116 



THE CELL AND PROTOPLASM 



These equations would imply that, 

 although the same product (CH2O) is 

 formed in all cases, the extent of its for- 

 mation may vary considerably. However, 

 one must bear in mind that these equations 

 are only first and somewhat rough approxi- 

 mations. The nature of the assimilation 

 product is unknown; it is certain that it 

 does not consist exclusively of carbohy- 

 drate, at least for Spirillum serpens where 

 Giesberger has ascertained the formation 

 of volutin. And thus it is conceivable 

 that different organisms may elaborate dif- 

 ferent assimilation products, and hence 

 that also the more intimate mechanism of 

 their synthetic reactions is not identical. 



Of great significance is the fact that 

 Clifton and Logan (1939) have demon- 

 strated the assimilatory reactions to be 

 functions of the chemical constitution of 

 the respiration substrate rather than of 

 the available energy. Their results have 

 shown that the assimilated fraction of 

 lactic and of pyruvic acid is exactly the 

 same on the basis of the number of carbon 

 atoms; the same holds for succinic and 

 fumaric acids: 



CH3CHOHCOOH + 2O2 -> 



(CH20)+2C02 + 2H20 (1) 



CH3COCOOH + li02 -^ 



(CH20)+2C02 + H20 (2) 



COOHCH2CH2COOH + 2^02 -» 



(CH20)+3Cd2 + 2H20 (3) 



COOHCH = CHCOOH + 2O2 -^ 



(CH20)+3C02 + H20 (4) 



Now the energy liberated in reaction (1) 

 is certainly considerably greater than that 

 set free in (2), just as that of (3) exceeds 

 the yield of (4). Also, the caloric effect 

 of (3) is definitely larger than that of (1). 

 And yet, from these four compounds it is 

 always only one carbon atom of each sub- 

 strate molecule that becomes incorporated, 

 at least temporarily, into the cell material. 

 Such results are, however, understandable 

 if it is assumed that during the stepwise 

 dehydrogenation of the four compounds 

 only one intermediate product originates 

 from which the occurring synthesis is pos- 



sible. It is clear that in the case of the 

 four substrates here discussed such a 

 mechanism is not at all improbable; their 

 gradual degradation can readily be formu- 

 lated in such a way that one molecule, e.g., 

 of formaldehyde, arises from one molecule 

 of each one of the raw materials. Hence, 

 if the synthetic reactions would function 

 through a condensation of formaldehyde, 

 the experimental results would fit in re- 

 markably well with such an assumption. 

 Different organisms may well use differ- 

 ent intermediate products for synthetic 

 processes. But also the gradual degrada- 

 tion of the substrate may, under the influ- 

 ence of specific agents, proceed by a dif- 

 ferent path. Thus, referring to the above- 

 mentioned differences in the assimiliatory 

 "efficiency" of e.g., Prototheca zopfii and 

 Escherichia coli, in the respiration of ace- 

 tate, it would be possible to formulate the 

 occurring reactions as follows: 



CH3COOH -^ CH2OHCOOH -^ 



CHOCOOH ^ CH2O + CO2 (5) 

 HOOCCH3 + H3CCOOH -» 



HOOC.CH2.CH2.COOH (6) 



Reaction (5) might represent the step- 

 wise oxidation of acetic acid under the in- 

 fluence of Prototheca; each molecule of the 

 substrate gives rise to the formation one 

 molecule of CH2O. If, however, the dehy- 

 drogenation of acetic acid by Escherichia 

 coli were to proceed by way of succinic 

 acid, as shown by equation (6), then one 

 molecule of CH2O would result from the 

 oxidation of two molecules of acetate, on 

 the basis of equation (3). 



These reflections do not aim at prema- 

 turely proposing hypothetical mechanisms 

 for the synthetic processes; they are given 

 here to show how the general idea of a 

 strictly chemical mechanism for assimila- 

 tory reactions seems to fit the available 

 experimental data better than one based 

 primarily on energy. They also indicate 

 how difficult it may be at the present time 

 to interpret correctly efficiency determina- 

 tions based upon measurements of the 

 energies involved without an accurate 

 knowledge of the internal mechanism. 



