PLANT METABOLISM 



207 



where ihe lice energy recjuired comes Iroiii the h'ght absorbed by the 

 pigmeiils, and tlie clieniical leactions involved are catalyzed by 

 enzymes. This reaction is the reverse of that shown on i)age 1G5 and 

 considers only one of the multitude of actual products. For other 

 products the quantities of energy, carbon dioxide, water, and oxygen 

 vary in accord with the structure obtained. 



Energy Absorption 



The initial event in photosynthesis is the absorption of radiant 

 energy. Apparently radiation of the long wave lengths of the infrared 

 region is readily absorbed by plants as heat but is not sufficiently high 

 in energy to induce the photosynthetic process. The available supply 

 of idtraviolet light is limited and probably not much of a factor 

 either. However, visible light is extensively absorbed by the specialized 

 pigments mentioned in connection with the above overall reaction. All 

 photosynthetic plants contain one or more of the group of pigments 

 called chlorophylls. 



Chlorophylls are structurally related to the porphyrins serving as 

 prosthetic groups in hemoglobin, catalase, and the cytochromes (pages 

 469 and 151). Chlorophyll a seems to be of special importance and has 



C2H5 



CH2 COOCH3 



C^oKagOOC 



chlorophyll a 



the structure shown. In chlorophyll b the uppermost methyl group 

 as written here is replaced by an aldehyde group. This latter pigment 

 occurs with chlorophyll a in the higher plants. Other chlorophylls are 

 associated with chlorophyll a in bacteria and some of the algae. 



Chlorophyll a strongly absorbs light in the blue and red regions, and 



