Approaches to the Analysis of Specific 

 Membrane Transport 



Peter Mitchell 



Chemical Biology Unit, Department of Zoology, 

 University of Edinburgh, Scotland 



Now that we have reached the last session of this Symposium, I notice 

 that the number of delegates has somewhat decreased, and this prompts 

 me to begin with some remarks about a fundamental thermodynamic 

 concept known as "escaping tendency". The escaping tendency of a 

 particle such as a molecule or an electron or a chemical group describes 

 the tendency of the particle to escape from one place and pass to another 

 by the thermodynamic process of diffusion. As a matter of fact, we are not 

 accustomed to thinking of people as ha\ing an escaping tendency in this 

 sense (at least, not a measurable one), for escape in the present context 

 can only occur by thermal movement, and thus the escaping tendency can 

 only be measured when the free energy necessary to move the particle is 

 not very much greater than the thermal vibration energy for each degree 

 of freedom. Nevertheless, every kind of transport process not involving the 

 absorption of radiant energy is primarily caused by diffusion. It may be 

 the diffusion of the molecules of hot gas that propel the piston or turbine 

 blade or air stream of the internal combustion or other heat engines that 

 are at this moment transporting some of our colleagues away from this 

 lecture theatre; it may be the diffusion of the filaments of actin and 

 myosin over one another in our hearts or skeletal muscles; or it may be 

 the diffusion of group donors to a glycosidase or synthetase located in a 

 membrane and the vectorial extrusion of a polysaccharide or other polymer 

 chain from it, as discussed in the session on polysaccharides yesterday 

 afternoon. Except in the case of photosynthesis, wherever there is transport 

 in biological systems it is the result of a spontaneous escape of particles 

 from a higher to a lower free energy state by thermally activated diffusion 

 in space. Of course, this fundamental fact will be very well known to many 

 of those present here, but I feel the necessity to mention it at the outset — 

 to make clearer what I am going to say later — because the concept of what 

 has come to be called "active transport" in biology has sometimes been 

 associated with the idea that the substrate specific transport systems of 

 living organisms can possess a special property that will actually cause 



