504 LIGHT AND LIFE 



isolated from several other species of higher plants. It now seems well 

 established, therefore, that the cytoplasmic particles, which in all 

 photosynthetic organisms contain the chlorophyll pigments, also con- 

 tain a closely bound phosphorylation system. 



Soon after the demonstration of photosynthetic phosphorylation in 

 isolated chloroplasts, attempts were made to compare its rate with 

 that of CO2 assimilation by illuminated whole cells. Since, as with 

 most newly discovered cell-free reactions^, the rates of photosynthetic 

 phosphorylation were rather low, there was little inclination at first 

 to accord this process quantitative importance (57, p. 292, 345) as 

 a mechanism for converting light into chemical energy. 



With further improvement in experimental methods we obtained 

 rates of photosynthetic phosphorylation up to 170 times higher (2) 

 than those originally described (11), and even these high rates were 

 exceeded by Jagendorf and Avron (76) . The improved rates of 

 photosynthetic phosphorylation were equal to or greater than, the 

 maximum known rates of carbon assimilation in intact leaves. It ap- 

 peared, therefore, that isolated chloroplasts retain, without substantial 

 loss, the enzymatic apparatus for photosynthetic phosphorylation — 

 a conclusion which is in harmony with evidence that the phosphorylat- 

 ing system is tightly bound in the water-insoluble grana portion of 

 the chloroplasts (see Section 11). 



Unlike the phosphorylation system, the enzymes catalyzing CO2 as- 

 similation are water-soluble (13, 173, 153) and are therefore readily 

 lost in the isolation of chloroplasts. This results in lower rates of 

 COo assimilation by isolated chloroplasts than by the intact parent 

 leaves. The difference between the rate of CO2 assimilation by 

 isolated chloroplasts and that of intact leaves may be made to appear 

 greater, though less relevant, if the comparison is made not between 

 isolated chloroplasts and their parent leaf tissue, but between isolated 

 chloroplasts and maximum rates of CO2 assimilation observed in un- 

 related leaf material and under different experimental conditions. 

 Nevertheless, the now known rates of CO2 assimilation in isolated 

 chloroplasts [10 to 20 per cent of that in parent leaf tissue (136, 96) ] 

 are substantial enough to strengthen the conclusion that photosynthesis 

 by isolated chlorophists mirrors that in the iiUact leaf. This con- 

 clusion is fortified by the identity of the photosynthetic products 

 found in both cases. 



'The most recent instance of this kind is the cell-free synthesis of DNA investi- 

 gated by Romberg. "The first positive results represented the conversion of only 

 a very small fraction of the acid-soluble substrate into an acid-insoluble fraction 

 (50 or so counts out of a million added)" (85). 



