CHAPTER 



10 



Measurements of Gas Exchange 



Frequently it is desirable to follow the progress of some biological reac- 

 tion in which gases are produced or used. The best-known of these 

 reactions, of course, are respiration and photosynthesis. Carbon dioxide 

 and oxygen are exchanged in both of these processes. 



Respiration occurs in all living cells and converts chemical energy to 

 a form usable by the cell. Some form of food, commonly the carbo- 

 hydrate glucose, is degraded to carbon dioxide which contains less poten- 

 tial energy than the carbohydrate. In a sense, the production of carbon 

 dioxide is incidental to the more important energy release. When oxygen 

 is available, most cells use oxygen and oxidize the sugar completely to 

 carbon dioxide and water. We might use the imaginary but useful ex- 

 pression {CH2O} to represent one carbon-atom's-worth of carbohydrate. 

 Then a summary equation for respiration is as follows: 



(CH2O} + O2 > CO2 + H2O 



If we wish to follow the progress of this reaction we could measure 

 any of the materials in the equation, at least in theory. However, measur- 

 ing the disappearance of carbohydrate is difficult, largely because the 

 common techniques interfere with the respiratory reaction itself. Meas- 

 uring the production of water is also unsatisfactory because the cell 

 contains a very large amount of water. The water produced in respira- 

 tion represents a small change in a large amount of water already present. 

 Generally, the most satisfactory method is to measure the carbon dioxide 

 or oxygen or both of these gases. 



The same general principles apply to photosynthesis. Green cells, 



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