130 VERTEBRATE RESPIRATION 



of this degradation is designed to facilitate the synthesis of 

 ATP molecules which can subsequently be used for the per- 

 formance of mechanical work, biosynthesis, active transport, 

 light production, etc. 



(c) THE SUPPLY OF ATP 



The different stages in the degradation of a glucose molecule 

 are not all of equal value in the production of ATP molecules. 

 The early parts of this process are usually termed Glycolysis and 

 may take place in the complete absence of oxygen, ending in the 

 production of two molecules of pyruvic acid from a single 

 molecule of glucose. At this stage of the breakdown, only two 

 molecules of ATP have been synthesised and therefore a mere 

 20,000 calories of the total 690,000 found in the glucose have 

 been captured in a form which the body can use (i.e. only 3 %). 

 Under anaerobic conditions the pyruvic acid is converted into 

 lactic acid which accumulates in the muscles or circulates round 

 the body. Under aerobic conditions, however, pyruvic acid 

 enters the so-called tri-carboxylic acid cycle. Furthermore, one 

 of the hydrogen acceptors (DPN) involved in glycolysis is 

 reduced, and under aerobic conditions can be oxidised via the 

 flavoprotein-cytochrome system. Glycolysis of each molecule 

 of glucose produces two molecules of this reduced nucleotide 

 which when oxidised results in the production of six ATP 

 molecules. Under aerobic conditions therefore the glycolytic 

 cycle may be summarised as follows : 



CeHisOg + 2 DPN + 2P > 



2CH3 COCOOH + 2 DPNH2 + 2 ^ P 



2 DPNH2 + O2 + 6P > 2 DPN + 2 H2O + 6 ^ P 



The remainder of the degradation of the glucose molecule 

 takes place according to the process described as the tri- 

 carboxylic acid cycle which involves oxidative phosphorylation. 

 This process may be summarised as follows : 



2CH3 CO COOH + 5O2 + 30P — > 6CO2 + 4H2O + 30 - P 



