From owner-biophysics@net.bio.net Wed Jan 07 22:00:00 1998 Path: biosci!agate!howland.erols.net!news.maxwell.syr.edu!nntp.news.xara.net!xara.net!server5.netnews.ja.net!daresbury!not-for-mail From: Pentcho Valev Newsgroups: bionet.biophysics Subject: Towards an agreement about chemical work Date: 8 Jan 1998 16:52:02 -0000 Lines: 31 Sender: lpddist@mserv1.dl.ac.uk Distribution: bionet Message-ID: <69307i$47i@mserv1.dl.ac.uk> Original-To: biophys@dl.ac.uk Delta G gives the chemical work only for a REVERSIBLE process, i.e. one for which dQ = TdS: Wu = -delta G = Tdelta S - delta H = Q - delta H /1/ where Q is the heat ABSORBED from the environment as work is done REVERSIBLY. It is said in textbooks that, if delta G for the ATP system is -50 KJ/mole, one can expect maximum 50 KJ/mole work to be done by the ATP system. As delta H for ATP hydrolysis is 20 KJ/mole, it follows from /1/ that Q, the heat absorbed as maximum work is done, is 30 KJ/mole. Everybody would agree that the ATP system NEVER does the maximum work, i.e. work production in this case cannot be REVERSIBLE. What is not clear however is whether the value of 50 KJ/mole can be APPROACHED. In other words, as the ATP system drives the reaction B -> C in ATP + B -> ADP + P + C /3/ when is the work production ALMOST REVERSIBLE so that almost 50 KJ/mole work is done and almost 30 KJ/mole heat are absorbed from the environment? Please note that the answer to this question is essential - the concept of "maximum work" makes sense only if this value can be APPROACHED under certain conditions. If it cannot be approached under any REAL conditions (e.g. delta G = -50 KJ/mole but really Wmax < 20 KJ/mole), we should certainly define a new concept giving us the REAL MAXIMUM WORK. Best regards, Pentcho .