THE CHEMISTRY OF PHO lOSYNTHESIS 157 



substance has a far higher concentration of labeled carbon than the former. 

 Thus, A2 should have its place in the sequence before Ai and not after A^, even 

 though A2 has a lower cpm value. 



For analysis, the labeled products are extracted from the whole cell, though 

 the reactions involved occur in certain areas of the cell (grana, mitochondria). 

 Only the specific activity at the site of photosynthesis is of importance. The 

 shorter the exposures to light, the less the likelihood that the decisive equi- 

 libria have been reached throughout the whole cell. The extracted com- 

 pounds therefore do not give a true picture of what happens at the site of 

 photosynthesis. 



As will be discussed in § 61 , Warburg in similar experiments found glutamic 

 acid to be one of the primary products in photosynthesis. Although Calvin 

 observed that Chlorella extracts are very rich in glutamic acid, he did not 

 find any noteworthy incorporation of labeled carbon in this amino-acid. It 

 could be concluded from this finding that the tricarboxylic acid cycle does not 

 participate in photosynthesis, as one of its intermediate products, a-ketoglu- 

 taric acid, is in a state of equilibrium with glutamic acid. It is highly prob- 

 able that the tricarboxylic acid cycle is not involved in photosynthesis, but 

 the exclusion of glutamic acid is certainly not justified. We shall see later 

 that glutamic acid, which is part of the chlorophyllproteid, is of the greatest 

 importance in photosynthesis (see § 64). 



There is some morphological evidence that chlorophyll occurs in the chloro- 

 plast in units each containing several hundreds of molecules. In some hypoth- 

 eses a physiological significance has been attributed to such units. Accord- 

 ing to Calvin and others, the chlorophyll aggregate having semiconductive 

 properties would be composed of about 1000 molecules. Arnold and Meek 

 (1) believe that one quantum absorbed deals with several hundreds of chloro- 

 phyll molecules. For the time being, the assumption of chlorophyll units 

 with physiological functions is still speculation, though such units may per- 

 haps exist from a morphological point of view. Emerson and Arnold (23) 

 determined with very short-lasting light flashes (10~-^ sec) of high intensities 

 and long dark intervals the maximal O2 production during one light flash. 

 They compared the amount of Oo observed with the chlorophyll content of 

 the cells, and calculated that 2500 chlorophyll molecules are involved in the 

 production of one molecule Oo. Warburg et al. (57) directly measured the 

 light reaction without intermittent light and found, per one molecule chloro- 

 phyll, the production of one molecule O2 (see § 39). Thus, a comparison of 

 the ratio chlorophyll/02 with intermittent and with continuous illumination 

 reveals a discrepancy of three powers of 10. Burk (16, 17) pointed out that 

 Emerson's calculations are based upon the assumption that enough light is 

 absorbed per flash to permit complete photosynthesis. In reality, however, it 

 is impossible to imagine light saturation during such short illumination times. 

 The light intensity of the flashes is some 1000 times too low, i.e., absolutely 

 insufficient to decompose the photolyte in 10~^ sec. Burk regrets with good 



