360 RESPIRATORY METABOLISM 



form of carbohydrate reserves, either dry or wet weight would be a poor 

 index of the amount of respiring protoplasm. If one wishes to compare 

 the rate per unit of protoplasm, the best index is probably the nitrogen 

 content, because protein and other nitrogen-containing compounds are 

 not ordinarily stored as reserve food. However, in the case of Protozoa 

 which secrete nitrogen-containing tests, this criterion might also be 

 very poor. Therefore any comparison of the absolute rate of different 

 species, even after all differences in technique, immersion fluid, and 

 physiological condition of the animal have been overcome, must usually 

 be made with reservations, or at least with an adequate understanding of 

 the limitations involved. Some authors have chosen to calculate O^ 

 consumption in terms of cubic millimeters of organisms, but the errors in- 

 herent in the methods of packing the animals for measurement (usually 

 centrifuging) or in calculating volume from linear dimensions are too 

 great to allow close comparison of data for different types of organisms. 

 However, for an extended series of experiments on the same or very 

 similar organisms, the use of dry weight (A. Lwoff, 1933) or of volume 

 measurements (Elliott, 1939) seems to be entirely satisfactory. 



If concentrated suspensions of organisms are used, the rate of shaking 

 should be carefully controlled. The importance of this factor is demon- 

 strated by the data of Hall (1938). If ammonia is produced by an 

 organism, it is necessary to maintain acid within the respiring chamber 

 in order to obtain true values of O, consumption. This very important 

 procedure is discussed by Specht (1935). 



Another question which arises in expressing results in absolute form 

 is that of measuring basal metabolism, i.e., the metabolism of rest. In a 

 mammal, for instance, there are certain well-defined limitations of condi- 

 tions under which Oo consumption may be termed a measurement of 

 basal metabolism. In a protozoan it is more difficult, if at all possible, 

 to apply these criteria, and in all known measurements we have a sum 

 of the total metabolic processes, i.e., of those to which we refer as "basal," 

 those due to movement of the organism, and, if the medium is nutrient, 

 those due to the manufacture of reserve food material and to growth. 

 The energy expended for each of these purposes will probably vary 

 with the species, the physiological state, and the environmental condi- 

 tions. 



If the metabolic substrates of an organism undergo complete oxida- 



