THE CHEMICAL BASIS OF THE BODY 759 



The carbohydrates may be divided into the 



Starch group, or polysaccharides. 

 Cane-sugar group, or disaccharides. 

 Dextrose group, or monosaccharides . 



Polysaccharides — Starch. — The formula for starch is unknown ; 

 it is considered to be (C 6 H 10 O 5 )w, where n is not less than 5 or 6, and 

 is probably very much larger. The molecular weight is also unknown. 



Starch exists in plants in the form of grains, the shape of which 

 depends upon the group from which they are derived ; thus potato, 

 bean, wheat, and other starch grains, have each a distinctive shape. 

 The grain is composed of two parts, an envelope known as cellulose, 

 and an interior called granulose. The granulose is the true starch ; 

 the cellulose is not, however, identical with the ordinary cellulose 

 of plants. Starch is insoluble in cold water, but when boiled the 

 grains burst, and a viscid, opaque, pasty mass results, which is not, 

 however, a true solution of starch. A solution of starch can be 

 obtained from this mass by careful and limited digestion with an 

 enzyme, such, for instance, as human saliva, or by the action of 

 dilute acid ; when this takes place the material becomes watery, 

 perfectly transparent, and niters readily, while previously this 

 was impossible. To this limpid fluid the name ' soluble starch ' 

 has been given. The characteristic test for starch is the blue colour 

 produced on the addition of iodine. Starch has no reducing action 

 on Fehling's solution. 



Dextrin. — When starch paste is acted upon by dilute mineral 

 acid, or the enzymes found in the saliva and pancreatic juice, soluble 

 starch is first formed as above described ; but if the process be 

 allowed to continue, further changes rapidly occur, leading to the 

 production of dextrin and finally of sugar. There are probably 

 several dextrins, though two are generally more particularly described 

 — viz., erythro-dextrin and achroo-dextrin. These are distinguished 

 from starch and from each other by their colour reactions with 

 iodine, erythro-dextrin giving a reddish colour, while achroo- 

 dextrin gives no colour. Much the same change which can thus be 

 brought about by acting upon starch out of the body takes place 

 in a more perfect and complete form within the body. The con- 

 version of starch into dextrin and finally into sugar under the in- 

 fluence of certain enzymes performs a most important physiological 

 function ; neither starch nor dextrin is capable of being absorbed 

 as such, whereas the sugar which results from this conversion is 

 readily assimilable. 



Glycogen closely resembles starch. It is found in several of the 

 tissues of the body, and its origin and use have been previously 

 discussed (see p. 247). It may be obtained as an amorphous white 

 powder, readily soluble in water, and gives with iodine a port-wine 

 colour instead of blue. By the action of acids or enzymes it is 

 readily converted into dextrin, and finally into sugar. The sugar 

 resulting from the action of acid is dextrose, whereas that produced 

 by the enzyme is maltose ; in the liver the sugar produced is dextrose 

 and not maltose, and the method by which this conversion is obtained 

 has been previously dealt with. 



Cellulose, though not found in the animal body, is of great interest 

 to the physiologist from its intimate relation to the feeding of the 

 herbivora. The food substance in. plants is locked up in a cellulose 

 envelope, and until this envelope is broken down the material within 



