Gray • Our Bridge from the Sun 



tcrial for the nourishment of the plants 

 themselves. 



No one had suspected that water 

 might play an essential part. But in 

 1804 the importance of this ingredi- 

 ent was recognized bv another Genevan 

 experimenter, Nicholas Theodore de 

 Saussure, and the picture changed to 

 one in which the light acted on both 

 the carbon dioxide and the water. 



Four decades later came a momen- 

 tous observation from Germany. There 

 the physicist, Robert von Mayer, 

 pointed out that the crux of the whole 

 photos}"nthetic process was the con- 

 version of light energy into chemical 

 energ}'. The green plant, illuminated 

 by sunlight, not only made organic 

 matter but it packed this matter with 

 chemical energ\'. 



With this discover)', the general 

 outline of photosynthesis was complete. 

 All the essentials of the process had 

 been identified, including the su- 

 premelv important energy factor, and 

 the products of the process— oxygen, 

 organic matter, and "the chemical dif- 

 ference"— had been recognized. But it 

 was only a rough exterior picture. 



For more than a century experi- 

 menters have been seeking to fill in 

 the details, and some of the giants of 

 biochemistn', including four Nobel 

 laureates, have worked on the problem. 

 There are still gaps in the picture, for 

 life does not easilv vield up the secrets 

 acquired in its billion years of evolu- 

 tion. But some of the features of pho- 

 tosvnthesis have been unveiled, hidden 

 sequences have been worked out, and 

 the picture of a cycle— or a series of 

 cycles— of chemical interactions is 

 slowly emerging. 



WHAT HAPPENS— AND WHERE? 



Wliat happens in photosynthesis 

 may be presented as an interchange in 

 which six molecules of carbon dioxide 

 combine with six molecules of water 



m 



in the presence of light and produce 

 sugar and six molecules of free oxygen: 



Light 

 Six CO. + six II..()^One CuH^.Oo and six O. 

 Chlorophyll 



Tliis is a perfecth' balanced chem- 

 ical equation. There are just as many 

 atoms on one side as on the other; but 

 those on the right are in different molec- 

 ular arrangements, and the kev ques- 

 tion is: How did they get that way? 

 Does photosvnthesis split the carbon 

 off from the carbon dioxide and com- 

 bine it with the water? Or does it split 

 hvdrogen off from the water and com- 

 bine it with the atoms of carbon di- 

 oxide? 



Both of these schemes have been 

 proposed from time to time and each 

 has had its advocates over the years, 

 although until the 1930's the argu- 

 ments were pure speculation. But early 

 in that decade a microbiologist at 

 Stanford University obtained experi- 

 mental data on the subject. This was 

 Keis B. van Niel, of Stanford's Marine 

 Station at Pacific Grove. While study- 

 ing purple and green bacteria, which 

 also have the power to trap sunlight 

 and make sugar. Dr. van Nicl turned 

 up evidence that photosvnthesis was 

 basically a process of transferring hydro- 

 gen atoms to carbon dioxide. 



Artificial light can power photos^Ti- 

 thesis, and earlv experimenters found 

 that it was possible to increase the rate 

 of sugar production by increasing the 

 intensity of the illumination. But F. F. 

 Blackman, a British botanist, obser\'ed 

 that eventually a saturation point was 

 reached, after which no intensification 

 of the light made any difference in the 

 production of sugar or output of oxy- 

 gen. From this, Blackman suspected 

 that photosynthesis was not a single 

 process activated by light, but that it 

 included a stage which did not require 

 light. 



Tliere have been various specula- 



