LIPIDS | classification, biomedical importance, fatty acids, essential fatty acids

Lipids

They constitute an essential heterogenous group of organic substances in plants and animals. Chemically lipids are different types of esters of different alcohols. They contain fatty acids, phosphoric acids nitrogenous base and carbohydrates in addition to alcohol.

Bloor’s Criteria

According to Bloor’s criteria, lipids are the compounds which have the following characteristics

• ·         Insoluble in H2O
• ·         Soluble in one or more organic solvents such as chloroform, acetone benzene etc.
• ·         They must have some relationship with fatty acids as esters either actual or potential. So lipids are fats, waxes, oil and related compounds.
• ·         An oil is a lipid that is liquid at ordinary temperature. This is the basic difference between oil and fats

Biomedical importance of lipids

1.          lipids are important dietary constituent
2.         They act as fuel in the body
3.          They produce more energy per gram in comparison with carbohydrates
4.          They can be stored in the body in almost unlimited amount in comparison to                          carbohydrates.
5.          Some of lipids may exert an insulating effect in the body, while the lipids present                  around vital organs provide padding and protection.
6.          Lipoprotein and phospholipids are main constituents of many natural membrane.
7.          Carrier of triglycerides, cholesterol in the body is lipoprotein.

Classification of lipids

¡  Bloor’s classification is usually adopted with few changes as follows

¡  Simple Lipids:  they are esters of fatty acids with various alcohols

¡  Compound Lipids: Esters of fatty acids consisting of groups other than and in addition to and alcohol and fatty acids

¡  Derived Lipids:  they are the derivatives obtained by hydrolysis of simple and compound lipids and still maintain the general properties of lipids.

¡  Miscellaneous:  Aliphatic hydrocarbons involve iso-octadecane present in liver fat and some particular hydrocarbons present in bees wax etc.

Fatty Acids

Fatty acids are carboxylic acids having long-chain hydrocarbon parts, obtained from the hydrolysis of fats. Fatty acids typically consist of between twelve and twenty carbon atoms.

Fatty acids generally contain an even number of carbon atoms because in nature they are made by combining two- carbon molecules.

Classification of fatty acids

They are sorted as saturated, monounsaturated and polyunsaturated fatty acids. second way by which the fatty acids are classified is by their stereochemistry. They can be sorted as cis or trans based on the stereochemistry of carbon-carbon double bond in hydrocarbon parts.

Omega notation and unsaturated fatty acid classification

The popular notation used for unsaturated fatty acids is Omega notation in nutrition literature. In this notation, the terminal carbon of hydrocarbon chain of a fatty acid is nominated as “omega carbon” or “ω-carbon.”

Essential Fatty acids (EFA)

Three polyunsaturated fatty acids, Linoleic acid, Linolenic acid and arachidonic acid are called essential fatty acids. These cannot be synthesized by the body so they must be provided to the body in diet. Deficiency of essential fatty acid in diet can produce growth retardation etc. Out of three EFA, Linoleic acid is the most important. 

Functions of EFA

Functions of essential fatty acids are following:

¡  They form the structural elements of tissues.

¡  They form the structural elements of gonads.

¡  They involve in synthesis of prostaglandins and other compounds.

¡   They are also involved structural elements of mitochondrial membrane.

¡  They maintain the serum level of cholesterol.

¡  They have effect on clotting time.

¡  Role of EFA in fatty liver.

¡  They have role in vision.

Melting points of fatty acids:

·         fatty acids are attracted to each other through hydrogen bonding and dipole-dipole interactions of their carboxylic groups and London forces. More larger the nonpolar hydrocarbon part, more stronger will be  London forces and higher will be the melting point.

·         Another factor that effects the melting point of fatty acids is their degree of saturation. The more C=C less will be the melting point