FOOD DIGESTION AND RESPIRATION 67 



themselves suffering any permanent change. They reappear un- 

 altered when their work is finished. 



That chemical reactions vary greatly in the rate at which 

 they proceed is familiar (E., p. 189). We have already noted that 

 comparative slowness of reaction is characteristic of carbon com- 

 pounds. In the processes of the living organism, it is of importance 

 that reactions should not go on at a perceptible rate except when 

 required. Hence the value of catalysts to quicken them up at 

 appropriate times. In the cell these catalysts, or enzymes, are 

 produced or brought into an active state as wanted. 



When we say that what we have to do with is an acceleration 

 of chemical changes which proceed of themselves, although but 

 slowly, we must remember that this rate may be so slow as to 

 seem not to take place naturally at all. Since, as we stated above, 

 the enzyme appears at the end unaltered, it is obvious that it does 

 not give up energy to the reacting substances, and therefore that 

 the final result must be the same as it would have been without 

 the presence of the enzyme. There is a certain qualification, how- 

 ever, which must not be passed over, although it does not alter 

 the general principle. Owing to the fact that enzymes, as we 

 shall see, form a separate phase of the system in which they act, 

 and the conditions at boundary surfaces differ from those in 

 homogeneous systems, the final state of equilibrium arrived at is 

 not necessarily the same in both cases. The explanation of the 

 fact is still obscure, and need not detain us at the present stage. 



An important consequence follows from such considerations, 

 and is found experimentally to be the case. An enzyme acts in 

 opposite ways, according to the state in which the reagents are 

 when subjected to its influence. This will be clear if we take 

 a special case. Fats, as we saw, are esters in their chemical nature, 

 and there are enzymes which bring about a splitting of esters in 

 general into their component acids and alcohols. This they do 

 by introducing water. Thus, taking ethyl acetate and putting 

 Et for ethyl (C 2 H 5 ) and A for acetyl (CH 3 COO) we have : 



Et A + H 2 O = EtOH 



Although this reaction only takes place rapidly under the 

 action of a catalyst, it proceeds at a detectable rate if allowed to 

 proceed by itself, but never completely. If we start with ethyl 

 acetate and water in the proportions in which they combine, that 

 is, if we take an equal multiple of the molecular weight of each, 

 we find that after some days we can detect the presence of acetic 

 acid (E., p. 190). After a long time it will be found that no 

 further change is going on, and that we have a state which we 

 call equilibrium. By estimating the amounts of the four com- 



