CHLOROPHYLL AND ENERGY 



163 



formula. The formulae of chlorophyll-c and chlorophyll-^? are 



not defined. 



H-C=0 



I H H 



I I I 



C C-C-C-H 



I 



=CH 



H H 



Chlorophyll-Z> 



The quantity of these secondary chlorophylls found in the 

 plant is generally a Uttle less than one-third of the quantity of 

 chlorophyll-a. An equiUbrium seems to have been estabhshed 

 between them and the passage from one to another seems to 

 present no difficulty. 



In the higher plants, the secondary chlorophyll is always 

 chlorophyll-Z?. In the lower plants, it is replaced by chloro- 

 phyll-c, in brown algae, and by chlorophyll-^^, in red algae. 



Beside the chlorophylls, carotenoids are always present — 

 carotene, at least. The formula for this substance is very 

 simple, for it contains only carbon and hydrogen, C^oHgg. 

 The formula for xanthophyll is the same with the difference 

 that at two points the group CHg is replaced by CHOH; two 

 alcohol radicals appear causing 2 atoms of oxygen in the 

 formula, C^qH^qO^. 



Carotenoids are often present in organs in which chloro- 

 phyll is absent; for example, in the carrot root they persist in 

 the autumn after the chlorophyll has disappeared. Carotene 

 is very important in animal nutrition because its formula 

 may change to that of vitamin A; this change can be effected 

 by the animal organism and that is why carotene is called a 

 precursor of vitamin A. 



