ACCUMULATION OF ENERGY 91 



CONCENTRATION OF SUBSTRATE 



The effects of concentration of substrate are practically 

 confined to one substance. As there is always an excess of 

 water, we can devote our attention to the concentration of 

 carbon dioxide. In land plants we can study the effect of 

 changes in the volume percentage of carbon dioxide in the 

 surrounding atmosphere, and in water plants changes in the 

 concentration of carbon dioxide in the solution. Even in 

 land plants the carbon dioxide must dissolve in the liquids 

 of the cell, hence we are ultimately reduced to the consideration 

 of a solution of carbon dioxide. 



The concentration of carbon dioxide in solution is pro- 

 portional to the volume (partial pressure) of the carbon 

 dioxide in the atmosphere in equilibrium with the solution, 

 hence the concentration of carbon dioxide in solution is 

 frequently expressed as tensions meaning the pressure in 

 millimetres of mercury percentage by volume of the carbon 

 dioxide in the gaseous mixture in equilibrium with the 

 solution. Thus if the total pressure of gas is 760 millimetres 

 of mercury and 2 per cent, by volume of the gas is carbon 

 dioxide the tension is 15-2 millimetres. 



Thus we have to consider the behaviour of a solution of 

 carbon dioxide. 



Carbon dioxide unites with water to form carbonic acid, 

 and the latter dissociates in two stages, giving rise to hydrogen 

 ions, bicarbonate ions and carbonate ions. We cannot 

 distinguish between the carbon dioxide in solution and the 

 carbonic acid formed from the carbon dioxide, but we do 

 know that the sum of the concentrations of these two is 

 proportional to the tension of carbon dioxide. 



Ewart has shown that both acid and alkali inhibit photo- 

 synthesis ; * thus it is not possible to draw any conclusion as 

 to which of the above dissociation products obtained by 

 dissolving carbon dioxide in water is used as the raw material 

 for photosynthesis. 



We require observations in which the hydrogen ion con- 

 centration is kept constant. If stabilising mixtures are used 

 the hydrogen ion concentration can be kept constant, then 

 the concentrations of bicarbonate ion and of carbonate ion 

 and of carbonic acid will each be proportional to the carbon 

 dioxide tension. By adding bicarbonate to a solution it is 

 possible to keep the carbon dioxide tension constant with a 



* A. J. Ewart, Joiirn. Linn. Soc. t 1896, vol. 31, p. 408. 



