CHAPTER XXII 

 CARBOHYDRATE METABOLISM 



The simple sugars synthesized in photosynthesis belong chemically to the 

 group of compounds known as the carbohydrates. As the name indicates 

 these compounds contain the elements carbon, hydrogen, and oxygen, the latter 

 two usually, but not invariably, being present in the same ratio in which they 

 are found in the water molecule, i.e. two hydrogen atoms for every oxygen 

 atom. Generally speaking the term carbohydrate refers to aldehydic or ketonic 

 derivatives of alcohols of the aliphatic series, and their condensation products. 

 The significance of this statement will become clearer in the light of the fol- 

 lowing discussion. 



A large number of different kinds of carbohydrates have been isolated 

 from plants, and doubtless many more remain to be discovered. They com- 

 pose the bulk of the dry matter of plants. Although some of the carbohy- 

 drates are of universal occurrence in plants, or practically so, others seem to 

 be restricted to a very few species. Certain carbohydrates are the important 

 structural components of the cell walls of plants, others are probably integral 

 parts of the protoplasm, some are in solution in the cell sap, while large 

 quantities of others accumulate in plant cells as insoluble storage products. 



Classification of the Carbohydrates. — A classification of the carbo- 

 hydrates is presented in Table 35. This table includes all of the more im- 

 portant types of carbohydrates, but only those known to be of considerable 

 significance in plant metabolism are listed by specific name in the right hand 

 column of the table. Those which are known to be of general and fairly 

 abundant occurrence in green plants have been further designated by prefixing 

 their names with a star. 



The monosaccharides are the group of carbohj-drates from which no 

 simpler carbohydrates can be produced by hydrolysis. They are classified 

 according to the number of carbon atoms which they contain. Although 

 monosaccharides of all of the seven groups included in Table 35 have been 

 identified, only the 5 -carbon atom (pentose) and 6-carbon atom (hexose) 

 monosaccharides are important in plants. Alost of the sub-groups of monosac- 

 charides can be further divided into aldoses and ketoses. Aldoses are monosac- 



H 



I 

 charides containing an aldehydic group ( — C=0), while ketoses contain a 



ketonic group (=C=0). 



367 



