56 RESPIRATION 



TYPES OF RESPIRATION 



Ordinarily respiration is arbitrarily divided into two types, 

 aerobic and anaerobic. Aerobic respiration occurs in the pres- 

 ence of atmospheric oxygen. Anaerobic respiration, on the other 

 hand, occurs in the absence of a supply of atmospheric oxygen 

 and proceeds at the expense of the oxygen that is combined in the 

 substance being respired. Presumably aerobic respiration is of 

 most common occurrence among fungi, but many species possess 

 the ability to respire either aerobically or anaerobically and are 

 spoken of as facultative anaerobes. Few, if any, species are known 

 to be strict anaerobes. The facultative anaerobes, because of the 

 products of their respiration, for example, alcohol, acetic acid, and 

 lactic acid, are of most interest and importance to man. 



Aerobic respiration. The most important reason that can be 

 given for elaborating upon aerobic respiration in this volume is 

 that such a discussion may help to clarify certain misunderstand- 

 ings of this process that are all too commonly prevalent and that 

 are sometimes transmitted from teacher to student. 



In the first place fungi, in common with all other living things, 

 release energy for their own metabolic activities during the proc- 

 ess of respiration. In the oxidation of glucose such aerobic release 

 is conventionally expressed as follows: 



C 6 H 12 6 + 60 2 = 6C0 2 + 6H 2 + 673 Cal 



This equation, the precise reverse of the reaction for photosynthe- 

 sis, is correct only in so far as it expresses the energy relations 

 and the final products. It merely indicates that the complete oxi- 

 dation of 1 molecule of glucose requires 6 molecules of oxygen 

 and that, while 6 molecules of carbon dioxide and 6 molecules of 

 water are formed, 673 calories of energy are released. Such an 

 equation leaves the erroneous impression that at one instant glu- 

 cose is present and at another, by some miracle, the sugar has be- 

 come carbon dioxide, water, and liberated energy. As a matter 

 of fact, the process is a complicated one, and intermediate prod- 

 ucts are formed. For this reason it is indefensible to indicate aero- 

 bic respiration as occurring in accordance with the foregoing 

 equation. 



Anaerobic respiration. Consideration will be given subse- 

 quently to some of the kinds of anaerobic respiration, products 



