88 



PHOTOSYNTHESIS 



been known that some heterotrophic organisms actually 

 require a definite concentration of CO2 for their growth. 

 Also it had been found that certain tissue preparations from 

 animals required the presence of bicarbonate in order to 

 give a high rate of respiration in vitro. The explanation of 

 these facts came as the result of a detailed analysis of the 

 actual substrates concerned in the respiration of animal 



COOH 



COOH 



CH3 



I 

 CO 



I 



COOH 



Pyruvic 

 acid 



CH2 + CO2 CH, 



I -— I " 



C(OH)COOH CH2 + CO, 



I I 



CH2 CO 



I I 



COOH COOH 



Citric a-Ketoglutaric 



acid acid 



COOH 



! 

 CH2 



I 



CO 



I 



COOH 



Oxalacetic 

 acid 



COOH 



I 

 CH2 



I 

 CHOH 



I 

 COOH 



Malic 

 acid 



COOH 



CH 



II 

 CH 



I 

 COOH 



Fumaric 

 acid 



I 



COOH 



I 

 CH2 



CH, 



+ CO, 



COOH 



Succinic 

 acid 



Krebs tricarboxylic acid cycle 

 FIG. 6.2 



tissues. Szent Gyorgyi showed that the four carbon di- 

 carboxylic acids were essential catalysts in the aerobic 

 respiration of animal tissue; Krebs established the presence 

 of the 'tricarboxylic cycle' and was able to complete the 

 analysis of the respiration of carbohydrate to COg and water 

 via the formation of pyruvate. The pyruvate is derived from 

 the carbohydrate as the result of fission of hexose into two 

 three carbon compounds. The oxidation of pyruvate via the 

 Krebs cycle is shown in Fig. 6.2 and it is clearly dependent 

 on the presence of oxalacetic acid. In organisms where the 



