90 PHYSIOLOGY OF THE FUNGI 



A knowledge of the amounts of oxygen consumed and carbon dioxide 

 evolved by organisms is the basis of a useful method of study in many 

 phases of physiology. The principles of such measurements are simple. 

 In aerobic respiration both the oxygen and carbon dioxide may be meas- 

 ured. The ratio of the moles, or volumes, of carbon dioxide evolved and 

 oxygen used is called the respiratory quotient (R.Q.) and is written 

 CO2/O2. From the respiratory quotient the nature of the substrate 

 being oxidized may be deduced. A respiratory quotient of 1 is character- 

 istic of aerobic oxidation of carbohydrate. The complete oxidation of a 

 fat may be represented as follows: 



(C,8H3602)3C3H5 + 81.50-2^ 57CO0 + 55H.2O 



The respiratory quotient for this fat is 57/81.5, or 0.7. If fungus cells are 

 suspended in a buffer in the absence of nutrients, and the respiratory 

 quotient determined, it is possible to deduce the type of compound within 

 the cells being used as a source of energy. Oxidation of the stored com- 

 pounds within the cell is called endogenous respiration. The oxidation of 

 substrate molecules from the medium is called exogenous respiration. 

 Since both types of respiration may occur simultanously in the presence 

 of nutrients, it is necessary, in order to determine exogenous respiration, 

 to subtract the value for endogenous respiration from that obtained in the 

 presence of nutrients. 



The rate and amount of respiration are determined by instruments 

 known as respirometers. Various types of respirometers have been used 

 to investigate different phases of fungus metabolism and nutrition. In 

 principle a respirometer is a closed vessel of known volume in which 

 fungus cells are suspended in a buffer or other solution. The carbon 

 dioxide evolved is absorbed in a concentrated solution of potassium 

 hydroxide. The change in volume due to the consumption of oxygen is 

 measured by the use of suitable manometers. At the end of the experi- 

 ment the amount of carbon dioxide evolved is measured after the potas- 

 sium hydroxide solution is treated with a mineral acid. Carbon dioxide 

 alone may be measured by passing a stream of carbon dioxide-free air 

 through a culture and absorbing the carbon dioxide evolved in barium 

 hydroxide or other suitable reagent. The results of such experiments are 

 reported on the basis of the volumes of oxygen used and carbon dioxide 

 evolved per milligram of dry weight per hour. These values are reported 

 as Q02 and Qco2 (see Umbreit et at., 1945). 



A modern respirometer is illustrated in Fig. 16. The various manipula- 

 tive details will not be discussed. For an adequate treatment of these 

 see Umbreit etal. (1945) and Dixon (1943). These methods are extremely 

 useful in studying a wide range of problems. Hawker (1944) used 

 manometric techniques in studying the effect of excess thiamine on 



