101 



15. The chemical structure of the starch and protein mole- 

 cules ; cleavage changes during digestion. The huge molecules 

 of starch and protein are believed by chemists to consist of 

 a large number of much smaller molecules linked together 



FIG. 53. Diagram of the structure of molecules of starch and protein 



Starch is represented as formed by the chemical linking together of many like 

 molecules of dextrose; protein, by the linking together of many unlike mole- 

 cules of amino-acids. Some of these chemical links (indicated by the arrows) 

 are broken by cleavage more easily than others. Hence cleavage first forms 

 smaller molecules of dextrines from starch and of polypeptids from proteins. 

 Ultimately each may be broken up into its constituent molecules of dextrose or 

 amino-acids respectively 



in chemical combination (see Fig. 53). By boiling in water 

 containing acid, these large molecules undergo a very simple 

 cleavage into their component molecules. Starch treated in 

 this manner yields only one substance, namely dextrose 

 (glucose, or grape sugar (C 6 H 12 O 6 )). Protein, on the other 

 hand, yields a much greater variety of compounds, some 



