Lipids

 The lipids are heterogenous group of compounds related to fatty acids. They are insoluble in water but soluble in organic solvents ether, alcohol, chloroform and benzene. They include fats, oils, waxes, cholesterol and related compounds. Lipids as hydrophobic compounds are components of cellular membrane. They are also used to store energy. Because of higher proportion of C-H bonds and very low proportion of oxygen lipids store double amount of energy as compared to same amount of any carbohydrates. Some of them provide insulation against atmospheric heat and cold and also act as water proof material. Waxes in the exoskeleton of insects and cutin an additional protective layer on the cuticle of epidermis of some plant organs for example leaves, fruits and seeds etc. are some main examples. They have been classified as acylglycerols, waxes, phospholipids, sphingolipids, glycolipids and terpenoid lipids including carotenoids and steroids.

Acylglycerols: Acylglycerols are composed of glycerol and fatty acids. The most widely spread acyl glycerol is triacyl glycerol also called triglycerides or neutral lipids. Chemically, acyl glycerol can be defined as esters of fatty acids and alcohol. An ester is compound produced as result of chemical reaction of an alcohol with an acid and a water molecule is released. Fatty acids are one of the most important components of triglycerides. Fatty acids contain even numbers(2-30) of carbon atoms in straight chain attached with hydrogen and having an acidic group COOH. They contain no double bond or up to 6 double bonds. In animals fatty acids are straight chain while in plants these may be branched or ringed. Solubility of fatty acids in organic solvents and their melting points increase with increasing number of carbon atoms in chain. Palmitic acid is much more soluble in organic solvent than butyric acid. Fats containing unsaturated fatty acids are usually liquid at room temperature and are said to be oils. Fats containing saturated fatty acids are solids. Animal fats are solid at room temperature whereas most of plant fats are liquids. Fats and oils are lighter than water and have specific gravity of about 0.8. They are not crystalline but some can be crystallized under specific conditions.

Waxes: Waxes are widespread as protective coatings on fruits and leaves. Some insects also secrete wax. Chemically waxes are mixtures of long chain alkanes, alcohol, ketones and esters of long chain fatty acids. Waxes protect plants from water loss and abrasive damage. They also provide water barrier for insects, birds and animals such as sheep.

Phospholipids: Phospholipids are derivatives of phosphatidic acid which are composed of glycerol, fatty acid and phosphoric acid. Nitrogenous bases such as choline, ethanolamine and serine are important components of phospholipids. They are widespread in bacteria, animal and plant cells and are frequently associated with membranes. Phosphatidylcholine is one of the common phospholipids.

Terpenoids: Terpenoids are a very large and important group of compounds which are made up of simple repeating units, isoprenoid units. This unit by condensation in different ways gives rise to compounds such as rubber, carotenoids, steroids and terpenes etc.

Lipids constitute major source of energy and play an important role in the structure of membranes of cell and of organelles found in the cell. They also provide insulation, mechanical protection and protection from water loss and abrasive damage.