Lipids: Definition , structure, function and classification

Lipids are a family of organic compounds, composed of fats and oils. These molecules yield high energy and are responsible for different functions within the human body. Listed below are some important characteristics of Lipids.

1. Lipids are oily or greasy nonpolar molecules, stored in the adipose tissue of the body.
2. Lipids are a heterogeneous group of compounds, mainly composed of hydrocarbon chains.
3. Lipids are energy-rich organic molecules, which provide energy for different life processes.
4. Lipids are a class of compounds characterised by their solubility in nonpolar solvents and insolubility in water.
5. Lipids are significant in biological systems as they form a mechanical barrier dividing a cell from the external environment known as the cell membrane.

Structure of lipids

Lipids are a diverse group of biological substances made up primarily or exclusively of nonpolar groups. Lipids are grouped together on the basis of solubility in oganic or non polar solvents. Lipids are insoluble in
water (or polar solvent).

Lipids vary greatly in structure and function. Lipids are nonpolar: As the hydrocarbon component (the alkyl group) of an organic compound increases in size, the relative contribution of a polar functional
group to the physical properties of the molecule decreases.

Lipids have larger nonpolar alkyl groups and are insoluble or poorly soluble in water. As the size of an alkyl group increases in an organic compound, the water solubility of the compound decreases. As a result of their nonpolar character, lipids typically dissolve more readily in nonpolar solvents such as acetone, ether, chloroform, and benzene, than in water.

This solubility characteristic is of extreme importance in cells because lipids tend to associate into nonpolar groups and barriers, as in the cell membranes that form boundaries between and within cells. Besides having important roles in membranes, lipids are stored and used in cells as an energy source. Other
lipids form parts of cellular regulatory mechanisms.

• Lipids link covalently with carbohydrates to form glycolipids and with proteins to form lipoproteins.
• hydrophobic or water hating– water insoluble nonpolar molecule.
• hydrophilic or water loving– water soluble polar molecule.

• They are classified on the basis of solubility not on any functional groups
• Insoluble or sparingly soluble in water
• Soluble in non-polar organic solvents

Within these two major classes of lipids, there are numerous specific types of lipids, which are important to life, including fatty acids, triglycerides, glycerophospholipids, sphingolipids and steroids. These are broadly classified as simple lipids and complex lipids.

Simple Lipids

Esters of fatty acids with various alcohols.

1. Fats:Esters of fatty acids with glycerol. Oils are fats in the liquid state

2. Waxes: Esters of fatty acids with higher molecular weight monohydric alcohols

Complex Lipids

Esters of fatty acids containing groups in addition to alcohol and fatty acid.

1. Phospholipids: These are lipids containing, in addition to fatty acids and alcohol, phosphate group. They frequently have nitrogen-containing bases and other substituents, eg, in glycerophospholipids the alcohol is glycerol and in sphingophospholipids the alcohol is sphingosine.

2. Glycolipids (glycosphingolipids): Lipids containing a fatty acid, sphingosine and carbohydrate.

3. Other complex lipids: Lipids such as sulfolipids and amino lipids. Lipoproteins may also be placed in this category.

Precursor and Derived Lipids

These include fatty acids, glycerol, steroids, other alcohols, fatty aldehydes, and ketone bodies, hydrocarbons, lipid-soluble vitamins, and hormones. Because they are uncharged, acylglycerols (glycerides), cholesterol, and cholesteryl esters are termed neutral lipids. These compounds are produced by the hydrolysis of simple and complex lipids.