CARBOHYDRATES 67 



stances of unknown chemical constitution, which act as catalytic 

 agents affecting a large variety of chemical reactions. They are 

 specific in their action, they are sensitive to changes in reaction 

 and temperature, being destroyed if heated to 60-80 C. Some 

 are reversible in their action, they are often secreted in inactive 

 form and become active only on coming in contact with some 

 other substance, their action is often incomplete in vitro, and 

 frequently is progressive in its character. 



On the activities of enzymes depend a very large number of 

 the processes by which the cells, and hence the tissues and the 

 body as a whole carry on their various activities. 



Individual Groups of Carbohydrates. 



Pentoses. The pentoses are found in both plants and 

 animals, usually combined with other substances as in nucleo- 

 proteins or in the form of polysaccharides made up of many 

 molecules of pentose. They are obtained by the hydrolysis of 

 these compounds. A pentose, probably arabinose, has been found 

 in the urine. This condition is known as pentosuria. Pentoses 

 may be either aldoses or ketoses. They reduce Fehling's and 

 other similar solutions, and give osazones with phenylhydrazine. 

 They usually do not ferment, however. The pentoses are 

 utilized by herbivorous animals but the extent to which they 

 may be utilized by man seems to be more limited, although the 

 subject is still a matter of some uncertainty. Pentoses may be 

 distinguished from hexoses by their osazones, by their failure to 

 ferment readily, and also by certain color reactions among which 

 are the orcin and phloroglucin tests. If a pentose is heated with 

 concentrated hydrochloric acid and a little orcin or phloroglucin, 

 a distinct color change results. With orcin the color is first vio- 

 let, then blue, red, and finally green, and a bluish green pre- 

 cipitate forms. With phloroglucin, the color is red. As other 

 substances will give similar colors, it is necessary to confirm the 

 result by observing the absorption spectrum of the colored sub- 

 stance after it has been dissolved out by shaking the liquid with 

 amyl alcohol. The orcin test gives an absorption band between 



