Preface vii PHYSIOLOGY OF BACTERIA isms is more fit for this purpose than the lowest fungi which Hve on dissolved food. That some of the human physiologists do not agree entirely with this viewpoint, may be seen from the following statement in Bayliss' 'Principles of General Physiology" (1924): ''Without denying the great value of the comparative method in ehminating merely incidental phenomena, it must be pointed out that this very simplicity is, in the majority of cases, a disadvantage. The same organ, or even cell, fulfils a variety of purposes which, in the higher organisms, are relegated to distinct groups of cells. Moreover, the size of the organism is of much importance. The science of nutrition would be almost impossible without the larger, warm-blooded animals. The advantage of the increased rate of reactions, owing to the higher temperature, is not to be undervalued. we have hardly any means to differentiate them chemically (see also Introduction to^^Growth'' p. 162). Besides energy formation and growth, all living organisms share one more property: they all must die. Starvation, poison, excessive heat will kill all living cells, and we know that organisms will die of old age in a natural way. Though there are great variations in to 0.6% of the protein nitrogen. 9 10 PHYSIOLOGY OF BACTERIA Normally chlorophyl plants do not excrete protein cleavage products. There are many observations indicating that green plants in the dark not only break down their carbohydrates, but also their proteins. Yet, their synthetic action seems to be so powerful that all 26 PHYSIOLOGY OF BACTERIA while those with a large energy yield are endo-enzymes. Lactase with twenty-three calories is sometimes soluble; sometimes, it is an endo-enzyme. Miquel's claim that urease is a soluble enzyme will be discussed on p. 36. Attention should be called to the fact that the entire amount of liberated energy does not always become available for growth, for thermodynamic reasons. These will be discussed in the chapter on Growth. Ordinarily, in organic fermentations, the difference between available and total liberated energy is small; furthermore, only a small fraction of the available energy is used for growth, even under ideal conditions. (c) METHODS OF MEASURING ENERGY YIELDS We have various means of measuring the amount of energy liberated in a fermentation. It is possible to compute it as the difference of the heats of combustion of the fermented material and of the products. This computation is not very accurate since the error in combustion heat determinations becomes greatly increased if we have to consider the difference of two combustion heats of nearly the same magnitude. The inaccuracy is increased by the necessary corrections for the heats of solutions of products and for gases escaping. A good compilation of combustion heats is given by Kharasch (1929). This method may be illustrated by the following example: * Another peculiar type of oxygen-free fermentation is the spUtting of fatty acids into CH4 and CO2 (Thayer, 1931). Bio...