In article <1993Dec23.130115.20856 at aber.ac.uk> hrs at aber.ac.uk (Herbert Martin Sauro) writes:
>Subject: Re: exothermic or endothermic?
>From: hrs at aber.ac.uk (Herbert Martin Sauro)
>Date: Thu, 23 Dec 1993 13:01:15 GMT
>In article <2fbpoo$shj at mserv1.dl.ac.uk> "" <unknown at 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 at AGRI.HUJI.AC.IL
The Hebrew University of Jerusalem | /\/ Bitnet: MARDER at HUJIAGRI
Faculty of Agriculture |/ \ Phone: (08 or +9728) 481918
