386 RESPIRATORY METABOLISM 



Glucose ^ 2 lactic acid -f 43,000 cal. (AH) 

 2 lactic acid -^ 60^ -^ 6CO,, -[- 6H2O -\- 634,000 cal. (AH) 



The first reaction is referred to as glycolysis, or cleavage. Glycolysis is 

 reversible, and it occurs under both aerobic and anaerobic conditions, but 

 the rate of the reverse reaction (lactic acid -^ glucose) is very much less 

 under anaerobic than under aerobic conditions. Consequently, in some 

 tissues (or in the tissue medium) lactic acid may accumulate aerobically, 

 but usually it accumulates only during anaerobiosis. If lactic acid does 

 tend to accumulate, it can be measured by allovv'ing it to displace CO2 

 from a bicarbonate immersion medium (usually glucose-bicarbonate- 

 Ringer). If it is assumed that the CO2 given oi? by oxidation is equal 

 to the O, consumed, then the amount of lactic acid can be calculated as 

 the "excess CO2," i.e., the COo evolved in addition to that released by 

 oxidation. This is expressed as Qo^ for the unit one cmm. of CO, per 

 mg. dry weight of tissue per hour. (Older authors use Q%, which is 

 easily confused with Qco,, the respiratory COo, and recent German au- 

 thors use Qo^ for the same quantity. ) If O2 is replaced by N2, all of the 

 COo evolved must come from glycolysis, and the unit is expressed as 

 Q^= (or Q^^ or Q^^ ). For comparative studies on various organisms, 

 it has been found to be useful to calculate the Meyerhof quotient (M.Q. ) , 

 which is defined as 



Qo. 



This is an index of the amount of lactic acid reconverted to glucose per 

 unit of oxygen consumption, i.e., a measure of the resynthesis of glucose. 

 Recent work on the interpretation of Meyerhof quotients is reviewed by 

 Burk (1937). Recent investigations of glycolysis in vertebrate tissue 

 indicate that glucose is converted into pyruvic acid, part of which is 

 oxidized and part of which is resynthesized into glucose, and that lactic 

 acid is a step in the resynthesis, rather than the end product of glycolysis. 

 It seems as if this may also be true for Protozoa. 



Occurrence of Anaerobiosis and Glycolysis 



Many Protozoa live in media which are almost if not entirely devoid of 

 oxygen. Examples are those which inhabit the bottom of stagnant ponds 



