MEASUREMENTS OF GAS EXCHANGE 131 



when exposed to light, perform this reaction which is the reverse of 

 respiration as far as net results are concerned, 



CO2 + HoO — > {CH2O) + O2 



In photosynthesis the very same gases are exchanged and either of these, 

 or both, can be measured to trace the progress of the reaction. 



Respiration and photosynthesis as described here represent only sum- 

 maries of complicated sequences of single reactions. Most of the individ- 

 ual reactions are enzyme-controlled. Many of the enzymes can be isolated 

 from the cell and can catalyze the same individual reaction under arti- 

 ficial conditions. If one of these separate reactions involved the produc- 

 tion or use of carbon dioxide or oxygen, there is no reason that we could 

 not use the same method of measurement used for the whole proc- 

 ess. 



Gases like carbon dioxide and oxygen can be measured in a variety of 

 ways. Some methods may depend entirely upon chemical principles, 

 others upon physical principles. As an example of a chemical procedure, 

 the carbon dioxide produced by cells may be analyzed by absorbing it in 

 a solution of alkali to produce a carbonate, 



CO2 + 2 KOH > K2CO3 + H2O 



By titrating with a standard acid we can determine the amount of alkali 

 neutralized, and from this amount we can calculate the amount of carbon 

 dioxide. A slight modification of this method depends on the fact that 

 divalent bases like calcium and barium form insoluble carbonate pre- 

 cipitates. From the weight of the precipitate, the amount of carbon 

 dioxide can be calculated. However, these methods frequently are cum- 

 bersome and may not be easily adaptable to continuous measurements of 

 gas exchange. Sometimes the chemical techniques destroy the gas being 

 measured, and this destruction might be undesirable in certain types of 

 experiments. There also are several chemical methods for analyzing 

 oxygen, but these are even less convenient than the carbon dioxide 

 analyses. 



The manometric technique 



The manometric method of measuring rates of metabolic gas exchange 

 is used in almost every cell physiology laboratory in the world. Instru- 

 ments similar in principle were developed by Barcroft and Haldane 



