Endotherms and Biochemical Reactions

Birds, mammals, and representatives from some other groups of organisms are thermoregulators called “endotherms.” Endotherms have the capability of internally generating sufficient amounts of heat to raise their body temperature considerably above the environmental temperature. Endotherms have been freed, to some extent, from the effects of environmental temperature. They can remain active, maintaining optimal conditions for their biochemical reactions, over a wide range oi environmental temperatures. Subtle changes in environmental temperature which would markedly affect the biochemical reactions of nonthermoregulators do not affect endotherms.

There is another group of thermoregulators which, unlike the endotherms, lack the metabolic machinery to generate internally large quantities of heat—the ectotherms. Reptiles, fishes, and representatives from many other groups of animals are ectotherms. An ectotherm relies primarily on behavioral adjustments to maintain a fairly constant body temperature. A lizard such as the desert iguana, for example, regulates its body temperature at 39°C ±10°C by moving into the sunlight when its body temperature falls below 38°C and into the shade when its body temperature rises above 40°C. This form of thermoregulation, which is energetically cheaper than generating the heat internally, nonetheless provides the advantages of biochemical stability found in the endotherms. However, the ectotherm can only regulate its body temperature in environments which have the appropriate thermal profile. At night, on overcast days, or during the winter, the ectotherm “slows down” as its body temperature falls toward the environmental temperature.

The regulation of body temperature, therefore, allows an organism to maintain a thermodynamically stable internal environment in which increases or decreases in the rates of millions of individual biochemical reactions can be changed by the organism (by changing the concentrations of enzymes or substrates) without the need to compensate for changes in environmental temperature.