Herbert Sauro (hrs at aber.ac.uk) writes that the entropy change in photosynthesis
is likely to be negative because CO2 and water are converted to glucose etc.
That is true for that part of the entropy change, but the total entropy
change involves a net flux of photons from the photosphere of the sun at 6000
degrees C + to a plant leaf at c. 25 degrees C. This favourable entropy
change is the ultimate driving source of photosynthesis.
However, I think there is no doubt that the photons are a source of enthalpy
(as calculated by hv), so that the question is a terminological one:
is a positive delta H equivalent to 'endothermic' if the energy is input in
a none-thermal form? My view is yes, even without considering the argument
that electromagnetic radiation is radiant heat, most obvious for infra red,
but true of the rest as well (why does the sun shine?). Therefore my view
is that photosynthesis is endothermic, supported by the observation
reported by Paul Schlosser (if I remember correctly) that leaves look
cooler in the infra red than their surroundings (though admittedly
transpirational cooling may play a role in this).
Additional argument: the dissociation of water to hydrogen and oxygen is
endothermic, and can be brought about at a sufficiently high temperature.
Does it cease to be endothermic if I carry out the same reaction by
electrolysis, because the energy input is in electrical form? And then
does it become endothermic again when I remember that the elecricity
was generated by a coal-fired power station? This makes a nonsense
of the First Law of thermodynamics and the principle of equivalence of
energy. The simplest way out is to accept a broad definition of
endo- and exo-thermic.
David Fell
daf at bms.brookes.ac.uk
