PHOTOSYNTHESIS AND RESPIRATION 



1927 



tional complications arise, however, from the possibility of incomplete 

 mixing of the products of photosynthesis and respiration with the pools of 

 respiratory substrates in the plant and of photosynthetic substrates in the 

 gas. 



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Fig. 37D.13. Time curves of CO2 exchange, showing C(12)-C(14) discrimination 

 in the photosjmthesis of barley (Warrington and Calvin 1951). 



From an approximate mathematical analysis of the specific activity 

 V8. time curve during one dark-light-dark cycle, in which complications 

 seemed to be least disturbing, Weigl and Calvin concluded that C*(14) 

 was taken up in light at a rate 17% slower than C(12) — a remarkably 

 strong isotopic discrimination — and that the rate of respiration in light was 

 only one half of that in light. Weigl, Warrington and Calvin also inferred 

 that the products of photosynthesis were not drawn into respiration as long 

 as the illumination was on, but began to be autoxidized immediately after 

 the cessation of illumination, in agreement with the conclusion drawn by 

 Calvin ei al. from C(14) distribution in the products of photosynthesis, 

 cf. chapter 36, pages 1645 and 1666. As mentioned on page 1666, Steward 

 and Thompson (1950) proposed another explanation of the C(14) findings. 

 They suggested that, in light, the Krebs cycle is maintained (in green cells) 

 at the cost of glutamic acid formed from proteins, rather than of pyruvic 

 acid formed from sugars. The C(14) fixed by photosynthesis must then 

 pass through the cellular pool of amino acids before it can be drawn into 

 respiration. 



A similar study was made by Van Norman and Brown (1952), using 

 the mass-spectroscopic method of continuous gas analysis described by 

 Brown, Nier and Van Norman (1952). Chlorella suspensions and barley 



