THE PRODUCTS OF PHOTOSYNTHESIS 195 



hexose sugars play an exceedingly important role. In general glucose 

 may well be considered as the chief source of energy which enables the 

 cells of most plants and animals to carry on their various forms of vital 

 activity. It should be stated at once, however, that this is concluded from 

 the almost universal presence of glucose or its condensation products 

 in greater or less concentration in the cells. Whether it is glucose itself 

 which is oxidized or a product of transformation of glucose, cannot be 

 considered as definitely established. In fact, glucose represents the most 

 stable form of the hexose carbohydrates and it is not improbable that be- 

 fore oxidation it is transformed into a more easily broken down form 

 as is required by the cell. Certain it is that glucose is the most com- 

 mon of all the hexose sugars, but it must also be remembered that glucose 

 is probably also the most stable of the hexoses found in nature. 



It is a very striking fact, and one which has puzzled chemists for a 

 long time, that of the 32 theoretically possible isomeric aldo-, 2-keto-. 

 and 3-ketohexoses there are found in nature only a very limited number. 

 These isomeric compounds have the same composition, all apparently con- 

 tain a straight chain of six carbon atoms and dififer principally in the 

 spatial distribution of the hydrogen, oxygen and hydroxyl groups attached 

 to each carbon atom. 



The hexoses which are found in plants are: the aldoses, d-glucose, 

 d-mannose and d-galactose, the 2-ketohexoses, d-fructose and d-sorbose 

 and there is some evidence that there are also small quantities of the 

 3-ketohexoses, d-glutose. Of these d-glucose is found in greatest abun- 

 dance, followed by d-fructose in varying proportions. These two hexoses 

 are also found in large quantities in the form of their various respective 

 di- and polysaccharides, notably sucrose, maltose, trehalose, raffinose and 

 especially starch and inulin. The other hexoses in the free form are 

 relatively rare components of leaves or other plant parts. It is very 

 difficult to account for the fact, at least on the basis of our present knowl- 

 edge of carbohydrate synthesis, that only this small number of carbo- 

 hydrates ajjpear in the plant as products of photosynthesis. Each one 

 of the hexoses found represents but one of two space isomers, they are 

 all (with the exception of d-glutose) optically active substances. This 

 is one of the most important points which must be met in any theory 

 of the mechanism of the synthesis of carbohydrates in plants. It has 

 led to extensive speculation, for not only in these genetically first organic 

 compounds is optical activity a striking characteristic, but it is appar- 

 ently maintained throughout the entire organic world. 



How to account for this optical activity of the products of photo- 

 synthesis is a problem for the solution of which we have not as yet 

 sufficient experimental data. However, the following possibilities sug- 

 gest themselves : 



1. That all the theoretically possible hexoses are formed and that these 

 are rapidly converted into the five or six actually found. While there 

 is some evidence of a conversion of hexoses within the cell, as for in- 



