PHYSIOLOGICAL 387 



will take up hydrogen, or do both. Its presence will strengthen the 

 oxidation processes in its neighbourhood, whether one side of the 

 tug-of-war is weakened, or the other strengthened, or both. A 

 reducing agent will have the opposite effect. It may further be 

 noted that each atom of oxygen is as strong as two atoms of 

 hydrogen, so that the neutral point is water, H^O, which is neither 

 an oxidising agent nor a reducer. This neutrality of water is not to 

 be confused with its neutrality in another sense, that it is made up 

 of the ions H and OH, thus balancing between acid character on 

 the one hand and alkaline on the other. 



Lavoisier's great contribution was the idea that living implies 

 oxidation, or, in other words, combustion; but whenever we think 

 of the cell as like an internal-combustion engine, a difficulty arises 

 in regard to the temperature. The burning of an organic compound 

 in the air evolves great heat, which is inconceivable for a living cell. 

 And if we picture the less complete oxidations that are often 

 brought about in the laboratory, we see that these require powerful 

 agents, such as nitric acid or chromic acid, which would irreparably 

 damage the living cell, even in small quantities. Or again, if we 

 think of very quiet oxidations, such as rusting, where the iron 

 combines with the molecular oxygen in the air, these take place 

 very slowly, whereas the oxidations in the cell are very rapid. We 

 see then that the idea of the combustion of the living body is far 

 from being easy; the facing of the difficulty is leading, however, 

 to a deeper understanding of the chemistry of the cell. 



The first point to be noted is that oxygen may momentarily exist 

 in what is called an active state, as free atoms, especially at the 

 instant at which it is liberated from some compound; and this 

 active oxygen will readily oxidise substances which resist ordinary 

 molecular oxygen, and will do this at a low temperature. 



The second point is the importance of catalysis, which is the 

 alteration of the speed of a reaction by some "foreign" substance 

 which does not enter into the products that result, and so there- 

 after is found unchanged. Especially important in connection 

 with the cell-combustion problem are those catalysts which speed 

 up such reactions as oxidations which would otherwise proceed 

 very slowly. In ordinary chemistry it is well known that spongy 

 platinum serves as a catalyst in the oxidation of sulphur dioxide 

 to sulphur trioxide, and finely divided nickel serves as a catalyst 

 in the reduction of unsaturated oils by molecular hydrogen. In 

 many cases, as in these two, the catalytic agents have very large 

 surfaces, on which the reaction they further has probably its seat. 

 So what catalysts has a cell ? 



It is characteristic of the cells of plants and animals that they 

 possess, and therefore presumably form, catalysts of a peculiar 

 and important type — the ferments or enzymes. It is impossible to 



