40 PROTOPLASM AND THE CELL 



their energy, from purely inorganic materials. These are the 

 so-called nitrifying bacteria, one of which, Nitrosomonas, con- 

 verts ammonia salts into nitrites while another, Nitrobacter, 

 changes the nitrites into nitrates. These organisms thus per- 

 form a most useful economic function in preparing food 

 material in the soil for use by the green plants. But the chief 

 biologic interest of these forms is that they^are able to build up 

 their own proteid molecules directly from inorganic substances 

 without the aid of chlorophyll, and- to get energy from such 

 compounds in which it is locked up for all other kinds of living 

 things. Thus urea, thrown off by animals and plants as a use- 

 less and to them harmful waste matter is a source of food and 

 energy for some bacteria which convert it into free ammonia, 

 carbon dioxide and water. 



Enzyme Action in General. We have chosen yeast and bac- 

 teria to illustrate one great field of physiological phenomena, 

 viz., ferment action, because in this matter yeasts are more 

 accurately known than any other type of cells. The so-called 

 endoenzymes are particularly important because of the growing 

 tendency in biology to associate the majority of vital activities 

 of metabolism, growth and differentiation with enzyme activi- 

 ties. With yeast all of these illusory substances must be formed 

 in the protoplasm by the interaction of the relatively few ele- 

 ments contained in the materials of Pasteur's fluid, and the 

 elements of yeast protoplasm. 



One peculiarity in the physiology of bacteria, as with yeast, 

 is that while individually invisible they produce effects in their 

 environment which are visible and conspicuous. For example, 

 alcohol manufactured by yeast in cider, is acted upon by the 

 vinegar bacteria and changed by oxidation into acetic acid and 

 water. Thus C 2 H 6 O + O 2 = C 2 H 4 O 2 + H 2 O. The same bac- 

 teria then may carry on the oxidation still further and convert 

 the acetic acid into carbon dioxide and water, thus: C 2 H 4 O 2 + 

 4 = 2C0 2 + 2 H 2 0. 



Such effects, like alcoholic fermentation, are due to the action 

 of ferments or enzymes. Here the effects are produced on sub- 

 stances foreign to the organisms which manufacture the fer- 



