Another hypothesis is that the tension of oxygen at the seat of oxidation affects the rate. Warburg has shown that cell oxidation is associated with structure and that no enzyme solution has been obtained that will account for the vital oxidation of foodstuffs. If the structure or surface responsible for the oxidations is designated as the catalyst, it seems possible that the tension of oxygen in the immediate vicinity of the catalyst influences the rate. This influence of tension on rate might still be a diffusion phenomenon, since the must diffuse toward the structure-catalyst or oxidase molecule.
Owing to the excellent review of the literature on the subject of this paper by Krogh, it seems unnecessary to multiply references. Roughly speaking, and within physiological limits, animal oxidation is about doubled with 10° rise in temperature, and this is shown here to be true of Cassiopea. In other words, oxidation is an exponential function of the temperature as expressed in the following equation: where V, is the velocity of oxidation at t° and V at 0°. This relation holds approximately true for a number of chemical reactions and is supposed to be due partly to change in diffusion-rate and partly to loosening of bonds in the reacting molecules and to ionization. Diffu sion depends on viscosity and tension (osmotic pressure). A fall of temperature from 30° to 20° increases the viscosity of water about 25 per cent, and of a 40 per cent sucrose solution 42 per cent, and of a 3 per cent gelatine solution 1,000 per cent (von Schroeder). The increase in viscosity of gelatine, however,does not cause a proportionate decrease in diffusion. We may assume that this fall of temperature may possibly cause about 30 per cent decrease in diffusion of 02 within the cell, due to viscosity alone. Tension is proportional to the absolute temperature. Although diffusion accounts for only about a third of the change in rate of oxidation with temperature, it is undoubtedly a factor. Since diffusion affects oxidation and concentration-gradient affects diffusion, it seems very probable that concentration of 02 should affect oxidation, even though every cell received some oxygen.
Krogh interprets the experiments on warm-blooded animals as show ing increased oxidation with increased oxygen tension, and reviews the work of Thunberg, showing the same effect to a greater degree on cold blooded animals. Henze (1910a) found that the oxidation-rate of sea anemones and annelids varies with 02 concentration (and of nudibranchs at low concentrations) and supposed that only a varying fraction of the cells received oxygen; his tables show that the oxidation-rate of crustacea and jelly-fish fell rapidly with time, but that in one series of determinations on Pelagia, the oxidation-rate varied reversibly with 02 concentration. In order to reduce the diffusion effect, he kept
sea-urchin eggs agitated in sea-water and found that the oxidation-rate apparently increased about 8 per cent on doubling the 02 concentra tion. Henze attributes this apparent difference in respiration to faulty technique, but since the experiments have not been repeated with improved technique, we may assume that a real difference exists.
It is interesting to compare the metabolism of Cassiopea with that of jelly-fish studied by Vernon. Since only the living cells metabolize, it would be an advantage to know the proportion of cellular tissue to the body-weight, but there is no data on this subject. The skeletal structure, mesoglcea, contains less organic matter than the cells, and hence the percentage of organic matter in the body is a partial indica tion of the cellular mass. The proportion of mesoglea increases with the size of the individual within the same species, but there can be no strict comparison between different species in this regard, and therefore the comparison is very crude. The cubic centimeters of per hour per kilogram of body-weight and per kilogram of organic matter ( = dry weight—weight of salts in equal volume of sea-water) at 20° is given in table 23. The agreement is about as close as could be expected.
Vernon has compared the metabolism (per unit weight of organic matter) of jelly-fish, molluscs, tunicates, and vertebrates, and shown it to be remarkably constant. Krogh (omitting jelly-fish, but including eggs and insects) obtained the greatest differences when the total body-weights were used, but the differences probably do not exceed the differences in water-content and in muscular activity. He found the metabolism of a young dog with body-temperature lowered to 20° during the experiment to be greater than that of cold-blooded animals at the same temperature; but if we calculate the metabolism of the average dog for 20°, using a reasonably high temperature coefficient, the agreement is more satisfactory. We should not expect close agree ment unless water and mineral salts and fibrous tissue are excluded from the weight and the activity of the nervous system is abolished. The chief factor in lowering the metabolism of hibernating mammals is probably the fall in body-temperature (the body-temperature may be as low as 6°).