Molecular Structure in Protoplasm 45 



with them argument by analogy creeps in perhaps a Httle too fre- 

 quently, and often in a rather far-fetched manner. 



In this discussion, then, we are attempting to form a conception 

 of a type mechanism as viewed on a molecular level. For this pur- 

 pose we have chosen the only one for which there seems to be enough 

 information available to give us at least a tentative conception. This 

 is associated with respiration, and while the conception of the 

 mechanisms involved is still in a very hypothetical stage, an attempt 

 at the comprehension of them may nevertheless serve as an avenue 

 of approach to an understanding of the mechanisms on still higher 

 levels. 



Investigations into the reactions and the materials necessary for 

 the processes or activities which we think of collectively as respira- 

 tion have given a fair idea of the various steps involved and of the 

 molecules required (10, 11) . Each step, it is thought, requires a 

 protein molecule with an attached small molecule; the two together 

 are considered as an enzyme. Many such enzymes are required for 

 the complete breakdown of a glucose molecule (10, 11) and there is 

 evidence (12, 13) that several of these may occur combined within 

 a single complex. That these enzyme systems are arranged in an 

 organized manner seems evident from the observations (14) that 

 certain activities are lost when the complex is disintegrated, and 

 that these enzyme systems are not rebuilt upon standing although 

 apparently all of the constituents are present. 



The frequency of occurrence of these respiration complexes 

 locally in the protoplasm is not known, but that many thousand such 

 mechanisms are distributed throughout the cell seems not too unrea- 

 sonable. In the active cell they occur in a water medium in which 

 molecules and ions of various kinds are also present, and since these 

 may be expected to influence the complex, it may be well to first 

 obtain a general picture of the cell as a whole on a molecular level. 



MOLECULAR CONSTITUENTS OF PROTOPLASM 



From analyses of the protoplasms of many organisms the com- 

 ponents of active cells may be listed in a general manner as in 

 Table 1. 



The proportions shown in Table 1 may be thought of as minima 

 of a sort but essential, nevertheless, for active protoplasm. By using 

 average molecular weights for the various constituents, a rough esti- 

 mate of the relative number of each may be obtained, as shown in 

 Table 2. 



