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Classification of Lipids

The term lipids actually refer to a collection of organic molecules of varying chemical composition. They are grouped together on the basis of their solubility in non polar solvents. Lipids are commonly subdivided into four main groups.

The classification of Lipids
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- CH3(CH2)14COOH
  • Stearic acid: CH3(CH2)16COOH
  • Oleic acid- CH3(CH2)7CH=CH (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 

saturated and unsaturated fatty acid



         Fatty acids have four major biological roles :
  *  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
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 (C27H45OH) 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.

Many hormones like testosterones, progesterone etc are synthesised from cholesterol. Vitamin D is formed in our body from a derivative of cholesterol.

Ergosterol (C28H43OH) 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.
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