BACTERIAL RESPIRATION 47 



The essential factor in any oxidation then is the removal of electrons ; in any reduction, 

 their addition. Regarded in this light, bacterial respiration covers those processes of 

 electron transfer in the mixture of substrate and bacteria which yield free energy. And 

 just as in simple inorganic reactions the oxidation of one substance implies the reduction 

 of another, so in a study of bacterial metaboUsm we must consider not only the substrate 

 to be oxidized by the bacterium, but the substances, either in the bacterium or in its 

 environment, which must be reduced in order that the biological oxidations may take place. 



Besides the fundamental conception of oxidation as electron transfer, we are con- 

 cerned in metabolic chemistry with the substances to and from which the electrons are 

 transferred. Thus biological oxidations may be expi'essed simply in terms of the transfer 

 of either hydrogen or oxygen ; but in using this interpretation it must be remembered 

 that it is a particularization of the general hypothesis of electron transfer. The addition 

 of oxygen to a molecule, or the removal of hydrogen from it, entails a decrease in electrons ; 

 both are oxidations. Similarly the removal of oxygen from, and the addition of hydrogen 

 to, a molecule are equally reductions. In the transfer of either oxygen or hydrogen, 

 at least two molecules are concerned. One, to supply the oxygen or hydrogen, is called 

 the donator ; the other, to which they go, is called the acceptor. 



In donating oxygen a molecule is reduced, in accepting oxygen it is oxidized. In 

 donating hydrogen a molecule is oxidized, in accepting hydrogen it is reduced. In both 

 cases the reaction is catalysed, and the catalyst is regarded as " activating " either oxygen 

 or hydrogen. 



The theory advanced by Wieland (1913, 1921, 1922) regards hydrogen activation, 

 and consequent hydrogen transport, as the essential mechanism of cellular oxidations. 

 The removal of hydrogen from a molecule may be preceded by the addition of a molecule 

 of water, in which case oxygen is in fact added ; or there may be no preliminary addition 

 of water, in which case the molecule is oxidized by the simple loss of hydrogen. In both 

 cases a suitable hydrogen acceptor must be provided. In general terms these reactions 

 may be expressed as follows : 



(a) X + HOH + A = XHOH + A = XO + AHg. 



(b) XH + A = X + AH. 



In (a) X rei^resents the substrate to be oxidized and A the hydrogen acceptor. Water 

 is first added to X and the hydrogen of the compound XH-OH is then activated and 

 passed on to A, leaving X oxidized. In (b) the compound XH is the substrate to be 

 oxidized. Oxidation occurs by the activation of the hydrogen and its transference to A. 



An example of the first type of reaction is afforded by the oxidation of an aldehyde 

 to an acid with previous hydi'ation, 



CH3CHO + H2O -> CHa-C^OH — > CH3COOH + 2H. 

 \0H 



An example of the second type of reaction is afforded by the oxidation of an alcohol 

 to an aldehyde by removal of hydrogen. 



CH3CH2OH — > CH3CHO + 2H. 



The hydrogen in such reactions is seldom liberated in the gaseous state. In almost 

 all the reactions with which we are here concerned, a hydrogen acceptor must be provided. 

 The enzyme that activates the hydrogen in the substrate to be oxidized, and so brings 

 about its transport, is known as a dehydrogenase. 



The Mechanism of Hydrogen Transport. — The phenomena of hydrogen trans- 

 port in bacteria may be demonstrated by a technique developed by Quastel and 

 his colleagues, which consists in observing the behaviour of washed bacterial 

 cells suspended in an appropriate buffer solution containing the necessary inorganic 

 ions, when incubated in an evacuated Thunberg tube in the presence of a substrate 

 and an indicator dye such as methylene blue. The reactions are usually completed 

 within 30 minutes or less, so that although a small degree of bacterial multiplication 



