PRINCIPLES OF MICROBIAL METABOLISM 371 



solution, as determined by the lowering of the freezing point, 3 change 

 in conductivity, 4 or other convenient method ; also of the hydrogen-ion 

 concentration, as determined by the electrometric or colorimetric 

 method, 5 and of the buffer content of the soil, as determined by the 

 curves which show the relation between addition of acid and alkali 

 and change in reaction. 



Enzymes of microorganisms. A large number of chemical reactions 

 are carried on by the microbial cell by means of enzymes. These are 

 either secreted outside of the cell (exo-enzymes) or remain within the 

 cell and can be separated from it only with great difficulty (endo- 

 enzymes). Among the different reactions, in which enzymes take an 

 active part, we need mention but four: (1) Hydrolysis. This involves 

 the transformation of various polysaccharides into sugars, of proteins 

 into amino acids, of amino acids into ammonia and oxy-acids; the 

 transformation of insoluble organic substances (starch, fibrin) into 

 soluble forms, and finally glycolytic decompositions, as the trans- 

 formation of sugar into lactic acid (C6H12O6 = 2C3H6O3). (2) Oxi- 

 dation, resulting in the liberation of energy. Some of the oxidation 

 processes, such as the formation of acetic acid from alcohol, citric 

 and oxalic acids from glucose, are frequently referred to as fermenta- 

 tions. These are distinguished from true fermentations, which result 

 in the liberation of energy by the decomposition of complex organic 

 compounds into simpler forms, without the intervention of free oxy- 

 gen, as in alcoholic and butyric acid fermentations. (3). Reduction. 

 Substances rich in oxygen are reduced to substances poor in oxy- 

 gen, such as the reduction of nitrates, nitrites and sulfates, and the 

 oxygen may be used for purposes of oxidation. The coupled reac- 

 tions of oxidation and reduction, whereby the one substance is oxidized 

 and the other reduced at the same time are common in microbio- 

 logical reactions. (4) Synthetic reaction, including anhydride forma- 

 tion and condensation. 



3 Bouyoucos, G. J., and McCool, M. M. The freezing point method as a new 

 means of measuring the concentration of the soil solution directly in the soil. 

 Mich. Agr. Exp. Sta., Tech. Bui. 24, 1915; 27, 1916; 31, 1916; 37, 1917; 43, 1918. 

 Hoagland, D. R. The freezing point method as an index of variation in the 

 soil solution due to season and crop growth. Jour. Agr. Res., 12: 369. 1918. 



4 Hibbard, R. P., and Chapman, C. W. A simplified apparatus for measuring 

 the conductivity of electrolytes. Mich. Agr. Exp. Sta., Tech. Bui. 23, 1915. 



6 Clark, W. M. The determination of hydrogen ions. The Williams & Wilkins 

 Co., Baltimore. 1922. 



