John Woods wrote that absorbing photons does remove energy from another
body - the sun. This is certainly the case, and if one wished to include
the sun in one's system then photsynthesis would be endothermic, but
including the sun is not very practical. It would seem to me that a
practical definition would be the plant (in which photosynthesis is taking
place) and it's immediate surroundings. In this case, photons are an
external source of energy (a flux of energy into the system). I assume
that the question of relevance is: does the process of photosynthesis
heat or cool the system (the plant and it's immediate environs)? The
absorption of photons does not remove heat from the system, and so, should
not be counted in answering *this* question (which might be technically
distinct from asking if the reaction is "exothermic" or not).
As to the point of a thermometer in a vacuum responding to photons, this
probably would occur (I'm not sure if the presence of photons was
included in Marc Roussel's discussion). I.e., the absorption of a photon
would raise one or more electrons to a higher energy state, part of which
could be converted to thermal energy. *But* while this does cool the photon
source, it again does not remove heat from the immediate environs of the
thermometer.
So, what is the real question here: does photosynthesis remove energy from
the universe? (probably yes) or, does photosynthesis remove heat from the
surrounding medium (cytosol and/or the air arround the plant - I would guess
not).
Paul
schlosser at beta.ciit.org
"I want my money back" -Meatloaf
