586 PETER MITCHELL 



membrane as an active participant in the metabolism of the cell as a whole 

 [2, 4]. To admit, however, that the membrane participates in the intra- 

 cellular metabolic processes is to pose a new question. What, we may well 

 ask, is the function of the enzymes and catalytic carriers that are located in 

 the membrane complex ? Why are these metabolic systems organized in 

 the surface of the protoplast instead of being tucked away safely in the 

 cytoplasm ? It occurred to me some time ago that this question might be 

 answered in the following way. During group transfer or substrate transfer, 

 the group transfer enzyme or catalytic carrier molecules of classical bio- 

 chemistry (in as much as they are anisotropic catalysts) catalyze a micro- 

 scopic vectorial movement or translocation of substrate or chemical group, 

 directed in space relative to the individual enzyme or catalytic carrier 

 molecules. We normally think of metabolism as a scalar substrate and 

 group transfer process (without direction in space) because we think of it 

 as though the enzyme and carrier molecules were orientated at random, 

 so that there would be no macroscopic vector component of the substrate 

 and group translocation processes. But if, as seems likely, the enzyme and 

 catalytic carrier molecules are specifically orientated in an organized 

 membrane structure, the microscopic translocations of the substrates and 

 chemical groups which represent the normal metabolic transfer processes 

 can show as concerted macroscopic transports of substrates and chemical 

 groups (including ions and electrons) across the membrane [2, 36-38]. 

 Thus, we might not need to stretch our imagination very far beyond the 

 bounds of classical biochemistry to conceive how the metabolic systems of 

 the membrane could function as the catalysts and controllers of membrane 

 transport. 



You will see now the reason for my opening remarks about escaping 

 tendency and diffusion. We are accustomed to thinking of the diffusion of 

 molecules and the chemical transformation of molecules in rather different 

 terms, but the processes involved in diffusion and chemical change are, in 

 fact, very similar. The diffusion of a solute particle such as a molecule or 

 molecular complex in a biological system describes the movement of the 

 particle by the thermally activated breaking and making of the secondary 

 bonds that tend to prevent the displacement of the particle relative to the 

 neighbouring atoms. The chemical transformation of a molecule or mole- 

 cular complex describes the movement of one of its constituent chemical 

 groups by the thermally activated breaking and making, not only of 

 secondary bonds, but also the primary bond that tends to prevent the 

 detachment of the group from its partner (or donor group) and its transfer 

 to an acceptor group. The enzymes and catalytic carriers of a membrane 

 complex must catalyze the movement of molecular complexes, molecules, 

 ions, electrons or chemical groups in the natural direction of the diffusion 

 or escaping tendency. It is convenient to call the catalysis of this natural 



