208 PLANT BIOCHEMISTRY 



these wave lengths are effective in pronioting photosynthesis. Green 

 and yellow lights are absorbed by chlorophyll much less effectively, 

 and reflection over this range leads to the characteristic color of the 

 green plants. Some species are highly pigmented with other colors but 

 nevertheless contain chlorophyll a masked by the additional pigments. 



Chlorophyll b has its short wave length band shifted a little toward 

 the blue-green. Thus it absorbs light not effectively absorbed by 

 chlorophyll a. Moreover, the energy absorbed by chlorophyll b is 

 transferred to chlorophyll a. Other photosynthetic pigments appear to 

 function in a similar way. The carotenoids of higher plants absorb 

 intermediate wave lengths, transferring at least part of the energy thus 

 acquired to chlorophyll a. Still other types of pigments perform this 

 function in the red, brown, and blue-green algae, providing for the 

 utilization of part of the radiant energy that would otherwise be 

 converted to heat and be of minor use in metabolic syntheses. 



The widely distributed carotenoids are divided into two groups, 

 carotenes and xanthophylls. Two of the most abundant representa- 

 tives are j8-carotene and lutein. Structural changes lead to other 

 carotenes, and the possibility of cis-trans isomerism at many positions 

 gives manv more. A large nimiber have been isolated. /3-Carotene is 



CH = CH— C=CH — CH =CH 



I I 



CHq CH'3 — c 



CH3 -^ "^ 



0-carotene 



related to the vitamin A of importance to animals. Lutein has hy- 

 droxyl groups at each of the two positions marked with * in the struc- 

 ture of /?-carotene. In addition, the double bond in one ring is shifted 

 downward one position, corresponding to a-carotene. Other xan- 

 thophylls are also hydroxy or keto derivatives of various carotenes or 

 ethers or esters of such compounds. 



