Gray • Our Bridge from the Sun 107 



electrons, any compound into which there were the tell-tale images marking 

 the isotopic carbon had been built the spots that contained the radiant 

 would be immediately tagged. Calvin's atoms. It was then a simple matter to 

 plan was to stop the photosynthesis at go back to the paper sheet, cut out the 

 different intervals of time, draw off a spots which had showed the radioae- 

 sample at each interval, and see if he tivity, and by chemical analysis de- 

 could spot the earliest stage at which termine what each compound was. 

 the C^-* betrayed its presence. So, after This neat scheme— so simple and 

 photosynthesis with the radioactive yet so imaginative— enabled Calvin to 

 material had been proceeding for ten look, as it were, into the algal cell at 

 minutes, he turned the stopcock at the different intervals of time and see how 

 bottom of the vessel, the suspension of far along its manufacturing processes 

 algae dropped through into a bath of had carried the radiant atoms. He first 

 alcohol which instantaneously halted allowed the photosynthesis to proceed 

 all biological action, and the experi- for ten minutes, and the photograph 

 menter then analyzed the algae and de- that resulted showed more than twenty 

 termined which compounds contained radioactive spots, meaning that the 

 radioactive carbon. carbon had been built into that many 

 To separate the medley of photo- different compounds. The next experi- 

 synthetic products, Calvin employed a ment dumped the algae into the alco- 

 technique known as chromatography, hoi bath after five minutes of photo- 

 In this a sheet of specially prepared synthesis, and again the photograph 

 absorbent paper is used. A drop of the showed many compounds. Then in- 

 mixture to be analyzed is placed on the tervals of one minute, half a minute, 

 paper in one corner of its blotter-like and finally ten seconds were tried- 

 surface, and then the paper is installed and now an interesting result began to 

 in an airtight tank and subjected to cer- turn up regularly. This was the domi- 

 tain treatments. As a result, the various nating presence of a substance which 

 compounds that are contained in the the chemists identified as phospho- 

 drop begin to migrate across the paper; glyceric acid. In all experiments with 

 but because of differences in molee- less than one minute of photosynthesis, 

 ular structure and in physical prop- this compound showed itself as the 

 erties they move at different speeds, principal holder of the radioactive car- 

 The effect is to separate the substances, bon. 



and after a few hours each appears as Now phosphoglycerie acid (PGA) 



an isolated spot on the paper. Often is a three-carbon compound— that is, its 



each such substance has, or can be molecule contains three carbon atoms 



made to have, a distinctive color— and —and because of certain considerations 



thus the compounds may show up as it seemed doubtful that it could be the 



islands of blue, pink, yellow, or other first step in the synthesis of sugar, 



tints. There must be some precursor to which 



It was not color that Calvin was the bubbling CO2 gas joined itself and 



looking for, but the content of radio- by which the tagged atom was passed 



active carbon. So he laid the paper on to form PGA. The search for this 



sheet on top of a sensitive photographic forerunner has led Calvin and his team 



film in the dark, and allowed the radio- over a wide field during the last five 



active spots to photograph themselves, years, but the chase finally ended in 



It was like radium, though millions of 1954 when the bits of circumstantial 



times weaker. After several weeks of evidence were pieced together. All 



exposure, he developed the film, and pointed to a five-carbon sugar known as 



