122 PHOTOSYNTHESIS 



photosynthetk activity of the unicellular green alga ChlorcUa. This 

 method makes use of the principle of the Haldane-Barcroft method of 

 hlood gas analysis and i)ermits rapid and accurate determinations to he 

 made. Ky using solutions of sodium carhonate and sodium bicarbonate 

 the concentration of carbon dioxide in the water could be controlled with 

 great accuracy. The source of light was a metal filament lamp of 1500 

 watts current consumption which at 15 cm. distance exceeded the light 

 intensity of direct sunlight. The concentrations of carbon dioxide in 

 solution ranged from that which is in equilibrium with air containing 34 o 

 to 10 times the normal amount. The temperature was 25°. It is thus 

 quite certain that carbon dioxide was at first the limiting factor. In 

 Figure 9 is given the curve which Warburg obtained under these con- 

 ditions. 



Warburg's curve indicates that at low concentrations of carbon dioxide 

 the rate of photosynthesis is closely proportional to the carbon dioxide- 

 concentration. Above a concentration of about 2 X 10"*' moles per liter 

 progressive increase in the carbon dioxide-concentration results in a con- 

 tinuously smaller increase in the rate of photosynthesis until finally the 

 latter seems to be independent of carbon dioxide-concentration. War- 

 burg interprets the form of the curve on the basis that the rate of photo- 

 synthesis is proportional to the concentration of carbon dioxide and to 

 the concentration of a second substance which reacts with carbon dioxide. 

 Thus if A represents the total amount of this absorbing substance in 

 a cell, x and A — x res]>ectively the amounts which are in the free and 

 combined condition and Ceo., the concentration of carbon dioxide, then 



in any steady state ^^- — would be constant. 



A — X 



Warburg's conception of the photosynthetic process thus involves that 

 of an absorbing substance for carbon dioxide, a conception the necessity 

 of which has been demonstrated in a number of dififerent ways and of 

 which there is further discussion in another part of this chapter. An im- 

 portant assumption in the idea that carbon dioxide first goes into com- 

 bination is that the rate of photosynthesis, even at the lowest carbon 

 dioxide-concentrations is governed by the rate of a chemical reaction and 

 not only by dififusion. It would seem that Warburg's experimental con- 

 ditions have certain advantages over previous ones for determining photo- 

 .synthetic rates, because the use of the unicellular organism reduces the 

 element of diffusion to a minimum. In higher plants possessing an in- 

 ternal atmosphere it is evident that these conditions are more complex. 

 This was already found by Blackman, who showed that the Bryophyte, 

 Fontinalis, has a photosynthetic rate about half that of the Phanerogam, 

 Elodea. The fact that carbon dioxide diffuses more rapidly as a gas than 

 in solution may account for this difference. In the higher plants, which 

 are of a more complex structure, under conditions of low carbon dioxide 

 supply, there would rarely be equilibrium between the carbon dioxide- 



