TEMPERATURE COEFFICIENTS IN BIOLOGY

Abstract

Summary.(a) The problem of temperature coefficients in biology was initiated by chemists and has suffered from the beginning from this circumstance. Attempts to apply chemical temperature‐velocity formulae (the Q10 rule and the Van't Hoff‐Arrhenius law) to biological processes failed, because none of the temperature constants used in chemistry (Q10, μ) can be said to hold good in biological reactions. It is shown that the value of μ in the Van't Hoff‐Arrhenius law varies, in biological processes and in simple enzyme reactions, with the temperature to about the same extent and for the same reasons as tlie value of Q10, these constants being closely related for the zone of biokinetic temperatures. The use of the Van't Hoff‐Arrhenius formula in biology, sponsored of late mainly by Crozier, is therefore of no greater advantage than the use of the simpler Q10 formula, both of them presenting the same low degree of accuracy in biology. A new empirical temperature formula, proposed by the writer, is shown to agree in many instances with sufficient accuracy with obscrved data. The constant of this formula (b) remains independent of temperature even in those cases in which the constants Q10 and μ show a steady decrease in value as the temperature increases. When b= 1, the formula becomes identical with the linear relation of Krogh and with the rule of thermal summation, which are thus special instances of a more general interrelation between tcmperature and the velocity of biological phenomena.None of the temperature formulae proposed up to the present in biology can be said to hold good in every case, nor is there a rational temperature law in biology. Even the constants Q10, and μ are nothing more than empirical and descriptive quantities, which might be used with this restriction so far as they remain relatively constant for the temperatures concerned.(b) The reason for which the velocity of biological phenomena is influenced by temperature in a way widely different from that commonly found in chemical reactions seems to lie in tlie heterogeneous nature of living matter and in the relatively high viscosity of the reacting protoplasmic phases. Even in enzyme reactions the formulae cited above proved unsatisfactory and one cannot expect that formulae which are not accurate for enzyme reactions should yield better results in biological processes.Some earlier attempts at a classification of biological phenomena and the undcrlying “master processes” by the aid of the temperature coefficient Q10, as well as a more recent attempt by means of the temperature characteristics μ (Crozier), has necessarily failed for a double reason. First, the formulae used up to now in biology are so inaccurate that they do not permit of any serious analysis. “Breaks” in biological temperature curves at definite “critical” temperatures (Crozier) are artefacts, due to improper use of the chemical temperature formulae in biology, and they do not exist in reality. Second, it is highly probable that diffusion is always or almost always the master process in biological reactions and that thus the viscosity of reacting protoplasmic phases is always or almost always the limiting factor which determines the velocity of biological processes. This thesis is supported by some experimental data from the physico‐chemical literature. If this is so, then the temperature coefficients of biological phenomena are merely temperature coefficients of the viscosity of reacting protoplasmic phases.(c) If this hypothesis is correct, it follows that temperature coefficients of analogous biological processes are indicators of protoplasmic viscosity. This is made probable by an analysis of J. Loeb's data on the viscosity of gelatin solutions, which shows that the temperature coefficient of viscosity regularly increases with concentration.The hypothesis can be tested on abundant numerical material, furnished by numerous investigators. It is shown that the temperature coefficients increase when a rise in protoplasmic viscosity may be assumed to take place. This is illustrated by the following facts: (1) the temperature coefficients of the heart‐beat frequency in various animals, of the developmental processes in vertebrates and invertebrates, and of the metabolic rate of animals and plants vary systematically with the age of the organisms; (2) insect development in dry air gives a higher temperature coefficient than when it takes place in a humid atmosphere; (3) the temperature coefficient of successive periods of the mitotic process changes in accordance with the variations of protoplasmic viscosity which have been shown to take place during mitosis.