THE CARBOHYDRATES 



They are amorphous white powders, easily soluble in water, forming solut , 

 which, when concentrated, are thick and adhesive. They are insoluble in 

 alcohol and ether. With cupric hydrate and caustic alkali they form blue 

 solutions, which reduce slightly on boiling. They are not precipitated by 

 saturation with ammonium sulphate. On boiling with dilute acids, they are 

 converted entirely into glucose. 



The changes undergone by starch during its hydrolysis by means of diastase have 

 been used by Brown and his co-workers as a method of arriving at some idea of the 

 size and structure of the starch molecule. Proceeding frcm the discovery that the 

 end-products of this reaction consisted of 81 per cent, maltose and 19 per cent, dext tin. 

 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 



t C 1 TT O "\ 

 hydrolysed to form malto-dextrin:] 12 22 n I one molecule of water being 



UC 12 H 20 O 10 ) 2 j 



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 i 

 sented as follows : 



the signc^ 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 ]0 5 H 2 0, and might be regarded 

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

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

 dextrin. 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 05). 



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 levulose. 



GLYCOGEN, or animal starch, is found in the liver, muscles, am 

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

 a white powder, soluble in water, forming an opalescent solution 

 precipitated from its solution on the addition of alcohol to 60 per cei 

 saturation with solid ammonium sulphate. On boiling with a 

 entirely converted into glucose. It is affected by the ferments 

 amylase, in the same way as vegetable starch, giving first dextrin 

 a mixture of maltose and dextrin. With iodine it gives a mal 

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

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

 more fully when we are dealing with the functions of the 1 



