acid which is often linked to additional nitrogenous groups like choline (in lecithin), ethanolamine (in cepalin), serine or inositol. These are amphipathic i.e. have
both polar and non polar groups. These form cell membranes along with proteins.
• Sphingolipids have amino alcohol sphingosine. Sphingomyelins are present in
myelin sheath of nerves. They have additional phosphate attached to choline, are present in nerve membrane. Cerebrosides are present in nerve membrane and have galactose.
• Gangliosides – have glucose, galactose, sialic acid and acetyl glucosamine, present in grey matter, receptors of viral particles, excess causes Tay-Sachs disease.
• Sterols or steroids contain 4 fused hydrocarbon rings called cyclopentane perhydrophenanthrene and a long side chain e.g. cholesterol, stigmasterol, campesterol, sitosterol, ergasterol.
• Cholesterol helps in absorption of fatty acids, sex hormones, vitamin D and bile
salts. Potato is rich in cholesterol. Excess of cholesterol causes atheroscherosis.
• Prostagladins are hormone modulators.
• Terpenes are lipid like carbohydrates formed of isoprene units e.g., menthol, camphor, carotenoids.
• Major function of lipids is to act as energy stores.
• Plants usually store oil than fats. Seeds, fruits and choloroplasts are often rich
in oils.
• Glycolipids are important components of cell membranes, chloroplast
membranes.
• Fatty substance in the cell wall (Wax, cutin, suberin) helps reduce transpiration
and provide mechanical protection from injury and parasites.
• Diosgenin is a steroid obtained from the plant called Dioscorea. It is used for
manufacturing antifertility pills.
Comments
Post a Comment