132 VERTEBRATE RESPIRATION 



of 38 molecules of ATP whereas anaerobic glycolysis produces 

 only 2 molecules. These considerations emphasise the very great 

 importance of the tri-carboxylic acid cycle together with the 

 flavoprotein-cytochrome system. Both of these systems are now 

 known to reside in the mitochondria of cells, but within these 

 structures they are spatially separate. The cytochrome system 

 is found on the surface of the membranes of the mitochron- 

 drium whereas the tri-carboxylic acid cycle enzymes are located 

 in the matrix or soluble portion of the inner contents. The 

 glycolytic enzymes are generally distributed throughout the 

 cytoplasm. 



(d) LACTIC ACID AND EXERCISE 



From this very brief summary of the basic mechanisms con- 

 cerned in the oxidation of glucose we can see that the production 

 of lactic acid is a very inefficient process for producing bond- 

 energy. Nevertheless, it has the advantage that it can take place 

 under conditions when oxygen is in short supply or completely 

 absent. At rest the body contains sources of readily available 

 energy in the form of ATP and another phosphate-containing 

 compound — phosphagen — which in vertebrates is creatine 

 phosphate (CP). This compound is able to transfer its energy- 

 rich phosphate bond to ADP and so produce a molecule of 

 ATP. These reserves are, however, soon depleted during rapid 

 exercise (ATP within 0-5 seconds, CP in about 5 seconds) and 

 glycolysis makes a significant difference even although it is only 

 capable of synthesising two molecules of ATP. Most of the extra 

 energy required during exercise lasting between 10 seconds and 

 2 minutes is derived from anaerobic glycolysis. When the animal 

 has finished the bout of exercise, the further oxidation of lactic 

 acid via the tri-carboxylic acid cycle and cytochrome system may 

 take place with the production of further molecules of ATP. 

 Some of this ATP may be used to replenish the store of ATP 

 and CP, and some for the performance of the chemical work of 

 biosynthesis, a portion (5/6ths) of the lactic acid being converted 

 back to glucose or glycogen in the liver. In other organisms, 

 including some fish, there is no attempt to pay off the debt 



