72 



Cellular Structure and Activity 



FOOD 



PYRUVATE OXIDATION 



Fig. 3. An outline model of a cellular system having anaerobic and aerobic mechanisms for carbohydrate 

 oxidation. Glycolysis (top section), Krebs-Szent-Gyot-gyi cycle (bottom right) and oxidase systems (lower 

 left) are diagrammed separately since they may well be separate morphological entities in cells. Each major 

 unit is linked by (1) raw materials and (2) coenzymes of the common energy pool. Synthetic and other 

 energy-using reactions should also be keyed in to the common energy pool. Within each unit process, indi- 

 vidual reactions are also linked together by raw materials and coenzymes. 



hydrate. In any case, the schemes built up 

 for the oxidative degradation of carbohy- 

 drate provide such generalized models that 

 they serve well to illustrate pathways which 

 may be available in any given type of liv- 

 ing system. It should, of course, be clearly 

 understood that some of the most interesting 

 physiological variations may be concerned 

 with additional systems not yet discovered, 

 and that there may also be both ontogenetic 

 and phylogenetic stages yet to be outlined* 

 (cf. Barron, '43). 



In Figure 3 is presented a simplified diagram of 

 pathways traversed by the elements of a common 

 hexose sugar, after a preliminary treatment in- 



* While the number of specific chemical reac- 

 tions taking place in cells in general is probably 

 finite, reporting on all of them would hardly be a 

 fruitful undertaking for the student of cellular 

 metabolism. The "Annual Review of Biochemistry" 

 serves as an excellent device for following details, 

 as also does "Chemical Abstracts." In this chapter, 

 examples which serve as models are deliberately 

 chosen in order to illustrate principles involved. 



volving phosphorylation, leading to the final forma- 

 tion of carbon dioxide and water (or, in glycolysis, 

 to lactic acid). Further details are given in Figure 

 4. The over-all reaction for the oxidative process is: 



C6H12O6 + 6 O2 ^ 



6 CO2 + 6 H2O + 683,000 calories 

 (at normal physiological concentrations) 

 (cf. Johnson, in Lardy, '50) 

 For lactic acid fermentation the reaction is: 



C6H12O6 ^ 2 CsHeOs + 50,000 calories 



The gas exchange resulting from oxidation is called 

 respiration; the process is mediated by a series of 

 enzymatically catalyzed steps so arranged that the 

 energy of oxidation is delivered in a series of small 

 "packets" (cf. Szent-Gyorgyi, '39). To oversimplify 

 the case, any single step might be pictured as fol- 

 lows: 



Reduced substrate -f cofactor >- 



(with energy 

 packet) 



Oxidized substrate -f reduced cofactor 



(wdth energy 



packet) 



Sugar, to feed the mechanism, may enter from 



the environment or from endogenous stores such as 



glycogen. In either instance, stages to be discussed 



