ENERGY EFFICIENCY IN PHOTOSYNTHESIS 260 



111 all methods of analysis it must be remembered that what is really 

 desired is the carbon dioxide and oxygen concentration within the bio- 

 logical material. These concentrations may be different in the solution 

 in which algal cells are suspended than they are in the gases bubbling 

 through the liquid phase. All methods that analyze the gases call for a 

 complete chemical equilibrium between the gas and the liquid phase, and 

 care must be taken to see that this eciuilibrium is reached rapidly. 



5. MEASUREMENT OF CHEMICAL CHANGE IN PHOTOSYNTHESIS 



BY MANOMETRIC METHODS 



The manometric method depends on the fact that carbon dioxide is 

 more soluble in water than oxygen is. In photosynthesis an oxygen mole- 

 cule is exchanged for a carbon dioxide molecule, and in respiration an 

 oxygen molecule is consumed within the cell and a carbon dioxide mole- 

 cule is liberated. If the volume is kept constant, there will be an increase 

 in pressure when the more soluble carbon dioxide is replaced by the less 

 soluble oxygen. If the solution is alkaline, the carbon dioxide will be 

 completely dissolved in the solution, and the changes in gas pressure will 

 be due to changes in oxygen alone. 



There is an inherent danger in any method of analysis which is not 

 specific for the chemical compound being determined. The manometer 

 records an increase in pressure whenever additional molecules of any kind 

 are introduced into the gas space. The manometer does not distinguish 

 between oxygen and carbon dioxide or even nitrogen or methane, whereas 

 chemical methods and some physical methods, such as infrared absorp- 

 tion, do. Physical chemists during the past have made serious errors in 

 some researches on chemical kinetics by following the rate of the chemical 

 reaction in the gas phase through measurements of the total gas pressure. 

 If one molecule decomposed into two molecules, the pressure doubled, 

 and if the reaction really went as assumed, the measurements were sig- 

 nificant. But sometimes the reaction did not go as assumed — competing 

 reactions used up the reactants in such a way that the pressure did not 

 double when the reaction was complete. Measurements of total pressure 

 in a gas mixture can be relied upon for determining the concentration of 

 a single gaseous substance only when it is proved experimentally that the 

 reaction in a specific case proceeds in the manner assumed in making the 

 calculations. 



Another danger in the manometric method in addition to this lack of 

 specificity is the complication of the time factor. All calculations for 

 the concentration of carbon dioxide and oxygen in the solution based on 

 measurements of gas pressure involve the assumption that an equilibrium 

 exists. But time is required for establishment of this equilibrium, and if 

 measurements are made during the transient period, when equilibrium 



