30 Inside the Living Cell 



inactive form, and it has been found that if this is exposed to a 

 source of ultra-violet light, such as bright sunlight, out of doors, the 

 active vitamin is formed. This is one of the reasons why sunshine is 

 health-giving. 



Vitamin B has been found to contain quite a number of different 

 substances, many of which have been identified and some synthe- 

 sized. Vitamin Bl, now called aneurin or thiamine, is the substance 

 of which a deficiency causes beri-beri. B2 was identified by Kuhn 

 and Karrer with riboflavin, a substance present in milk, eggs, or 

 liver, the lack of which produces dermatitis, facial sores, etc. B3 is 

 possibly a substance called pantothenic acid, which was first found 

 to be required by yeasts and later recognized as a food requirement 

 of animals and insects. B6 was identified with pyridoxine in 1938, 

 and in 1944 a more active form of the vitamin was discovered in the 

 compound pyridoxal. 



Other substances which are essential to the life and growth of 

 many organisms are choline, a simple substance required by rats 

 and birds and possibly by human beings; biotin, originally recog- 

 nized as a food requirement of yeasts, but since known to be 

 required by the higher animals; inositol, a particular type of sugar, 

 and folic acid, a substance abundantly present in green leaves, which 

 stimulates growth. 



Having discovered all these substances, which must be present 

 in our food in minute quantities, to keep us healthy, working 

 organisms, scientists naturally began to wonder what their action is. 

 If the lack of them produces such serious effects, they must perform 

 a very vital function. 



The clue to this came from quite a different direction. It will be 

 remembered that the juice squeezed out of living yeast cells contains 

 enzymes which can by themselves ferment sugars. It was discovered 

 by Harden and Young in 1906 that if this juice was divided into two 

 parts, one containing the big (protein) molecules and the other small 

 molecules, neither part separately could bring about the fermenta- 

 tion, but when they were again mixed the fermentation went on as in 

 the original juice. Hence both the big molecules and the smaller 

 molecules were necessary to bring about the fermentation. The big 

 molecules, as we have seen, are proteins and some of them enzymes. 

 The smaller molecules include some which are also needed for 

 enzyme action, and these were called co-enzymes. 



In the course of time a number of co-enzymes have been separated 

 and distinguished. It has been found that frequently one co-enzyme 

 is shared by several different enzymes. This appears to be a device of 

 Nature to economize materials which are rather scarce. The yeast 



