138 MEASUREMENTS OF GAS EXCHANGE 



sure change. Obviously a solution for this complication must be found. 

 Some quite simple remedies are in common use, as well as some that are 

 fairly complex. Perhaps the most obvious answer to the problem is to 

 prevent the change in one of the two gases. If we wish to know the rate 

 of the whole respiratory reaction, we may be satisfied to measure only 

 oxygen. We must assume that respiration is the only cell process that uses 

 or produces any gas, but often this assumption is safe enough. We can 

 place the living material in the main part of the vessel and then, in a 

 separate compartment, place a small amount of alkali solution. The car- 

 bon dioxide produced in respiration diffuses through the gas space to 

 the alkali where it is absorbed. Thus the only gas exchange that will 

 aflFect the pressure is the change in oxygen. In many cases the amount 

 of carbon dioxide produced is about the same as the amount of oxygen 

 used. 



In photosynthesis another method is necessary because carbon dioxide 

 is needed in the reaction. Ordinarily this measurement is performed by 

 suspending the green cells in a bicarbonate solution. As the cells use 

 carbon dioxide, more of this gas is provided by the bicarbonate. Since 

 the carbon dioxide concentration in the gas phase does not change, we 

 measure only the production of oxygen. This technique is used quite 

 commonly even though the high pH of the bicarbonate solution may 

 influence the activity of the cells. It might seem convenient to absorb 

 the oxygen, just as the carbon dioxide was absorbed in respiration. How- 

 ever, oxygen absorbers are expensive, dangerous or difficult to handle, or 

 react quite slowly. If we wish to avoid the alkaline conditions of the 

 bicarbonate solution, we must use one of the more complicated tricks of 

 the manometric technique. 



Anyone who wants to use the manometric technique extensively 

 should consult the standard reference book by Umbreit, Burris, and 

 Stauffer, Manometric Techniques. It contains complete discussions of 

 many applications of manometry and a number of variations that are 

 possible. The 1957 edition also incorporates information on several other 

 useful laboratory methods. 



This description has been restricted to carbon dioxide and oxygen. 

 However, exchange of other gases may result from some of the metabolic 

 reactions of cells. Some algae, for example, can perform a reaction in 

 which molecular hydrogen is involved. Ammonia, nitrogen, methane, 

 hydrogen sulfide, and perhaps other gases might appear or disappear in 

 certain specialized biological processes. One of the great advantages of 

 the manometric method is that it could be used for anv of these. 



