Biomolecules — BIOLOGY STD 11 Science — Question

Lipids are fatty acids esters of alcohols and related substances which are insoluble in water but get dissolved in a number of non-polar organic solvents like ether, benzene, chloroform, acetone, etc.Depending upon their composition and characteristics lipids are often classified into. Simple lipids, compound lipids and derived lipids.

1. Simple lipids– These are formed from fatty acids and alcohol. They do not have any additional group, e.g., fats, suberin, cutin, wax.
   1. Neutral or true fats– They are triglycerides which are formed by esterification of three molecules of fatty acids with one molecule of trihydric alcohol, glycerol (glycerine or trihydroxy propane). Three molecules of water are eliminated.fatty acids, e.g., dipalmito-stearin, palmito-oleio-stearin, steario-oleio-palmitin. In fats the three fatty acids are only rarely similar (e.g., tripalmitin, tristearin, triolein). They are called pure fats. Usually they are dissimilar or two of the three fatty acids are similar. They are known as mixed fats, e.g., Butter. Fats are named after the names of fatty acids, e.g., dipalmito-stearin, palmito-oleio-stearin, steario-oleio-palmitin.
      
   2. Waxes– They are fatty acid esters of long chain monohydric alcohols like cytyl, ceryl or mericyl.
      
   3. Cutin– It is a complex lipid produced by cross-esterification and polymerisation of hydroxy fatty acids, as well as other fatty acids with or without esterification by alcohols other than glycerol.
      
   4. Suberin– It is a mixture of fatty material having condensation products of glycerol and phellonic acid or its derivatives.
      
2. Compound or conjugated lipids: These are the esters of fatty acids and alcohol but contain other substances also, e.g., phosphor lipid, glycol lipids, sphingo lipids etc.
   1. Phospholipids- They are triglyceride lipids where one fatty acid is replaced by phosphoric acid which is often linked to ‘additional nitrogenous groups like choline (in lecithin), ethanolamine (in cephalin), serine or inositol.”
      
   2. Sphingo lipids– They are lipids having amino alcohol sphirigosine. Sphingomyelins contain an additional phosphate attached to choline like phospholipids.
      
   3. Glycol lipids– These are sugar containing lipids, in which the lipids portion of themolecule is usually based on glycerol or sphingosine and the sugar is typically galactose, glucose or inositol.
      
3. Derived lipids: These are lipid-like substance such as sterol or derivatives of lipids, e.g., steroids, prostaglandins and teapenes.
   1. Steroids– They are a group of complex lipids that possess a hydrogenated cyclopentano-perhydrophenanthrene ring system.
      
   2. Prostaglandins– They are derivatives of arachidonic acid and other 20 carbon fatty acids.
      
   3. Terpenes– They are lipid like hydrocarbons formed of isoprene (C₅H₈) units. Steroids like cholesterol are also derived from terpenes having 6 isoprene units. Fats are also differentiated into two main types, on the basis of their melting points at room temperature as follows. Hard Fats are solids at room temperature and contains long chains of fatty acids, e.g., Animals fat.