396 PLANT METABOLISM 



and has obtained essentially the same results as before. Warburg and 

 Burk have consistently observed quantum efficiencies of about 4 with 

 several modified techniques. 



In red light with 43 Cal. per Einstein, a quantum efficiency of 4 repre- 

 sents almost 70 per cent efficiency ^ in storing the energy of the incident 

 light. This is a higher efficiency than that of any known photochemical 

 reaction which can be performed under controlled conditions. Even a 

 quantum efficiency of 8 to 12 is good in terms of known photochemical 

 reactions, and if photosynthesis actually has a quantum efficiency of 4, 

 it is unique. It is impossible at present to make a categorical state- 

 ment of the quantum efficiency of photosynthesis, for the matter remains 

 highly controversial. 



Intermediates and products of photosynthesis 



One of the most interesting problems of photosynthesis is the determina- 

 tion of the organic compounds produced by the sequence of reactions. 

 What substances are formed between the first reduction of carbon dioxide 

 and the final appearance of the finished sugar molecule? There is an 

 extensive literature on this subject, much of which has centered around 

 the formaldehyde hypothesis. In 1870, Baeyer suggested formaldehyde 

 as the first product of photosynthesis, and though there never has been 

 substantial evidence to support it, the hypothesis has been perpetuated 

 in textbooks of biochemistry and botany to the present. 



Current interest centers around phosphoglyceric acid as the first demon- 

 strable product of photosynthesis. Pioneering work from 1939 to 1942 

 with the short-lived radioisotope C^^ resulted in the introduction of a 

 number of new concepts and the discard of certain old ideas on photo- 

 synthetic intermediates. Since 1946, the use of the long-lived (5,568 ± 30 

 year half life) radioactive isotope C^^ has led to real advances in our 

 knowledge of intermediates. Although it remains incomplete, a general 

 picture of the reaction sequence has now emerged. Calvin and co- 

 workers have made particularly notable contributions. By skillful use 

 of a combination of C^'*, paper chromatography,^ and "radioautography" ^ 



1 Calculation : /^^'^^ x 100 = 69.5%. 

 4 X 43 



^ In paper chromatography a small drop of a mixture of compounds in solution is 

 placed near one end of a long strip of filter paper and is dried. The end near the 

 point of application is immersed in a suitable solvent. As this solvent moves by 

 capillary action past the added mixture of compounds and on toward the other end 

 of the paper, it carries with it the individual components of the mixture varying dis- 

 tances and thus achieves their separation. 



^ "Radioautography" is a procedure for locating radioactive substances separated 

 by paper chromatography. After the chromatographic separation is completed, the 

 paper is dried and allowed to "take its own picture" by placing it on a photographic 

 film in the dark. Black areas on the developed film correspond to the location of 

 radioactive substances on the paper strip. 



