ACTIVE TRANSPORT AND MEMBRANE EXPANSION- CONTRACTION CYCLES 643 



suggested, only a matter of words ; for, when you come to analyse the transport 

 process you can often distinguish between the free-energy changes invoh-ed in 

 making and breaking the intermediate which is going to travel, and the free-energy 

 changes associated with the change of configuration that must happen in between : 

 for example, in the transport of oxygen by the heart and circulatory system. You 

 have two energy considerations. One is the overall free-energy step, up which (or 

 down which) the carried particle must travel — and this simply represents the free- 

 energy difference between the carried species of particles in the phases on either 

 side of the membrane. The other is the activation energy for diffusion (or for 

 "contraction-expansion") of the carrier-carried particle complex — and this will 

 determine the rate of the process. The elastic process can represent, as it were, a 

 local heating, due to a locally catalyzed exothermic reaction, and this, as I said 

 before, might facilitate the transport. But the elastic process — being no more than 

 a diffusion process at the molecular level of dimensions — cannot actually drive the 

 transport in the normal energetic sense. My criticism of Dr. Schoffeniels' model, 

 and my statement that it represents, as it stands, a Maxwell demon is based on the 

 fact that it does not include an adequate description of the vital part which must 

 be played by primary or secondary chemical bonding in any ion selective active 

 transport system: he has substituted demons for bonds. 



Schoffeniels : Just a few words about Dr. Mitchell's comment. I do not believe 

 that the mechanical model I have drawn on the board, or any model presented so 

 far, really tells us what is happening in the cell. As a working hypothesis I prefer 

 the carrier hypothesis, but I want to point out that the mechanical model proposed 

 could work at the cellular level. We have first to postulate that the pore is vibrating 

 by means of some kind of contractile machinery, and this would account for the 

 energy spent by the cell in the process of active transport. Then of course the pore 

 must exhibit specificity towards a certain ionic species. This model could be taken 

 as a working hypothesis for active transport, as well as the carrier hypothesis or 

 any other kind of hypothesis since we really do not know anything about the 

 molecular mechanism underlying active transport. Any model thermodynamically 

 sound is then just as good as any other. We nevertheless must be aware of the fact 

 that it is possible to fit the results obtained experimentally with living cells in a 

 great variety of models. This is especially true with regards to the energy require- 

 ments of a given model since it is always possible to make ad hoc hypothesis as far 

 as efficiency is concerned. 



Davis: I am not altogether convinced that something involving a contractile 

 element may not be a satisfactory model, and one could even support the proposi- 

 tion that at small enough dimensions a contractile element that changes the shape 

 of a molecule and a group transfer that changes the surface of that molecule 

 converge and become essentially the same thing. It may well be that our thorough 

 understanding of active transport is going to depend on our study of the physical 

 chemistry of macromolecules in such a subtle way that we can really recognize 

 such a convergent phenomenon if it occurs. ^^"^ i/»^~"~^ 



