CHLOROPHYLL AND ENERGY 173 



molecule of this acid is fixed at the other end of the chain on 

 the first atom of carbon to give fructose- 1-6-diphosphate. 



Rendered fragile by these two phosphoric appendices, the 

 chain of glucosides breaks in the middle, through a diastase, 

 aldolase, and we have a mixture of phosphoric dihydroxy- 

 acetone and glyceraldehyde-3-phosphate. In this last sub- 

 stance, the aldehyde is transformed into alcohol, the phos- 

 phoric acid passes from the 3rd atom of carbon on the 2nd 

 and is finally liberated, giving rise to pyruvic acid. 



No molecule of carbon dioxide has yet been given off" and 

 the degradation of the glucose has consisted of scarcely 

 more than splitting its carbon chain in two, but this degra- 

 dation has provided much more energy than it has required. 



The second stage takes place entirely in the citric acid 

 cycle, a gigantic millstone to pound the molecules to extract 

 carbon dioxide from them. 



Pyruvic acid enables this to be done. With its 3 atoms of 

 carbon, and by means of a diastase, aconitase, it combines 

 with oxaloacetate which has 4; the first molecule of carbon 

 dioxide is given off", and we have a substance with 6 atoms of 

 carbon or, rather, several substances. An equihbrium is, in 

 fact, established, in which citrate (80 per cent) is predominant, 

 with a little aconitate (4 per cent), but more particularly 

 isocitrate (16 per cent). The last-named enters into the cycle 

 in losing 2 atoms of hydrogen, then, successively, 2 molecules 

 of carbon dioxide, the second and third, and we arrive at 

 succinate which has no more than 4 atoms of carbon. It loses 

 hydrogen, fixes a molecule of water, loses hydrogen again and 

 brings us back to oxaloacetate. The cycle can begin again and, 

 by a single turn of it, the 3 atoms of carbon brought by a 

 new molecule of pyruvic acid will be oxidized and trans- 

 formed into carbon dioxide. From these oxidations comes the 

 energy which life needs for its functions. 



Respiration could be represented as a thermal power 

 station in which the combustion of glucose by its progressive 

 transformations into pyruvic acid and carbon dioxide turns a 

 dynamo — the citric acid cycle — generating a current of energy 



