44 PHOTOSYNTHESIS 



Fig. 4.1 illustrates the results obtained by Blackman and 

 Smith (191 1) with Elodea and Fontinalis for the relationship 

 between the rate of photosynthesis at a high light intensity 

 and the concentration of carbon dioxide. The results show 

 that the rate is proportional to the concentration of carbon 

 dioxide over a certain range, but that it is independent of 

 concentration at the highest concentrations. Other workers 

 repeated this type of experiment (sometimes using flowing 

 bicarbonate solutions instead of solutions of carbon dioxide) 

 and found that there was a considerable range during which 

 the rate was neither directly proportional to nor independ- 

 ent of the carbon dioxide concentration. An example of such 

 observations are those of Harder (1921), using the water 

 moss Fontinalis (Fig. 4.2). It gradually became clear that a 

 whole variety of types of relationship could be observed and 

 that the observations of Blackman and Smith were appro- 

 priate to only one set of experimental conditions. 



How such a variety of apparent relationships can come to 

 exist may be considered with reference to enzyme kinetics. 

 Michaelis and Menten suggested that catalysis in an enzy- 

 matic reaction was due to the formation of an enzyme-sub- 

 strate complex which decomposes to give the products of 

 reaction. If S is the concentration of substrate, and the total 

 concentration of enzyme is assumed to be small in compari- 

 son to that of substrate, then the relationship between rate 

 and substrate concentration is given by 



where Ra^ is the maximum rate and Kq a constant. Let us 

 suppose that in photosynthesis we have a very simple 

 mechanism in which carbon dioxide combines with some 

 enzyme and the rate of breakdown of this complex is deter- 

 mined by the light intensity. Obviously the rate of photo- 

 synthesis will be a function both of the concentration of 

 carbon dioxide and of the light intensity. It will only be a 

 simple function of either variable if we limit the experimen- 

 tal conditions. Hence the advantage of studying the effect of 

 light intensity when the concentration of carbon dioxide is 



