DISCUSSION ON BACTERIAL RESPIRATION 253 



many the term implies construction of cell material, enzymes, and 

 perhaps organic compounds involving a carbon chain. Clearly this 

 simple demonstration of carbon dioxide fixation is not a reliable cri- 

 terion of assimilation so defined. Obviously the carbon dioxide could 

 have been fixed in the simple one-carbon compound urea or formic 

 acid, a fixation far different from that which the term "assimilation 

 of carbon dioxide" suggests to most people. 



Assuming that one has completed a simple demonstration of fixa- 

 tion of carbon dioxide in a biological process, what additional criteria 

 must be met then to determine the mechanism of fixation? It is here 

 that the real problems of isotopic work are met, and the problems 

 are by no means simple. Isotopes are a very valuable tool to the 

 investigator, but even isotopes involve many uncertainties. Frankly, 

 the field of isotopic investigation is not fully enough developed to 

 permit a clear understanding of all the criteria that must be met. 

 Therefore, in this short presentation only a few of the possible 

 sources of error will be presented as a starting point for discussion. 



In any investigation, whether it involves isotopes or not, the gen- 

 eral experimental procedure must be reliable if results are to be 

 valid. In fact, assuming that one is cooperating with a competent 

 physicist, the actual isotopic analysis and separation of isotopes will 

 be the minor problem; the general experimental procedure will offer 

 the real problems. A specific experiment may be cited as an example 

 of a case in which the error resulted from the general procedure and 

 not from any isotopic considerations. In the propionic acid fermen- 

 tation, propionic acid is formed in which carbon dioxide-carbon is 

 incorporated. To obtain information on the possible mechanism of 

 the reactions concerned in the fixation of carbon dioxide in propionic 

 acid, it was necessaiy to determine the location of the fixed carbon 

 in the propionic acid molecule. Thus the problem was to select a 

 reliable chemical reaction for the degradation of the molecule. In 

 doing this Carson and his co-workers degraded radioactive propionic 

 acid obtained from the propionic acid fermentation by alkaline 

 permanganate oxidation and obtained oxalic acid and carbon diox- 

 ide: 



CH3CH2COOH -> COOHCOOH + CO2 



From 70 to 75 per cent of the radioactive carbon was found in the 

 oxalate fraction and 25 per cent in the carbonate. Since the workers 

 believed that the carbonate arose from the carboxyl carbon and the 

 oxalate from the alpha and beta carbons of propionic acid, it ap- 



