CARBOHYDRATES 49 



The monosaccharides are sometimes called the simple sugars. 

 The disaccharides are so called because they are formed by the 

 union of two molecules of monosaccharide, with elimination of 

 water. Polysaccharides are composed of several molecules of 

 monosaccharide united in a similar manner. The monosac- 

 charides are subdivided into groups according to the number of 

 carbon atoms in the molecule, thus the bioses are sugars hav- 

 ing only two carbon atoms in each molecule, the trioses, three, 

 etc. The first six of these groups are found in nature. The 

 octoses and nonoses have been built up in the laboratory. Of 

 the monosaccharides, the pentoses and hexoses are the most im- 

 portant. With the exception of the bioses, there are two 

 classes of sugars in each group, aldehyde sugars and ketone 

 sugars. Of the biologically important sugars all are aldehyde 

 sugars, or aldoses, with the exception of fructose, which is a 

 ketone sugar or ketose. The individual carbohydrates will be 

 discussed later. 



Origin and Synthesis. The ultimate dependence of the 

 animal world upon plants is well illustrated by the carbohy- 

 drates. These compounds which are an important fuel for 

 the body are obtained from plants, in which they are built up 

 from the very simple substances carbon dioxide and water. The 

 exact mechanism by which the plant brings about this important 

 synthesis is a matter of some uncertainty, but it is probable that 

 the carbon dioxide from the air is first reduced to formaldehyde. 

 Several molecules of formaldehyde then condense to form a 

 carbohydrate. The reaction might be represented as follows: 



C0 2 +H 2 0-H-CHO+0 2 



6 HCHO-^C 6 H 12 6 



The energy for this synthesis is derived from sunlight by the 

 agency of chlorophyl, the green coloring matter of plants. 

 The hexose so formed may then be built up into more complex 

 substances such as starch, which is laid away as reserve food 

 in the seeds, tubers and other parts of the plant. Sunlight is 

 not necessary for the second part of this process, as starch can 

 be built up in the roots and tubers, which are underground. By 



