Temperature: Metabolic Aspects and Perception 343 



t2. However, the velocity of enzymatic reactions is not a linear function of 

 temperature, and the Qio diminishes at higher temperatures. 



A temperature characteristic with a sound theoretical basis is the Arrhenius 

 /M. If velocity constants Ki and K2 (proportional to measured velocities) at 

 absolute temperatures Tj and T2 are known, the fx is obtained from the 

 following relation, where R is the gas constant 1.98 calories: 



M /"l — 1 



Ko=Kie ^ V^^ '"■ 



M=4.6 log (K2-KO /( -1 -^ 



(See discussions by Barnes,^! HoagIand,^^and Sizer.^^'-*) Definite values of ju. 

 characterize specific catalysts irrespective of the substrate.** When the Arrhen- 

 ius fj. is calculated for a wide variety of biological reactions, certain values 

 appear regularly, and there is reason to associate these with underlying chemi- 

 cal reactions.^ ■''''> The velocity of all reactions in animals varies with the abso- 

 lute temperature. Animals subject to extreme variations, and particularly to 

 very low body temperatures, are limited metabolically. A frog swimming in ice 

 water cannot possibly go faster than energy is made available in its muscles. 



LETHAL EFFECTS OF COOLING 



As the temperature of living cells goes down, many life processes become so 

 slow as to be ineffective. Three low-temperature ranges may be considered: 

 freezing temperatures, vitrifying temperatures, and lethal temperatures above 

 freezing. Protoplasm as an aqueous solution freezes at a few degrees below zero, 

 and freezing of a cell with resultant ice crystals kills. Resistance to freezing 

 temperature is affected by several factors, the most im^jortant of which is water 

 content and the state of contained water; prior desiccation or large content of 

 bound water lowers the lethal temperature. If cooling is slow many organisms 

 become reorganized largely by dehydration and sometimes by acquiring a tough 

 insulating coat. Some Protozoa encyst, and sponges form gemmules. Prolonged 

 subcooling (cooling below freezing point without freezing) also favors survival 

 in the freezing range. Capillarity and existence of body fluids in small spaces 

 favors subcooling and in many insects the free body fluids diminish in win- 

 j-gj|. 135 Whether or not an animal freezes at a given low temperature depends 

 on the rate at which it is cooled and the length of time it is kept at the low 

 temperature. The grain weevil Sitophrilus granarins dies in 875 hours at 

 7.2°, in 100 hours at —6.6°, and in 2.5 hours at —\7.7°M'^ (See also the 

 discussion of supercooling in Wigglesworth.^^-) Insects collected in winter 

 freeze at much lower temperatures than do summer specimens. The winter 

 animals are cold-hardened. Blood of wood-boring larvae freezes at a higher 

 temperature than do some other tissues ( — 22°: freezing point of fat of cold- 

 hardened oak borers), and death is associated with freezing of other tissues 

 rather than of the blood. ^"^ Certain insects enter a state of dormancy, a reor- 

 ganized state in which the protoplasm does not freeze at temperatures several 

 degrees below zero (Table 59). Aquatic insects are not subject to temperatures 

 below zero. The water content of terrestrial insects decreases while cold- 



