ANAEROBIC AND AEROBIC RESPIRATION 541 



Alcohol may be produced in the process in a quantity equivalent to that indi- 

 cated by the equation, but part of it may be immediately converted into other 

 compounds by subsidiary reactions. Another possibility is that in the higher 

 plants the process takes a slightly different course, certain other products 

 being formed in addition to or instead of ethyl alcohol. 



3. The complex of enzymes known as zymase is present in the cells of a 

 number of species of the higher plants as well as in yeast (Stoklasa and 

 Czerny, 1 903). It is now considered that this enzyme is of widespread and 

 probably of universal occurrence in plant cells. 



Causes of Injury to Cells as a Result of Anaerobic Respiration. — As 

 has already been noted, prolonged anaerobic respiration has a detrimental or 

 even lethal effect on many plant tissues. At least two probable reasons for 

 this effect have been recognized. As the equations already presented show, the 

 oxidation of one mol of glucose in aerobic respiration results in the liberation 

 of 673 kg.-cal., while the energy released from the same quantity of glucose 

 in anaerobic respiration is only about 25 kg.-cal. Aerobic respiration is 

 therefore more than twenty-five times as effective an energ^^-releasing process 

 as anaerobic respiration. This is probably one of the reasons why many plant 

 tissues, especially if metabolically active, cannot endure prolonged anaerobiosis. 

 When anaerobic is substituted for aerobic respiration the rate of energ}^ release 

 is inadequate for the maintenance of cell processes, and deleterious effects are 

 soon produced. 



A second probable cause of injury during anaerobic respiration is the 

 accumulation of substances toxic to the protoplasm. In anaerobically respiring 

 tissues alcohol and other substances described later usually accumulate in the 

 cells. These compounds are toxic to protoplasm and probably account, at 

 least in part, for the injurious effects produced in many plant tissues by con- 

 tinued anaerobic respiration. It is perhaps significant that senescent tissues 

 such as ripe fruits, in which metabolic activity is sluggish, can usually endure 

 a more prolonged period of anaerobic respiration without injury than metabol- 

 ically active tissues. 



The Relation between Anaerobic and Aerobic Respiration. — There 

 are a number of reasons for believing that a close relationship exists between 

 these two types of respiration as they occur in the higher green plants. In 

 1878 Pfeffer proposed a theory which involved this concept. He postulated 

 that aerobic respiration takes place in two steps. In the first sugar was sup- 

 posed to be split anaerobically into alcohol and carbon dioxide and in the 

 second alcohol was supposed to be aerobically oxidized into carbon dioxide 

 and water. 



For various reasons this theory proved to be untenable but a modifica- 



