Gray • Our Bridge from the Sutt 109 



that the chloroplasts are made up of ox}-gen, and therefore demonstrated 

 still smaller bodies called grana, and it the light phase of photosynthesis; but 

 is these that contain the chlorophyll. A it did not carry the hydrogen atoms 

 t^'pical square inch of leaf surface has over to the carbon dioxide and there 

 about a billion cells; there are, on the was no production of sugar. It blazed 

 average, from 20 to 100 chloroplasts in a trail, however, which other investi- 

 each cell, about 100 grana to each gators followed, and one group of them 

 chloroplast, and it is estimated that was able to announce to the American 

 each grana holds about 10,000,000 Association for the Advancement of 

 chlorophyll molecules. One can extract Science in December 1954 that chloro- 

 the chlorophyll as a pure chemical, and plasts in a test tube had carried out 

 chemists have found it possible to pro- complete photosynthesis, and had pro- 

 mote various photochemical reactions duced sugar with no assistance from 

 by exposing solutions of pure chloro- outside except the energy brought by 

 phyll to light— but none has ever been the rays of light, 

 able either to split water or to reduce The leader of this group is Daniel 

 carbon dioxide by this means. I. Arnon, professor of plant physiology 



Early experimenters broke up cells at the University of California. For a 

 to obtain chloroplasts, even carried the number of years Dr. Arnon and asso- 

 separation down to the grana, and from ciates have been experimenting with 

 time to time they obtained small bursts isolated chloroplasts under light. Their 

 of oxygen when these fragments were first results were to repeat the Hill re- 

 illuminated. Such bursts were attrib- action and obtain the release of oxygen, 

 uted to the decomposition of residual Later they were able to demonstrate 

 oxygen compounds left on the leaves, that as the chloroplasts split the water 

 But in 1937 it occurred to an English they converted the light energy into 

 biochemist that if photosvnthesis were chemical energy and stored it in the 

 possible outside the cell, it might be form of high-energy phosphorus com- 

 assisted by providing a substance that pounds (the very same compounds 

 had a special affinity for hydrogen. This which function as energy-holders in 

 was Robin Hill, of Cambridge Univer- human muscle). Finally, only a few 

 sity. Such a substance, he reasoned, by years ago, they obtained unmistakable 

 attracting hydrogen would promote the evidence of sugar and starch which the 

 decomposition of water and thus speed chloroplasts had fashioned and tagged 

 up release of the oxygen. There are cer- with radioactive carbon that had been 

 tain iron compounds which have this fed to them in the test tube, 

 affinity. So, after crushing a batch of The amounts of sugar produced 

 leaves, separating out the chloroplasts, were admittedly small, almost micro- 

 and suspending these green bodies in a scopic, but, says Arnon, "The quantity 

 vessel of water, Professor Hill added a is not the significant thing. The fact 

 feric salt. When the mixture was il- that the process has been transferred 

 luminated, oxygen came off at a steady from the living cell, with it multiplicity 

 rate. of metabolic reactions and other vital 



The Hill experiment is a milestone phenomena, to the simpler and less 

 in the laboratory attack on the secret of complicated environment of the labo- 

 the green bodies, because there, for the ratory vessel, is the important gain. To 

 first time, a photosvnthetic process was the physiologist and biochemist such 

 achieved outside the living cell. It was a transfer is prerequisite to the unravel- 

 only half a loaf that Hill got, however, ing of the detailed mechanism of a 

 His experiment split water and released complex process." 



