106 PHYSIOLOGY 



tions as to the nature of this "Blackman a lowering of the efficiency of the over- 

 reaction," but nobody doubts that part all process. 'Hiis explains why an experi- 

 of tiie photosynthetic process in fact nienter, using intermittent flashes, can 

 requires no light. Numerous experi- make a plant produce more sugar for a 

 ments have demonstrated this. In 1905 given amount of light than it can turn 

 two other British botanists, II. T. out under continuous illumination. 

 Brown and F. Escombe, rigged up a What goes on in the light is photo- 

 rotating sector (a slotted disk) and chemistr)', i.e., chemical reactions 

 found that three quarters of the light which are energized and promoted by 

 could be shut off in each revolution light; whereas in the dark the reactions 

 without decreasing the rate of photo- are purely chemical, energized and pro- 

 synthesis. In 1919-20 a still more con- moted by molecular forces. The goings- 

 vincing demonstration was given by on in the dark have recently been 

 Otto Warburg, then working at the brought into the open in amazing de- 

 Kaiser Wilhelm Institute for Biology tail, thanks to the ingenuity, enterprise, 

 in Berlin. Instead of using leaves, as and persistence of a group of chemists 

 Brown and Escombe did, Warburg at the University of California, 

 flashed his light on green algae of the 



genus Chlorella, illuminating these chemical detective at work 

 water plants with intermittent rays of 



ver)' high intensity from a rotating The leader of this group is Melvin 



sector that gave periods of light and Calvin, professor of chemistry in the 



dark every four-thousandth of a second, university. The problem was to track 



and he found that the efEciency of down the intermediate steps between 



photos\nthesis doubled. the plant's receipt of a molecule of 



Clearly, there is a sequence of reae- carbon dioxide and its incorporation 



tions involved in the green plant's into a molecule of sugar. To catch the 



manufacture of sugar, some requiring first step, identify the most primitive 



light, others not; and apparently it is intermediate product, and trace the 



possible for the reactions promoted by course of the synthesis piece— that was 



light to produce their parts of the sugar the goal. 



molecule faster than the dark reactions The study was made largely with 



can handle them, thus clogging the as- green algae. In Calvin's project, a sus- 



sembly line and slowing down the over- pension of algae in water was placed in 



all process. a closed glass vessel. The vessel was en- 



Tlie specific structure which per- tered at the top by a tube, and could 

 forms this office has not yet been iden- be emptied quickly through a stopcock 

 tified, but the chemists postulate a at the bottom. Carbon dioxide was bub- 

 catalytic molecule— or team of mole- bled in through the tube, light was di- 

 cules— which has a special affinity for rected upon the vessel from the oppo- 

 hvdrogen and yet will readily release site sides, and soon photosynthesis was 

 the hydrogen to the carbon-dioxide re- proceeding at a steady rate, 

 ducing system when it reaches that part At a given time, the incoming 

 of the mechanism. stream of carbon dioxide was changed. 



It is clear that if the light phase Up to now it had been ordinary' COo, 



splits water faster than the dark phase but at this moment a switch was turned 



can supply the necessary escort, more and suddenly the stream contained a 



hydrogen will be freed than can be used labeled COu in which the carbon atom 



in the synthesis, lire result will be an was the radioactive isotope, C^'*. Since 



imbalance between the two stages and these C^'* atoms continually discharge 



