PHYSIOLOGICAL 397 



reactions are influenced by catalytic agents, in some cases by special 

 enzymes; but the action of a catalyst is to hasten only. It does not 

 matter whether we start with pure ester or with acid and alcohol, 

 in the presence of a catalyst the equilibrium position will be reached 

 more rapidly than before, but it will not be passed. So that under 

 different conditions the same enzyme may aid either the formation 

 or the breaking down of a compound molecule. This fact is generally 

 true of enzymes: that their action is reversible. In reactions of this 

 kind, the process of attaining the position of equilibrium, from 

 either side, is a process which yields energy. Enzymes have no 

 power of supplying energy to drive a reaction in the "wrong" 

 direction; Bayliss aptly compared them to a lubricating oil which 

 enables a weight to slide downhill (converting potential into kinetic 

 energy) more swiftly and more easily, but has no power to help the 

 weight to go uphill. We may think of a reversible reaction as 

 V-shaped, with the position of equilibrium at the bottom. 



The energy-yielding reactions within plant or animal cells, as we 

 have seen, are first and foremost of the nature of oxidations, e.g. 

 the conversion of lactic acid to carbon dioxide and water in the 

 case of muscle. Animal cells generally have no power to reverse 

 these reactions, because they have no form of energy available to 

 "push the weight uphill". But exactly these reactions are reversed 

 in the life of the green cells of plants, which from carbon dioxide 

 and water form sugars, starches, and fats. It has been already noted 

 that cells cannot get rid of their waste nitrogen in the form of the 

 pure gaseous element, but only in the form of simple compounds; 

 in the reverse processes, plants (with the exception of certain 

 bacteria) are unable to start with pure nitrogen, but are able to 

 build up simple nitrogenous products (nitrates and so on) into 

 complex amino-acids and proteins. These reactions can only go on if 

 energy is supplied to them, and the source of this energy is the sun. 



The reactions themselves are by no means clear, and it is even 

 less certain how the green pigment, chlorophyll, plays its certainly 

 essential part. What is certain is that with light and chlorophyll, 

 the plant cell can form sugar from carbon dioxide and water, the 

 former obtained from the air, the latter from the soil. It is generally 

 agreed that the first step is the reduction of the fully oxidised 

 carbon atom of carbon dioxide to formaldehyde ; it is likely that at 

 the same time water is oxidised to hydrogen peroxide. The formalde- 

 hyde may possibly appear in an "active state" in which its molecules 

 readily join together to form molecules of sugar. These reactions 

 can be imitated to some extent in the laboratory if ultra-violet 

 light is substituted for ordinary daylight, and it is conceivable that 

 the chlorophyll is able to convert one kind of light into the other. 

 But this question is discussed in another section along with the 

 chemistry of chlorophyll. 



