Cell Constitution 



71 



and synthesis separately. They apparently may be 

 dissociated in cells by such agents as dinitrophenol, 

 and off-on switching devices between the two 

 phases of metabolism may well be operative in nor- 

 mal physiological control. 



There is abundant evidence that at temperatures 

 in the physiological range, the downhill release of 

 energy to be used by the uphill synthetic paths 

 cannot involve a simple transfer of heat such as 

 one finds in the usual heat expansion type of ma- 

 chine. Rather, it has become obvious that chemical 

 forms of linkage energy must be made available. 

 These chemical links must be such that they can 

 be formed by the oxidative pathways and then enter 

 into reactions with the synthetic pathways, giving 

 up their high energy content for purposes of 

 chemical synthesis, transport of materials, gross 

 movements and so forth (cf. Johnson, in Lardy, 

 '50).* A good rule-of-thumb to keep in mind is that, 

 in order for an energy-yielding (exergonic) reac- 

 tion to drive an energy-using (endergonic) reaction, 

 the two reactions must have a common component. 



It is apparent that the whole study of 

 cellular metabolism involves a number of 

 aspects. Starting with the outside of the ani- 

 mal cell and working inward we may list 

 the following: 



1. The exchange of foods and waste prod- 

 ucts between cell and environment. Under 

 this heading are frequently listed such sub- 

 jects as permeability, secretion, and excre- 

 tion. Although of obvious importance, these 

 subjects will be treated only incidentally 

 here. 



2. The oxidative breakdown of the re- 

 duced carbon chains of the foods taken in. 



3. Formation of energy linkage compounds. 

 It is here that the coenzyme systems become 

 of greatest importance. Three systems will 

 be invoked frequently (cf. Meyerhof, '49) : 

 the coenzyme I and II systems (nicotinamide 

 coenzymes) concerned with hydrogen (elec- 

 tron) transfer; the adenylic acid systems 

 (ATP and ADP) concerned with phosphate 

 bond energy transfer; and the cocarboxylase 

 system (thiamine pyrophosphate) relating to 

 decarboxylation and carbon dioxide for- 

 mation. 



4. The synthetic mechanisms themselves, 

 or perhaps better, the transducer mecha- 

 nisms whereby one form of energy is con- 

 verted to another. Of the biochemical syn- 



* Frequent reference will be made to the provoc- 

 ative review by Barron ('43) and to the volume 

 "Respiratory Enzymes," edited by Lardy. In many 

 instances, rather than referring to original papers, 

 citations will be given by name of the author of the 

 appropriate chapter in "Respiratory Enzymes." 

 Thus, citations given in this way do not necessarily 

 mean that the author cited is the originator of the 

 work mentioned. 



thetic systems, the formation of polysaccha- 

 ride is best adapted to serve as a model sys- 

 tem (cf. Hassid and Doudoroff, '50) ; for other 

 types of processes, perhaps the best model is 

 found in Szent-Gyorgyi's muscle protein 

 preparations (cf. Szent-Gyorgyi, '51). 



Finally, an aspect of cell metabolism that 

 is perhaps the most intriguing and mysterious 

 of all must be treated: that of the control 

 of the reactions in the protoplasmic sub- 

 stance. It is at this point, as well as with item 

 number four in our list, that cytology and 

 general physiology come closest together, 

 since certainly a large proportion of the trans- 



-C-C— -C-- 



ORGANIC \, 

 SUBSTRATES A 

 POP QURNING 

 FOODS 



MOVEMENT 



Fig. 2. Schematic model to illustrate some gen- 

 eral principles of cellular metabolism. Organic sub- 

 strates (foods or products of assimilation) at a high 

 energy level are represented as reduced carbon 

 chains. These chains are oxidized stepwise, each 

 step potentially an energy-transferring link to 

 physiological functions. 



ducer and controlling mechanisms must re- 

 side in the formed elements of the proto- 

 plasm. 



This preliminary general outline could fit 

 almost all types of cells, with, of course, 

 special modifications. The cells of green 

 plants have the photosynthetic mechanism 

 added to the scheme outlined; for these living 

 units energy is taken directly from the en- 

 vironment as radiant energy so that reduced 

 carbon chains are built up. Presumably from 

 here on their metabolism is similar, in gen- 

 eral outline, to that of animal cells. Simi- 

 larly, some microorganisms can obtain the 

 energy for chemical energy storage from the 

 oxidation of simple, sometimes elementary, 

 substances. 



GLYCOLYTIC AND OXIDATIVE 

 MECHANISMS 



The most common, but by no means ex- 

 clusive, fuel for cells appears to be carbo- 



