Lipids

Lipids are organic substances present in all living organisms. They include fats, oils, waxes and other related compounds. They are insoluble in water and hydrophobic, and soluble in organic solvents. They are related to fatty acids and are not polymers. They are esters of long chain fatty acids and alcohols.

Classification of lipids

They are broadly classified into simple lipids, complex lipids, derived lipids and miscellaneous lipids based on their chemical composition.

Simple Lipids

These lipids are the esters of fatty acids with alcohols. They are of two types.

Triglycerols and waxes.

Triglycerols (Fats and oils)

They are esters of fatty acids with glycerol. They are insoluble in water, non-polar in character, and are commonly called neutral fats.

Waxes

They are the esters of fatty acids long chain monohydroxy alcohols other than glycerol. They have higher melting points then neutral fats.

Complex Lipids or Compound Lipids (Heterolipids)

Complex lipids are Lipids with additional groups are called compound lipids.

These lipids are esters of fatty acids with alcohols and other groups such as phosphate, nitrogenous base, etc. They are again divided into 3 types.

Phospholipids (Phosphatids)

These lipids contain phosphoric acid, fatty acid, nitrogenous base and alcohol. They are again sub-divided into

Glycero phosphlipids

These contain glycerol as alcohol. They are also called phosphoglycerides.

Sphingophospholipids

These contain sphingosine as alcohol. They are also called as sphingomyelins.

Glycolipids

They contain a fatty acid, carbohydrate and nitrogenous base but no phosphorus. They are also called as glycosphingo lipids. They have sphingosine as alcohol.

Lipo proteins

These are the macromolecular complexes of lipids with proteins.

Apart from the above lipids some others like sulfolipids, aminolipids, and lipo polysaccharides also come under complex lipids.

Derived lipids

These lipids are obtained on hydrolysis of simple and complex lipids. These lipids contain glycerol and other alcohols. This class of lipids include steroid hormones, ketone bodies, hydrocarbons, fatty acids, fatty alcohols, mono and diglycerides, terpenes and carotenoids. These are sometimes present as waste products of metabolism.

Miscellaneous lipids

These include compounds, which contain characteristics of lipids. They include squalene, terpenes, hydrocarbons, carotenoids, etc.

Chemical Structure

Simple lipids

1.Glycerides

The triglycerides are esters of glycerol with long chain fatty acids. Fatty acids always have an even number of carbons and may be saturated e.g, palmitic acid (C₁₅H₃₁COOH) and stearic acid (C₁₇H₃₅COOH) or unsaturated e.g., oleic acid (C₁₇H₃₃COOH) and linolenic acid (C₁₇H₂₉COOH).

The three fatty acids in triglycerides may be same or different. Fats are glycerides of saturated fatty acids e.g., triplamitin and tristearin. Oils contains unsaturated fatty acids e.g., tristearin. Oils contain unsaturated fatty acids e.g., triolein. a-oleo-b-palmito-a'- stearin is an example of mixed glyceride.

The presence of double bonds with less stable 'cis' stereochemistry in unsaturated fatty acids e.g., at C-9 in oleic acid (C₁₇H₃₃COOH) at C-9 and C-12 in linoleic acid (C₁₇H₃₁COOH), at C-9, C-12 and C-15 in linolenic acid (C₁₇H₂₉COOH) is of vital biological significance. In solid state, the molecules of saturated fatty acids fit closely together due to their zig-zag tetrahedral structure. The cis unsaturated acid chains have a bend at the double bond and do not fit closely resulting in lowering of the resulting point of the fat.

Waxes

There are esters of long chain saturated and unsaturated fatty acids with long chain monohydroxy alcohols. The fatty acids range between C₁₄ and C₃₆ and the alcohols range from C₁₆ to C₃₆. Most of the waxes are mixtures.

Functions in Biosystem

Lipids perform several important functions in biosystems.

• Phospholipids are the constituents of cell mebrane and regulate membrane permeability.

• Phospholipids are also used as detergents to emulsify fat for transport within the body.

• They act as cellular metabolic regulators.

• They protect internal organs, serve as insulating materials and give shape and smoothness to the body.

• They serve as a source of fat soluble vitamins.

• Essential fatty acids are useful for transport of cholesterol, formation of lipoproteins, etc.

• Triacyl glycerols are the concentrated fuel reserves of the body.

• Phospholipids in mitochondria are responsible for confirmation of electron transport chain components.

• Accumulation of fat in liver is prevented by phospholipids.

• Phospholipids help in removal of cholesterol from the body by participating in reverse cholesterol transport.

• Cholesterol is a constituent of membrane structure and it synthesizes bile acids, hormones and vitamin D. It is the principal sterol of higher animals, abundant in nerve tissues and gallstones.