CARBOHYDRATE METABOLIC PATHWAYS 73 



CO2 



Fig. 3.6 The Krebs cycle— an energy-producing pathway. 



These experiments demonstrate what was said before 

 about the value of a concept: the idea gradually developed 

 that the mitochondria, and the mitochondria alone, could 

 furnish the crucial enzymes necessary for the oxidative 

 breakdown of the major foodstuffs. The scheme in Fig. 3.6 

 formalizes this; the Krebs cycle may also be considered as 

 an energy cycle, whose enzymes reside in the mitochondria 

 —as Claude has said, these are the power plants of the cell. 

 We will also see how concepts may become top-heavy; i.e., 

 as students of comparative biochemistry added their find- 

 ings during the 1940's to the pool of knowledge, namely 

 that virtually every organism examined possessed an active, 

 functioning Krebs cycle, the notion began to prevail that 

 this pathway was the "prairie fire" of all terminal oxidation. 



That the importance of the Krebs cycle may have been 

 oversold to biochemists (or oversubscribed by them) is 

 revealed in their reaction to the discovery of the Zwischen- 



