From owner-metabolic-reg@net.bio.net Thu Dec 23 22:00:00 1993 Path: biosci!CS.Arizona.EDU!organpipe.uug.arizona.edu!uunet!cs.utexas.edu!howland.reston.ans.net!newsserver.jvnc.net!gmd.de!dearn!barilvm!vms.huji.ac.il!marder.agri.huji.ac.il!MARDER Newsgroups: bionet.metabolic-reg Subject: Re: Photosynthesis exothermic or endothermic? Message-ID: From: MARDER@agri.huji.ac.il (Jonathan B. Marder) Date: Fri, 24 Dec 1993 09:27:23 References: <2fbpoo$shj@mserv1.dl.ac.uk> <1993Dec23.130115.20856@aber.ac.uk> Distribution: bionet,world Organization: The Hebrew University of Jerusalem Nntp-Posting-Host: marder.agri.huji.ac.il X-Newsreader: Trumpet for Windows [Version 1.0 Rev A] Lines: 45 In article <1993Dec23.130115.20856@aber.ac.uk> hrs@aber.ac.uk (Herbert Martin Sauro) writes: >Subject: Re: exothermic or endothermic? >From: hrs@aber.ac.uk (Herbert Martin Sauro) >Date: Thu, 23 Dec 1993 13:01:15 GMT >In article <2fbpoo$shj@mserv1.dl.ac.uk> "" writes: >. . . Having talked to a few other people about this, my feeling is >that photosynthesis is most probably exothermic. >The delta G must be negative (otherwise it wouldn't go of course) but >delta G = delta H - T delta S and I imagine the delta S change is >negative because we're changing things like co2 and water into gluose >so there is much less freedom? This is the bit I'm not too happy about >because I suppose one should consider what's happenning to the >environment etc so even just deciding the sign of delta S is probably >difficult. But if it is negative that obviously delta H must be negative >for delta G to be negative, hence photosynthesis is exothermic? >Humm.. The delta H *is* the effect on the environment (call it change in environmental entropy change if you like). Delta S is the change WITHIN the photosynthetic system being considered. I was going to step into this discussion sooner but was too puzzled by the question to contribute. Now that I think about it, it seems to me that photosynthesis is clearly EXOTHERMIC. First let's deal with the photons; they come not as raw heat, but with definite energies, so it is best to think of them as *substrate* for the reaction. Once you do this, it is clear that some of this substrate energy (light) is conserved in the photochemical + biochemical conversions while the rest is release as heat (hence EXOTHERMIC). An endothermic reaction would draw heat from the surroundings. Nevertheless, within the multistep process there are probably some endothermic steps. >Herbert Sauro __ Jonathan B. Marder ' Department of Agricultural Botany | Internet: MARDER@AGRI.HUJI.AC.IL The Hebrew University of Jerusalem | /\/ Bitnet: MARDER@HUJIAGRI Faculty of Agriculture |/ \ Phone: (08 or +9728) 481918 .