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Lipids

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Published in: Chemistry
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Lipids as macromolecules

T H / Kochi

2 years of teaching experience

Qualification: M.Sc, M.Phil

Teaches: Bio Technology

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  1. LIPIDS Synopsis v/ Lipids v/ General Formula v/ Classification of Lipids v/ Fatty acids v/ Glycerids v/ Nonglycerides v/ Complex lipids v/ Functions
  2. Lipids: Lipids are esters of long chain fatty acids and alcohols. They are waxy or oily substance present in all living organism. They are non-polar (hydrophobic) compounds, soluble in organic solvents (chloroform, benzene, ether etc.). Fatty acids have a long hydrocarbon chain with a terminal carboxylic acid group. General formula: CH3 (CH2) n COOH where n is an even number. Even though lipids exhibit a variety of structures, they have certain common characters: All lipids are formed of Carbon (C), Hydrogen (H) and sometime Oxygen (O). Small amount of phosphorus, nitrogen and sulphur are also present in some lipids. They are insoluble in water but soluble in non polar solvents like benezene and choloroform. Classification of Lipids Lipids Fatty acids Sphinholipids Steroids Waxes Glycerides Neutral glycerides Phospho glycerides Lipoproteins Lipids are commonly subdivided into 4 main groups 1. Fatty acids (saturated & unsaturated) 2. Glycerids(Glycerol containing lipids)
  3. 3. Nonglyceride (Sphingolipids, steroids, waxes) 4. Complex lipids (Lipo proteins) 1. Fatty acids They are long hydrocarbon chain organic acids with a terminal carboxyl group (COOH). The length of the hydrogen (H) chain differs from one fatty acid to another. Fatty acids are saturated and unsaturated carboxylic acids containing between 12-24 carbon atoms. Fatty acids with even numbers of carbon atoms occur most frequently in nature. Examples: Palmitic acid- Stearic acid: Oleic acid- (CH2)7COOH Saturated & Unsaturated Fatty acids It depends on the kinds of covalent bonds in the fatty acid parts of the molecule. A Saturated fatty acids has only single bonds between its carbon atom Mono and poly unsaturated fatty acids have one or more double bonds Olive oil (mono -unsaturated) and corn oil (poly -unsaturated) Differences between saturated and unsaturated fatty acid
  4. HO HO Saturated Fatty Acid H— —H H- -H Palmitic acid Saturated Fatty acid Contain only single C—C bonds Closely packed Strong attractions between chains High melting points Solids at room temperature Fatty acids have four major biological roles : • Unsaturated Fatty Acid - -H Linoleic - -H acid Unsaturated Fatty acid Contain one or more double C=C bonds Nonlinear chains do not allow molecules to pack closely Few interactions between chains Low melting points Liquids at room temperature They are components of membrane (glycerophospholipids and sphingolipids) Several proteins are covalently modified by fatty acids They acts as energy stores (triglycerols) Serve as hormones (Prostaglandins) and intracellular second messengers (DAG and IP3). 2. Glycerids: Glycerol containing lipids
  5. They are the most abundant lipids. The triesters of glycerol (triglycerides) are of greatest importance. Neutral triglycerides are important because of their ability to store energy. Triglycerides are esters of fatty acids with alcohols like glycerol. Fats and oils are generally called simple lipids. They are the main form in which fat is stored in plants and animals. Phospholipids: They are the common lipids found in cell membranes. Phosphoglyceride belong to a class of phospholipids. These contain glycerol, phosphates, two fatty acids and compound that may be choline, ethanolamine, serine or inositol. Lecithins and and ciphalins are representative of the phospholipids. Glycolipids: Glycolipids is similar to Sphingolipids. Instead of the phosphoric acid esterified to the amino alcohol, an oligo saccharide made usually of glucose or galactose is attached to the sphingosine. Galactosyl diglycerides and sulpholipid form are the major glycolipids of plants. Galactocerebroside is the main glycolipids found in the myelin sheath of nerve cells. 3. Nonglycerides: Sphingolipids, steroids, waxes Nonglyceride lipids consist of Sphingolipids, steroids and waxes. Sphingolipids They are similar to phospholipids, but Sphingosine is present instead of glycerol. Steroids They do not contain fatty acids but are included in the lipids because they have lipid like properties. They are composed of four fused carbon rings. They include bile salts, sex hormones etc. Waxes They are esters of fatty acids with long hydrocarbon chains. The alcohols in waxes have only one hydroxyl group. Cuticle is a waxy coating covering the epidermis of leaves and stem of plants. It prevents excessive evaporation of water. Sebum, the secretion of sebaceous glands in our skin is a mixture of waxes and triglycerides Sterols Sterols are alcohol derivatives of steroids. Cholesterol (C27H450H) is a common steroid found in the animal body. It may occur free or as an ester with fatty acids. Cholesterol are insoluble in water, so they are deposited in the arteries and veins when the amount increases. This may cause high blood pressure and heart diseases.
  6. Many hormones like testosterones, progesterone etc are synthesised from cholesterol. Vitamin D is formed in our body from a derivative of cholesterol. Ergosterol (C28H430H) is a sterol found in plants. Diosgenin is a steroid produced by the yam plant Dioscorea. It is used in the manufacture of anti fertility pills. 4. Complex lipids: Lipoproteins Lipoproteins are complexes which contain lipids and proteins in association. Plasma lipoproteins are complex lipids that transport other lipids through the blood stream. Lipoproteins provide the surface for biochemical reactions and energy production and also help in the transportation of lipids and proteins to the various parts of the plant or animal body. Chylomicrons, which have a density of less than 0.95 g/Ml, carry dietary triglycerides from the intestine to other tissues. The remaining lipoproteins are classified by their densities. Very low density lipoproteins (VLDL) have a density of 0.95-1.019 g/mL. They bind triglycerides synthesised in the liver and carry them to adipose and other tissues for storage. High density lipoproteins (HDL) have a density of 1.019-1.063 g/mL. They transport cholesterol from peripheral tissues to the liver. Low density lipoproteins (LDL) have a density of 1.063-1.210 g/mL. They carry cholesterol to peripheral tissues and help regulate blood cholesterol levels. Functions: 1. Major source of energy: Fats form the highly concentrated source of energy for man. Energy is released by the oxidative break down of fat if necessary. They are producing more than twice as much energy per gram as do the carbohydrates. 2. Food reserves: The neutral fats form the important food reserves in both plants and animals. In plants, they are stored in the seeds to provide nourishment for the embryo during germination. Eg: Oil seeds like ground nut, mustard, coconut and castor. The oil i extracted from the seeds is used for cooking and other purposes. 3. Protection and Insulation: In animals they are stored in adipose cells. Deposited beneath the skin and around the internal organs prevent the loss of body heat. They are act as shock absorbing cushions around the eye ball, kidney and gonads. About four percentage of the total body fat is reserved for this critical function.
  7. 4. Cell membrane structural components: Phospholipids, Glycolipids and sterols form the important components of biomembranes. Some glycolipids are found in myelin sheath. Waxes form a protective coat on plant parts and animal furs. 5. Hormones: Cholesterol IS used in the synthesis of steroid hormones, vitamin D and bile salts. Prostaglandins are hormone like compounds derived from fatty acids like arachidonic acid (20:4). 6. Vitamins: The lipid soluble vitamins (A,D E and K) play a major role in the regulation of several critical biological processes, including blood clotting and vision. 7. Vitamin absorption: Dietary fat serves as a carrier of the lipid soluble vitamins. All are transported into cells of the small intestine in association with fat molecules. Therefore a diet that is too low in fat can result in a deficiency of these four vitamins.

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