536 ANAEROBIC RESPIRATION 



Since the oxygen released is in the molecular rather than the active state, 

 it cannot be utilized in oxidations. It is also supposed that catalase can 

 liberate molecular oxygen from organic peroxides. The presence of catalase 

 in plant or animal tissues can be demonstrated by bringing some of the 

 tissue into contact with a dilute solution of hydrogen peroxide. If a small 

 block of a potato tuber, for example, is dropped into a test tube containing 

 hydrogen peroxide solution, evolution of bubbles of oxygen ensues immediately. 



This enzyme can be determined quantitatively by carrying out the reaction 

 in an apparatus in which the rate of evolution of oxygen can be determined. 

 Because of the ease with which such quantitative measurements of catalase 

 activity can be made, many extensive studies of this enzyme have been carried 

 out. 



Considerably more time and attention have been devoted to precise studies 

 of the occurrence and action of catalase than the very inadequate knowledge 

 of its role in living organisms would seem to warrant. Despite its almost 

 universal presence in living cells, the metabolic significance of this enzyme, if 

 any, is obscure. It has been suggested that catalase may act as a sort of "safety 

 valve" by preventing the accumulation in cells of hydrogen peroxide or other 

 peroxides produced as a result of cell metabolism. There is, however, no 

 positive evidence in support of this suggestion. 



Metabolically active plant tissues usually exhibit a high rate of respiration 

 and a high general level of enzymatic activity. Hence a correlation usually 

 exists between the catalase activity of a tissue and its metabolic status. Meas- 

 urements of the catalase activity of a tissue are therefore often accepted as an 

 index of the intensity of metabolic activity in that tissue. 



Fermentation. — Of all the many known types of fermentations by far the 

 most important, both from the theoretical and practical point of view, is the 

 process of alcoholic fermentation. Although this process has been familiar to 

 the human species for time out of mind, it was not until the classical researches 

 of Pasteur, begun in 1857, that it was recognized that alcoholic fermenta- 

 tion resulted from the metabolic activities of yeast plants. Yeasts, it should 

 be recalled, are single-celled organisms belonging to the Ascoinycctcs. Yeasts 

 may multiply by budding and under certain conditions produce ascospores 

 (Fig. 116). 



Pasteur also realized that alcoholic fermentation was an anaerobic process 

 and in fact characterized fermentation as "life without oxygen." The final 

 basic step in the understanding of the alcoholic fermentation was furnished by 

 Buchner's demonstration in 1897 that an active agent or enzyme — zymase — ■ 

 could be extracted from j'east cells, and that this enzyme could catalyze the 

 process in the total absence of yeast cells. 



