Cell Respiration

Abstract

Cell respiration may be defined as the sum total of all oxidative processes in the cell which lead to the complete desmolysis of the metabolites and which involve the uptake of oxygen as well as the evolution of carbon dioxide. The bulk of the substrates of cell respiration is formed by sugars or their decomposition products. Consequently, the respiratory quotient (R Q) of most tissues approaches unity under normal conditions. The burning of deaminized amino acids and of fatty acids tends to lower the R Q somewhat. Processes with a greatly different R Q do not come under the scope of cell respiration proper; but are either anoxybiontic reactions or experimentally isolated partial stages of oxybiosis such as pure dehydrogenations by oxygen (e.g. of succinic acid) or mere additions of oxygen (e.g. to unsaturated fatty acids) or the oxidative deamination of amino acids. All these reactions form no CO₂ and have therefore no R Q. In some instances where a dehydrogenation with a predominant decarboxylation is studied the R Q may approach the value 2. On the other hand, it has already been pointed out (p. 241) that the path of cell respiration is essentially characterized by the transfer of metabolic hydrogen; and that the production of CO₂ is automatically coupled with this main process by the fact that there arise in the course of dehydrogenation certain carboxylic acids which are capable of decarboxylation. The latter may either occur as a pure carboxylase reaction or it may be bound up with a further dehydrogenation (p. 252). The basic feature of cell respiration, then, is an interplay between hydrogen donators and hydrogen acceptors, including oxygen, organized and regulated by a series of oxidation-reduction catalysts.