68 PHYSIOLOGY 



and many of their reserve materials consist of members of this group. In 

 the animal body, where the supporting tissues are composed chiefly of deriva- 

 tives of proteins, the sole significance of polysaccharides lies in their value as 

 food-stuffs. In plants, anhydrides both of hexoses and pentoses occur in 

 bewildering variety. Here however we may confine our attention to those 

 members of the group of polysaccharides which are important as food-stuffs. 



STARCH (C 6 H 10 8 ) is present in large quantities in nearly all vegetable 

 foods, and is an important constituent of the cereals, from which flour and 

 bread are derived, as well as of tubers, such as the potato. In the plant cells 

 it occurs as concentrically striated grains within minute protoplasmic 

 structures the amyloplasts, the office of which it is to manufacture starch 

 from the glucose present in the cell sap. When freed, by breaking up the 

 cells and washing with water, it forms a white powder consisting of micro- 

 scopic grains, each of which presents the characteristic concentric stria tion. 

 It is insoluble in cold water. In hot water the grains swell up and burst, 

 forming a thick paste, which sets to a jelly on cooling. This semi- solution, 

 as well as the original starch-grains, gives an intense blue colour on the 

 addition of iodine. On treating starch with cold alkalies or cold dilute acid, 

 it is converted into a soluble modification, the so-called soluble starch or 

 amylodextrin, which also gives a blue colour with iodine. This modification 

 is also produced as the first stage of the action of diastatic ferments upon 

 starch. On boiling with dilute acids, starch is converted first into a mixture 

 of dextrins, then into maltose, and finally into glucose. On acting upon 

 starch with various ferments, such as the diastase which may be extracted 

 from malt or germinating barley, or with the amylase occurring in saliva or 

 pancreatic juice, it undergoes hydrolysis, the final result of the action being a 

 mixture of four parts of maltose to one part of dextrin. As to the inter- 

 mediate stages in this reaction opinions are still divided. The first product 

 is soluble starch, amylodextrin, giving a blue colour with iodine. This 

 breaks up into a reducing sugar, and another dextrin, erythrodextrin, which 

 gives a red colour with iodine ; and this dextrin, on further hydrolysis, 

 yields reducing sugar and achroodextrin, which is not coloured by the 

 addition of iodine. Thus there are a series of successive hydrolytic decom- 

 positions of the molecule, each resulting in the splitting off of a molecule 

 of sugar and the production of a lower dextrin. 



The DEXTRINS are ill-defined bodies which are difficult to separate. 

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

 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 reduc.e 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 from the discovery that the 

 end-products of this reaction consisted of 81 per cent, maltose and 19 per cent, dextrin, 



