THE CARBOHYDRATES 69 



they concluded that starch must consist of five dextrin-like groups, four of which are 

 arranged symmetrically round the fifth. At each stage one of these groups is split off and 



{CHO ) 

 ,p VI 2 3. ^ [ one molecule of water being 



taken up. The malto-dextrin group is then changed into maltose by the further 

 assimilation of two molecules of water. The central dextrin-like group is attacked 

 with great difficulty by the ferment, and therefore remains at the end of the reaction as 

 achroodextrin. The malto-dextrin, the penultimate stage in the action of diastase, 

 can be regarded as formed by the condensation of three molecules of maltose attached 

 by the oxygen of two CHO groups, so that one CHO group remains free and determines 

 the reducing power of the malto-dextrin molecule. Its formula may therefore be 

 represented as follows : 



>.C 12 H 21 O 10 



< 



yC 12 H 20 9 



<o,, 



the sign ( being used to denote the open terminal CHO group. 



They further found that the stable dextrin remaining at the end of the diastatic 

 hydrolysis of starch probably had the formula of 40C 6 H 10 5 H 2 O, and might be regarded 

 as a condensation of forty glucose molecules with the elimination of thirty -nine mole- 

 cules of water. The starch molecule cannot be less than five times that of the stable 

 achroodextrin. Since the latter has a molecular weight of 6498, the molecular weight of 

 starch cannot be less than 32,400, and its empirical formula can be represented by : 



100C 12 H 20 10 , or (80C 12 H 20 10 .40C 6 H 10 5 ). 



INULIN. Another kind of starch, known as inulin, occurs in dahlia 

 tubers. It is easily hydrolysed by weak acids, and is entirely converted into 

 d-fructose, or laevulose. 



GLYCOGEN, or animal starch, is found in the liver, muscles, and other 

 tissues of the body, and occurs in large quantities in all foetal tissues. It is 

 a white powder, soluble in water, forming an opalescent solution. It is 

 precipitated from its solution on the addition of alcohol to 60 per cent., or by 

 saturation with solid ammonium sulphate. On boiling with acids, it is 

 entirely converted into glucose. It is affected by the ferments diastase and 

 amylase, in the same way as vegetable starch, giving first dextrins and finally 

 a mixture of maltose and dextrin. With iodine it gives a mahogany-red 

 colour which, like the blue colour produced in starch, is destroyed by boiling, 

 to return again on cooling. We shall have occasion to consider its properties 



I more fully when we are dealing with the functions of the liver. 

 THE ^CELLULOSES. Cellulose (C 6 H 10 6 ) X is a colourless, insoluble 

 material, or mixture of materials, which forms the cell walls of the younger 

 parts of plants, and is therefore a constituent of most of our vegetable 

 foods. It is insoluble in water or dilute acids or alkalies, its only solvent 

 being an ammoniacal cupric oxide solution. On boiling with strong acids, 

 it gradually undergoes hydrolysis and yields sugar, the nature of which 

 varies according to the source of the cellulose. In herbivorous animals cellu- 

 lose undergoes digestive changes and forms an important constituent of their 



