30 PHOTOSYNTHESIS 



method has shown the large number of carotenoid pigments 

 present in the leaf. 



Ratio of carotenoids to chlorophyll 



The molecular ratio of the carotenoid to chlorophyll 

 present in leaves is very variable. In normal green leaves 

 there are more molecules of chlorophyll than carotenoid, in 

 shade leaves the ratio sometimes approaching 5 : i. In etio- 

 lated plants the carotenoids are present before greening 

 has occurred, although they may increase in amount con- 

 currently with the chlorophyll. 



Chemical properties 



The carotenoids contain a large number of double bonds, 

 many of which form a conjugated system. This gives rise to 

 the possibility of a large number of cis-trans isomers, a 

 number of which have been isolated. 



In leaves the main carotenoid constituent is ^-carotene, a 

 member of the class of hydrocarbons the carotenes, C4QH56. 

 It crystallizes easily in steel-blue crystals, which are deep 

 red by transmitted light. In most organic solvents it forms a 

 yellow solution but in carbon disulphide it forms an orange 

 solution. Both the pure solid and the solutions are oxidized 

 on exposure to air, giving amongst the products /?-ionone. 

 In common with all other carotenoids carotene gives a deep 

 blue colour with concentrated sulphuric acid. The xantho- 

 phylls constitute the class of carotenoids which contain 

 oxygen. The most common compound of this type in green 

 leaves is luteol, a dihydroxy-carotene, C4oH5g02. During 

 autumn, when changes occur in the carotenoids of the leaf 

 an isomer of luteol, zeaxanthol, becomes the most abundant 

 constituent. 



Like most unsaturated compounds the carotenoids are 

 very reactive and are easily oxidized by molecular oxygen. 

 However, in the living cell the carotenoids appear to be 

 stable, although on breaking the cell rapid oxidation some- 

 times takes place. The oxidative breakdown is less under 

 conditions where enzymes are destroyed and this has led 

 to the suggestion that the cell contains enzymes catalysing 



