RESPIRATORY METABOLISM 353 



under the subjects of movement, permeability, and excretion. Therefore 

 the problem of respiratory metabolism of a protozoan organism, at least 

 for the present discussion, is easily resolved into a problem comparable 

 to that of cellular respiration in the Metazoa. 



Purposes of Studying Respiration 



One of the first questions to be considered is: "What can be learned 

 by studying respiration?" The measurement of gaseous exchange is not 

 an end in itself, but it is a tool which, when used singly or in combina- 

 tion with other tools, may help us to obtain answers to the following 

 types of questions. 



1. What is the rate of energy expenditure of the organism.-' How does 

 the metabolic rate (basal and otherwise) of one species compare with 

 that of another.^ How does it change during starvation? Or conjugation? 

 Is this rate dependent upon the Oo or CO^ tension of the environment? 

 How is it affected by narcotics? How does it vary with temperature? Or 

 with other chemical and physical factors of the normal environment? 



2. What is the source of this energy? Is it obtained by oxidation of 

 fats, carbohydrates, or proteins? Or by anaerobic oxido-reductions? Can 

 this source be shifted by changing the chemical or physical environment 

 of the organism? What are the intermediate products formed during 

 oxidation of the substrate? Is oxidation of the substrate complete (i.e., 

 to COo and H.O), or may these intermediate products be excreted by 

 the cell? 



3. What is the mechanism by which energy is obtained from the sub- 

 states? Is this brought about through the intermediary action of dehydro- 

 genase, cytochrome, and Warburg's respiratory enzyme? Or is it brought 

 about through dehydrogenase and reversible oxidation-reduction systems, 

 such as yellow pigment or pyocyanine? Or by glutathione? Can these 

 substances be replaced by artificial oxidation-reduction systems? Can a 

 shift in this mechanism of respiration be induced by changing the avail- 

 able substrate? How is this mechanism related to the degree of anaerobio- 

 sis which the organisms can endure? Is the organism capable of synthesiz- 

 ing the respiratory enzymes from simple substances, or must they be 

 obtained from complex outside sources, i.e., from vitamins? 



Measurements of the respiratory metabolism of an organism may be 

 used as indices of the rate at which it uses energy, the substrate from 



