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.

Body Temperature Regulation

The energy expended by “warm-blooded” organisms, such as ourselves, to regulate our body temperature is enormous: temperature regulation is costly. For example, at low environmental temperatures, people might have to expend 1800 kcal/day, or more, solely to generate the heat necessary to maintain a body temperature of 37°C and perhaps 10% more at febrile temperatures. This expenditure of energy often amounts to over 90% of the total energy used in any given day for performing external work. The energy, of course, comes from the food we eat, and as a result, we must eat an equivalent amount of kcal of food each day just to regulate body temperature. On days when our food intake falls below our daily energy expenditure, we rely on our stores of fat for this source of energy, resulting in a loss of weight. One can calculate, approximately, the amount of energy saved by a resting human-sized organism which does not regulate its body temperature, and therefore remains at a constant environmental temperature, say 20°C. This organism would have a metabolic rate (energy expenditure) roughly equivalent to that of the American alligator, or about 60 kcal/day. In other words, regulating can cost about thirty times more than not regulating body temperature f 1800 kcal/day vs. 60 kcal/day). Expending and therefore procuring such large amounts of energy have led to numerous adaptations in birds, mammals, and other “warm-blooded” organisms. These adaptations have increased the efficiency of these organisms to obtain, digest, and utilize large volumes of food. Based on the enormous energy cost of regulating body temperature, it is often speculated that there must be some adaptive value in maintaining body temperature at a high and fairly constant level rather than allowing body temperature to fluctuate with the environmental temperature. The adaptive value of regulating body temperature is thought to be related to the effect of temperature on biochemical reactions.

The physiology of any organism can ultimately be reduced to a series of chemical reactions. Most of these reactions are strongly influenced by temperature. The effect of increasing temperature on the rate of increase in biochemical reactions is often greater than can be explained simply by the thermally induced increase in the average kinetic energy of the reacting molecules. For example, many biochemical reactions increase their reaction rate two to threefold over a 10°C rise in temperature. Based on simple molecular kinetics, a 10°C rise in temperature should have increased the reaction rate only a few percent. In the late 1800s, the Swedish physical chemist Arrhenius proposed that the often logarithmic increase in the reaction rates of biochemical reactions is related to their activation energy. Arrhenius mathematically characterized the effects of temperature on biochemical reactions, pointing out that most biochemical reactions tend to increase logarithmically with increasing temperature to a point of maximization. Above this optimal temperature, the reactions decrease. Examples of this profound effect temperature has on biological systems can be seen in its effect on the growth rates of various organisms.

Organisms which regulate their body temperature maintain a degree of biochemical stability not found in the non-thermoregulators. Not only have their biochemical reactions evolved to function optimally at or near the regulated body temperature, but, perhaps more importantly, these reactions can now occur in comparative independence of the environmental temperature. A normal reduction in the environmental temperature does not slow the metabolic processes of a thermoregulator like a bird or mammal.