From owner-biophysics@net.bio.net Sun Jan 18 22:00:00 1998 Path: biosci!daresbury!not-for-mail From: Pentcho Valev Newsgroups: bionet.biophysics Subject: Chemiosmotic events Date: 19 Jan 1998 19:08:08 -0000 Lines: 39 Sender: lpddist@mserv1.dl.ac.uk Distribution: bionet Message-ID: <6a08ao$hov@mserv1.dl.ac.uk> Original-To: biophys@dl.ac.uk Jonathan Marder wrote:>>>>>>>>>>>>>>>>>> I meant that if we consider the protons moving between the two buk phases (on either side of the membrane), the only consideration in net energy change is the p.m.f. between these two phases. The exact path is irrelevant. Actually, I would expect the p.d. to be the same at the ATPase as across any other point locus on the membrane - otherwise we would have large lateral p.d.'s which are not evident. You can probably work out what that means in terms of your above electrochemical definition of p.d.<<<<<<<<<<<<<<<<<< There is an OBJECTIVE contradiction here which suggests extremely interesting conclusions. Let us idealize the situation: pH 2 M pH 7 where M is a membrane permeable only to H+ and whose capacitance varies considerably. At equilibrium there is a potential difference between the bulk phases on the two sides of the membrane. Let us also assume that the potential is uniform throughout each bulk phase (close to the membrane). I accept the above description, but we should somehow reconcile it with another obvious result - that the transmembrane potential difference is equal to the ratio of the surface charge density and the capacitance. Let us imagine two adjacent spots on the membrane: -------------low-capacitance------high-capacitance---------------------- For the transmembrane potential difference to be the same across the two spots, the surface charge density (of H+ and the respective counterion) must be different. But this is extraordinary - consider only H+ on one side of the membrane - we have, laterally, a constant potential but a H+ concentration gradient, i.e. we can expect a lateral H+ flow at equilibrium. What do you think? (BTW, your postings come rather distorted. Can't you remove the bug? I will comment on the rest of your last posting separately). Best regards, Pentcho .