ACTIVE TRANSPORT AND MEMBRANE EXPANSION-CONTRACTION CYCLES 64 1 



accumulate there and fold or wrinkle because of the reduced surface area available. 

 If you have a stream of water running into a lake, the natural surface film carried 

 down by the streani collapses at the point of entry and you can see this collapse in 

 myriads of parallel striations usually over several metres near the point of entry. 

 In a stream of cytoplasm or water running into a deeper region, unit volume will 

 have an area, if the depth is D^ in the shallow part, of 1/-D1, and if the depth is D.y 

 in the deep part, of i!D.,, so the ratios of the surface areas will be D./Dj ; the ratio 

 of these thicknesses may be 10 to i or a 100 to i in the cell, which means that 

 when cytoplasm with its associated membrane reaches the end of the cell at these 

 turning points, 90 to 99",, of the area disappears giving almost complete desorption 

 of anything that would be adsorbed on it. 



Allen : I am very curious to ask Dr. Goldacre exactly how he changed the 

 charge of the particles in the experiments he just described a moment or two ago. 



Gold.acre: Carmine particles were suspended in a solution of polyethylene 

 imine hydrochloride (a cationic polymer, which is adsorbed by the carmine) and 

 then washed, and it was shown that the charge had been reversed with wires and a 

 battery; the particles moved in the opposite direction. 



Davis : It seems to me that the possible mechanism of active transport involving 

 a contractile element does not critically depend on the kind of contractions that are 

 grossly visible. Are these not more of a model system for the micro-contractions 

 which would change the affinity of the substance for the carrier ? 



Goldacre: Yes, there might well be submicroscopical or near submicroscopical 

 contractions in cells such as the red cell ; in the red cell I imagine you might 

 possibly have something like a protein chain or fibre perhaps running along the 

 surface and absorbing things and then folding up in the middle of the cell some- 

 where. There is not much evidence of that, but the shimmering movements 

 associated with active transport, which can be inhibited by various chemicals 

 (described by Pulvertaft and Maizels) indicate that something must be going 

 on there. 



Mitchell: I was worried a bit by Dr. Schoffeniels' model. Just as we can be 

 too easily convinced by what we see as cytologists when we look down the micro- 

 scope and are confronted by a very attractive picture ; in the same way, we must 

 be very careful indeed in drawing conclusions about an attractive everyday macro- 

 scopic model and imagining that what happens in the model can happen at the 

 molecular level. The model described by Dr. Schoffeniels, as it is stated without 

 all sorts of additional specifications, is really a Maxwell demon and can't work at 

 the molecular level. And perhaps it would be appropriate to say that this is true 

 of most of the contractile mechanisms of transport that have been proposed. I am 

 not sure that all of those proposed so far are Maxwell demons, but certainly 

 nearly all. 



You could legitimately say that the contraction itself can increase the rate at 

 which the transport takes place : but it simply cannot be responsible for the change 

 of free-energy of the molecule which is eventually regenerated after carriage of the 

 molecule through the membrane as a result of its formation of a compound or 

 complex. It simply cannot be responsible for that change. 



Goldacre: Schoffeniel's model would require a membrane with a valve-like 

 action on the molecular scale. Such asymmetry does not appear to exist in non- 



VOL. n. — 2 T 



