NEET Biology Notes Biomolecules and Enzyme Lipids
Lipids
The term ‘lipid’ was first of all used by Bloor (1943). Lipids are water insoluble, esters of fatty acids and alcohols, which consist of C, H and O but the ratio of H and O is more than 2:1.
A fatty acid has a carboxyl group attached to an fl-group. The 11-group could be a methyl (—CH3) or ethyl (—C2H5) or higher number of —CH2 groups. Fatty acids could be saturated (without double bond) or unsaturated (with one or more C=C double bonds). They can be monoglycerides, diglycerides and triglycerides. Based on melting point, they are also called as oils and fats. Oils have lower melting points and hence remain oil in winters.
Lipids include fats, phospholipids and steroids and can be classified as follows
Lipids are less dense than water and therefore, float.A major function of lipids is to act as energy stores.Lipids have higher calorific value than carbohydrates,therefore, a given mass of lipid will yield more energyon oxidation than an equal mass of carbohydrate.Triglycerides are the commonest lipids in nature,Tristearin is an example of a lipid, which consists ofthree stearic acid molecules with 17 carbon atom tail in each stearic acid.Fats are larger molecules but they are not polymers.A fat consists of glycerol and fatty acids. Glycerol is an alcohol with three carbons, each bearing a hydroxylgroup. A fatty acid is a long carbon skeleton varying insize but are commonly 14-24 carbons long. At one end of the fatty acid is a ‘head’ consisting of a carboxylicgroup. Head is attached to a long hydrocarbon ‘tail’.Three fatty acids can be joined to three OH groups of glycerol by an ester linkage, a bond between a hydroxyl
group and a carboxyl group can form a fat or triacylglycerol.
Glycoconjugate is the covalent linkage of a carbohydrate, to a protein or lipid.
Proteoglycans are glycosaminoglycan chains covalently linked to protein. Carbohydrate moiety is the main site of biological activity, e.g. cartilage (connective tissue). Glycoproteins have one or more oligosaccharide chains joined covalently to a protein. They are used as receptors for cell signaling. Glycolipids are membrane lipids, in which, the hydrophilic group is an oligosaccharide.
Fatty Acids
- Most of the fatty acids found in nature have an even number of carbon atoms (usually 14 to 24). The general formula of a saturated fatty acids is CH3 (CH2) nCOOH. If a fatty acid has only one double bond (palmitoleic acid and oleic acid), it is said to be monounsaturated.
- Fatty acids lacking double bonds are called saturated fatty acids, e.g. palmitic acid and stearic acid.
- Fatty acids some times contain one or more double bonds (C=C) such as oleic acid and are known as unsaturated fatty acids.
If a fatty acid has more than one double bonds (2 in linoleic acid, 3 in linolenic acid and 4 in arachidonic acid), it is said to be polyunsaturated.
Unsaturated fatty acids are slightly more abundant in nature than saturated fatty acids, especially in higher plants.
A diet rich in saturated fats is one of several factors that may contribute to the human cardiovascular disease known as atherosclerosis. A layer of fat beneath the skin insulate the body. This subcutaneous layer is especially thick in whales, seal and most other marine mammals.
Fats that are generally liquid at room temperature are called oils. Oils are rich in^msaturated fatty acids and have low melting points. In hydrogenation, the unsaturated fatty acids become saturated and oil becomes solid fat (hydrogenation). Fat is a molecule used by most animals for long term energy storage.
Rich source of polyunsaturated fatty acids in the diet is vegetable oils. Sunflower oil is rich in linoleic acid and minimum fatty acid content is in coconut
oil. Mustard oil is one of the most unsaturated.
Essential fatty acids serve as a precursor for synthesis of prostaglandins. Essential fatty acids are some polyunsaturated fatty acids, which cannot be synthesized in the animal body and must be supply with food to avoid ir deficiency. Linoleic, linolenic and arachidonic ^^ids are essential fatty acids for man.
Linolenic acid is an essential fatty acid with three double bonds. Maximum number of double bonds present in essential fatty acids is four.
Phospholipids are main constituents of plasma membrane because they contain both polar and non-polar portions.
Lecithin is a phospholipid. Lecithin is an excellent emulsifying agent, which is produced commercially from soyabean (Glycine max) seeds.
Steroids are derivatives of a four membered ring known as phenanthrene.
Cholesterol is insoluble in water and chemically unreactive. Cholesterol is synthesised from acetyl Co-A. or acetate in liver.
Phytosterol is a steroid found in plants. Cholesterol content is minimum in vegetable oils. Diosgenin is a steroid obtained from the plant called Dioscorea. It is used for manufacturing antifertility pills.
Animal hormones such as androgens, oestrogens, progesterons, glucocorticoids, mineralocorticoids, etc, are steroids, which are derived from cholesterol. Bile salts are derivatives of cholesterol, which are synthesized in liver. They help in emulsification of fat in small intestine.
Saponins are glycoside having steroids (e.g. sapogenin) which form foam when their watery solution is shaken. Saponins are widely distributed in plants (e.g. Saponaria, Quillaja). They can dissolve RBCs even in high dilution.
Prostaglandin are derivatives of arachidonic acid and other C20 fatty acids, which have several functions like vasodilation, vasoconstriction, bronchoconstriction, acid production in stomach, cell communication and hormone modulation. In most fungi, ergosterol replaces cholesterol in the cell membranes. Ergosterol inhibitors are antifungal compounds. A high concentration of cholesterol in .blood may contribute to atherosclerosis.
The complex lipids may be of following types:
- Chylomicrons which transport triglycerides from intestine to other tissues except kidneys.
- Very Low Density Lipoproteins (VLDL) which bind triglycerides in liver and carry them to fat tissue.
- Low Density Lipoproteins (LDL) which carrry cholesterol to peripheral tissues.
- High Density Lipoproteins (HDL) which bind to plasma cholesterol and transport cholesterol to liver.
The main biological functions of lipids are
- Lipids are concentrated source of energy and provide fuel for aerobic respiration.
- Phospholipids form the structural framework of cellular membranes.
- Waxes and oils are secreted on surfaces to provide waterproofing in plants and animals.
- Fats absorb shocks, organs that are prone to bumps and shocks (e.g., kidneys) are cushioned with a relatively thick layer of fat.
- Lipids are a source of metabolic water. During respiration, stored lipids are metabolized for energy, producing water and carbon dioxide.
- Stored lipids provide insulation, increase body fat levels in winter and reduce heat losses to the Lipid constructed from five carbon compound isoprene are called terpenes.
- Isoprene and its derivatives are joined in various combination to produce substances such as Vitamin-A and carotenoids.
Carotenoids are isoprenoid hydrocarbon a plant pigment containing 40 carbon atoms. - Natural rubber is a polyterpenes.