VELOCITY OF REACTIONS 



the sugar which has been formed from starch is 

 immediately removed. 



The contrary effect, viz. that further formation 

 of compounds is hindered when the storage of this 

 compound has reached a certain stage, is also a 

 frequent phenomenon in living organisms. When 

 leaves are cut off from the branch and are exposed 

 to sunlight under favourable conditions of life, 

 for a certain time they continue their assimilation 

 of carbon dioxide, and starch is formed to a con- 

 siderable extent. Even more starch is stored in 

 such leaves than in normal leaves which have not 

 been separated from the plant. But, after a time, 

 carbon dioxide assimilation diminishes and ceases 

 entirely. The concentration of sugar in the leaf 

 cells.becom.es too great and the assimilation process 

 is hindered by the reaction products. 



The mechanism accelerating and ceasing re- 

 actions in living cells is very often simply regulated 

 by the general laws of reaction velocity, and we 

 need not assume any special power of living proto- 

 plasm. The next chapter will touch on one of the 

 most important influences on the reaction velocity, 

 and will show that living cells possess most effective 

 means to accelerate reactions and to cause sur- 

 prising chemical results. 



