CONSTITUENTS OF THE BIOSPHERE 



41 



becomes a cholestene and the position of the double bond is indicated by 

 the first (in the order of increasing numbers) carbon atom Hnked by the 

 double bond. 



One of the most widely distributed sterols, for example, is cholesterol or 

 3 (m)-hydroxy-5-cholestene. 



HO 



CH2— CH2— CVU— CH 



CH. 



CH3 



Cholesterol 



The numerous sterols in the biosphere differ in the number and position 

 of the double bonds, and in the number of carbon atoms in the side-chain. 

 There are saturated sterols, and sterols having one, two or three double 

 bonds. In the higher animals, cholesterol is the characteristic sterol, but 

 this is not the case for lower animals, in which cholesterol is only one 

 sterol among many. 



Ergosterol is the chief sterol in mushrooms, and fucosterol in the brown 

 algae. The higher plants contain complex mixtures of sterols. Bacteria, 

 in general, do not contain sterols, or other steroids. 



Werner Bergmann classes the sterols according to their rotatory power, 

 which is intimately related to their structure : 



system of conjugated double bonds in 

 ring B. 



a 5:6 double bond. 



I. [a] greater than —90' 



II. [a] from -30° to -70° 



III. [a] from -20° to +10° 



IV. [a] from +10° to +30° 



V. [a] from +40° to +50° 



a 7:8 double bond 



saturated ring and side chain (excep- 

 tion : neospongosterol). 



an 8:9 double bond. 



The sterols in group I differ from all the others in one important respect : 

 the conjugated double bonds in ring B confers upon them the property of 

 undergoing molecular rearrangement under the influence of ultra-violet 

 light. 



Ergosterol, the chief sterol of mushrooms, belongs to this group, the 

 members of which differ from each other in the side-chain (presence or 



