THE CARBOHYDRATES 75 



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 microscopic grains, each 

 of which presents the characteristic concentric stria tion. It is in- 

 soluble 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 intermediate 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 decomposi- 

 tions 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 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 from the 

 discovery that the end-products of this reaction consisted of 81 per cent, maltose 

 and 19 per cent, dextrin, 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 hydrolysed to form malto-dextrin : 



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1222n one molecule of water being taken up. The malto-dextrin 



