CHAPTER XIII 



MECHANISM OF CHEMICAL CHANGES PEODUCED BY 

 MICE00EGANISM8— ENZYMES AND FEEMENTATION 



The bacteria, yeasts and molds must secure all of the 

 materials useful to the cell by a process of absorption or of 

 osmosis/ All organisms grow only in contact with water. 

 The substances to be used must be dissolved in the water 

 outside the cell and diffuse through to the interior. There 

 is nothing corresponding to a mouth through which solid 

 particles may enter. It is apparent, therefore, that micro- 

 organisms must bring about changes in substances outside 

 the cells in order either that they be made soluble in 

 water, thus capable of diffusing into the cell, or so changed 

 that even though soluble they may diffuse into the cell and 

 be utilized. Further, the cell must make use of materials 

 which diffuse to the interior in various ways. Sometimes 

 these must be broken down into simpler compounds. Usually 

 they must be oxidized directly or indirectly and the energy 

 thus secured utilized for growth purposes. Certain of the 

 compounds introduced in some instances may be reduced. 

 Carbon dioxide and water, for example, diffusing into the 

 cell may be combined to form carbohydrates. 



The fact that microorganisms can bring about changes in 

 the chemical nature of compounds led many years ago to 

 their designation as organized ferments. They were con- 

 trasted with so-called body secretions such as the gastric 

 juice, which could also bring about fermentative changes and 

 digestion, and which were termed unorganized ferments. 

 Later it was discovered that many microorganisms excreted 

 substances which brought about changes outside the cell 



157 



