492 LIGHT AND LIFE 



The ability of isolated whole chloroplasts to carry out the synthesis 

 of starch from CO2 and water, without the aid of external enzyme sys- 

 tems and substrates, provided the final link in the chain of evidence 

 for the conclusion that chloroplasts are autonomous cytoplasmic 

 bodies containing the complete cellular apparatus needed for photo- 

 synthesis (cf. also 157, 73, 147, 66, 148, 65, 136, 137). 



In later experiments, COo fixation was investigated not Avith whole 

 chloroplasts but with disrupted chloroplasts (13, 173, 153, 151, 96, 

 152) . The disruption of chloroplast structure impeded starch syn- 

 thesis, but was very useful in the study of the soluble products of 

 COo assimilation. Under suitable experimental conditions the same 

 soluble compounds were formed in light by chloroplasts isolated from 

 a number of different species; spinach, sugar beet, sunflower, Phy- 

 tolacca americana, and Tetragonia expansa (174, 175) . The soluble 

 products of CO2 assimilation by broken sugar beet chloroplasts are 

 shown in Fig. 1. 



The close similarity of photosynthetic products of intact cells and 

 of chloroplasts isolated from different species left little doubt that 

 chloroplasts are the sites of the complete photosynthetic process in 

 green plants. It seemed legitimate therefore to explore the pattern 

 of energy conversion in photosynthesis, using isolated chloroplasts 

 with the expectation that it wovdd also be relevant to the experi- 

 mentally much more complex situation in whole cells. 



3. The Role of Light in COo Assimilation 



It is an interesting aspect of the history of photosynthesis that the 

 energy conversion process appeared settled in broad outline for well 

 over a hundred years. Few hypotheses in biology seemed as secure 

 with the passage of time, and had a more tenacious hold on the think- 

 ing of biologists for over a century, as the one that the fimction of 

 light in photosynthesis is the splitting of CO^, with a residtant libera- 

 tion of O2 and the retention of "C" for forming plant substance. 

 From the time it was first proj^osed by Ingcnhousz in 1796 that plants 

 absorb from "carbonic acid ... in the sunshine, the carbon, throw- 

 ing out at that time the oxygen alone, and keej^ing the carbon to 

 itself, as nourishment" (72), this hypothesis grew in the 19ih cen- 

 tury to the stature of a fixed principle which served to divide all 

 living organisms into two main groups: green plants that lived by 

 using radiant energy for the assimilation of CO^ and organisms with- 

 out chlorophyll that were incapable of using radiant energy for CO2 



