CARBOHYDRATES 49 



enzyme for completion, the reaction taking place in successive 

 stages, each one of which is brought about by a different enzyme. 

 Thus the breakdown of starch into glucose requires at least two 

 and possibly more enzymes. The first breaks down the starch 

 through various stages to maltose, the second breaks the maltose 

 into glucose. 



Summary. In summary it may be said that enzymes are sub- 

 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. 



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 



