Classification of Lipids

LIPID

Lipids more commonly known as fats and oil, are integral part of our food. They are insoluble in water but soluble in organic solvents. They occur in both plant and animals. Lipids are a concentrated source of energy.

CLASSIFICATION

Lipids are classified into simple, compound and derived lipids which are further subdivided as follows.

1.Simple Lipids

a) Fats and oils

b) Waxes

2.Compound Lipids

(a) Phospho lipids

(b) Glyco lipids

(c) Lipo proteins

3.Derived Lipids

(a)Sterols

(b)Fatty acids

Simple lipids

Fats and Oils are included in this type. At room temperature, oils are liquids and fats are solids. Fats and oils contain esters of fatty acid and glycerol, a form in which lipids are present in food.

Compound lipids

They are esters of fatty acids containing phosphorous carbohydrate or protein. Phospholipids contain a phosphoric acid in addition to the alcohol and fatty acids.

Glycolipids contain a fatty acid, carbohydrate and a nitrogenous base. Phospholipids and glycolipids form part of the cell membrane and the nervous system.

Lipoproteins are macromolecular complex of lipids with proteins.

Derived lipids

These are substances liberated during hydrolysis of simple and compound lipids which still retain the properties of lipids. The important members of this group are sterols, fatty acids and alcohol.

Sterols

Sterols are solid alcohols and form esters with fatty acids. In nature they occur in the free state in the form of esters. Based on their origin sterols are classified as cholesterol (animal origin) and phytosterol (in plants).

Cholesterol is a complex type of lipid that is regularly synthesised by and stored in the liver. It is present in all animal products.

Fatty acids

Fatty acids are the main building blocks of fat. They have a methyl group (CH₃) at one end and a carboxyl group (COOH) at the other end with a chain of carbon and hydrogen atom in the middle. They have a basic formula CH₃(CH₂)n COOH. Where ' n' denotes the number of carbon atoms which may vary from 2 to 2l.

Fatty acids can be classified into Saturated Fatty Acids(SFA) & Unsaturated Fatty Acids (UFA)

Saturated Fatty Acids

Saturated fatty acids are those that are unable to absorb more hydrogen. They are usually stiff and hard fats. Eg. Ghee, Butter.

Unsaturated fatty acids

Unsaturated fatty acids have one or more double bond in their molecule and are thus not saturated with hydrogen. They are liquid at room temperature. Eg. Sunflower oil.

Unsaturated fatty acids may be monounsaturated or polyunsaturated depending on the number of double bonds.

Monounsaturated fatty acids (MUFA)

MUFA have only one double bond in their molecule. Eg. oleic acid found in olive oil, peanut oil.

Polyunsaturated fatty acids(PUFA)

PUFA have 2 or more double bonds in their molecule. Eg. linoleic acid, linolenic acid. They are present in corn, safflower, soyabean, sunflower oils and fishoils.

Monounsaturated and polyunsaturated fats are usually soft or liquid at room temperature.

Triglycerides

Fatty acids combine with glycerol to form a glyceride, When only one fatty acid combines with glycerol, it forms a monoglyceride, diglycerides have 2 fatty acids and triglycerides have three fatty acids attached to glycerol.

Most of the fatty acids present in the body and absorbed from foods occur in the form of triglycerides.

During digestion triglycerides are hydrolysed to form free fatty acid, monoglycerides and glycerol which are absorbed by the intestinal wall and the majority of these are rebuilt as triglycerides.

Long and short chain fatty acid

The number of carbon atom in fatty acids decides the chain length. Thus short chain fatty acids contain 4 to 6 carbon atoms, medium chain 8 to 12 carbon atoms and long chain fatty acid have 14 to 18 carbon atom.

Essential and Non - Essential fatty acid

Essential fatty acid (EFA) are those which cannot be synthesized by the body and need to be supplied through diet.

Linolenic acid, linoleic acid and arachidonic acid are essential fatty acids.

Non -essential fatty acids