16 



On the Function of Chlorophyll. 



combining with the readily oxidisable chlorophyll needs further investi- 

 gations, but the views put forward seem to give a reasonable temporary 

 working hypothesis of the function of chlorophyll, xanthophyll, and carotin 

 in the assimilation of carbon dioxide. A continuous liberation of oxygen 

 would only be possible when stages 2 and 3 were carried out. The chemical 

 reactions involved form a series of chemical changes which in part at least 

 are reversible, and whose continuance in a definite direction is determined 

 by light and by a continued supply of carbon dioxide at one end of the 

 equation and by a continued removal of the sugar, formaldehyde, and surplus 

 oxygen at the other end of the equation. 



Summary. 



No peroxides, organic or inorganic, are produced during the photo-oxidation 

 of chlorophyll, xanthophyll, or carotin, but these substances, when exposed 

 to light in the presence of an abundant supply of oxygen, may act as oxidases 

 not only to themselves but also to substances with which they may be in 

 contact, such as hydriodic acid, litmus, or guaiacum. Hence arises the 

 "iodoxidase" reaction of bleaching chlorophyll, carotin, xanthophyll, and 

 other pigments oxidising in light. 



Chlorophyll and xanthophyll decompose during photo-oxidation into 

 (a) solids and (b) a gas. The solids consist of colourless waxy substances 

 and hexose sugars. The waxy solids are relatively small in amount in 

 the case of xanthophyll. The gas is formaldehyde gas. With dry films in 

 dry air free from carbon dioxide, relatively more formaldehyde is produced 

 and less sugar, and the bleached residue weighs much less than the original 

 solid. In moist air more sugar is formed and the residue may weigh nearly 

 as much as the original solid. 



Carotin oxidises more rapidly than xanthophyll or chlorophyll, and yields 

 a little formaldehyde and a large amount of a colourless waxy solid, which may 

 be a form of phytyl or of phytosterin. 



Carbon dioxide combines with chlorophyll forming xanthophyll and a 

 colourless waxy solid. The combination only takes place actively in the 

 presence of water, and is accelerated by sunlight. Portion of the oxygen 

 liberated by this reaction may oxidise the xanthophyll in the presence of 

 sunlight to formaldehyde, sugar and phytyl, the latter retaking its place 

 in the tricarboxylic chlorophyll grouping. No oxygen is set free when 

 extracted chlorophyll is used. Some special means must exist in the 

 chloroplastid of liberating the remaining oxygen without its oxidising the 

 chlorophyll. Carotin may aid in protecting chlorophyll from photo- 

 oxidation, and the reductase action of magnesium may be of importance. 



