164 GENERAL BIOCHEMISTRY 



-OOG— GH2— N G— NH2 + ATP -> 



1 II 



GH3 +NH2 



-OOG— GH2— N G— NH— P03= + ADP 



1 II 



GH3 +NH2 



creatine phosphate 



in muscle. Some creatine phosphate accumulates during periods of 

 rest and serves as a reserve used during muscular activity. When the 

 muscle is working, the free energy of the creatine phosphate is con- 

 verted into mechanical energy and creatine reappears for reaction with 

 more ATP. Muscle action in invertebrates involves a related reaction 

 depending on arginine instead of creatine. 



Some cells, yeast being the best known, store a complex material 

 called metaphosphate. The linkages resemble those of pyrophosphate 

 and may be represented thus: 



o o- 



11 1 



-O— P— O— P— o— 



I II 



o_ o 



The formation of this grouping from ordinary orthophosphate, 

 H0P03=, requires free energy, AF = +10,000 cal. Hence metaphos- 

 phate contains a large potential store of chemical energy for use during 

 periods of starvation or any other stress. 



Utilization of Energy from Metabolism 



Energy transfers of the various types provide for the utilization of 

 the chemical energy of the foods or substrates consumed. The overall 

 efficiency of this utilization is a matter of concern in the economics of 

 food sources and the selection, development, and processing of food 

 materials. These problems all become acute in dense populations 

 where the competition for food is severe. Given an equal food supply, 

 the most efficient organism will tend to survive the stresses of starva- 

 tion. 



A biochemical understanding of the situation is difficult to reach at 

 the level of the metabolic reactions involved because of the complexity 

 of metabolism and incompleteness of the knowledge of the reactions. 

 Nevertheless, some estimates of efficiency have been made. Probably 

 vertebrates do not differ much in this regard, so selection must occur 

 on other bases. Undoubtedly there are differences between vertebrates 



