THE PRODUCTS OF METABOLISM 9I 



for members of the Chlorophyceae may become important 

 where growth of these algae takes place on a large scale. 

 The direct nutritional value to most other organisms of 

 algal excretion products is probably slight^^^' ^^ but their 

 indirect effects on organic production are perhaps sometimes 

 considerable.^^ 



CARBOHYDRATES AND RELATED SUBSTANCES 



Carbohydrates may occur in considerable proportions in 

 algae either as intracellular reserve materials or as cell wall 

 constituents and more is known of this class of algal product 

 than of any other. Most of the carbohydrates that will be 

 mentioned are built up from units containing a heterocyclic 

 six-membered, or pyranose, skeleton. It may be helpful to 

 the reader of this section to note that, in order that the 

 spatial relationships of the atoms may be shown, this ring 

 is conventionally represented thus: 



the plane of the ring being supposed to be at right angles 

 to that of the paper, with the thickened edge foremost. 

 The formation of this ring from a simple chain results in a 

 further carbon atom, that numbered i, becoming asym- 

 metric. Thus there are two optical isomers of the pyranose 

 form of D-glucose: 



HO X^^ 'V OH HO 



a-D-glucopyranose j3-D-gIucopyranose 



