Date: Winter 2009-2010
I remember studying biochemistry and learning that
photosynthesis is an endothermic process. As I remember endothermic
processes are only possible if they result in an increase in entropy
within the reaction. Is this true in photosynthesis? It seems to be
that the reactants are more disordered than the products, what am I missing?
Photosynthesis is a marvelous example of how much more complex chemical processes
can be in nature than in the chemistry lab! You are right that reactions will
generally only be spontaneous if they either are exothermic or endothermic with
an increase in system entropy. Overall, a spontaneous process generally gives an
increase in the entropy of the universe (through giving out heat quanta so
increasing the entropy of the surroundings, or having an increase in its own
An endothermic reaction that has a decrease in system entropy should not happen
on its own, we call it "endergonic" meaning that work must be done to achieve it,
it is not spontaneous. However, we have seen that endergonic processes can occur
if they are coupled with exergonic (spontaneous) processes and the net result is
exergonic. This happens fairly often in nature as enzymes form relatively weakly
bound intermediates with very limited arrangements (low entropy) before decomposing
to form products. The first step is endergonic but it still happens because the
second step is exergonic enough to make the whole process spontaneous.
In the case of photosynthesis the true "reaction" is much more complex than the simple
CO2 + H2O + light gives sugar + O2 that we all remember from school. This, by itself
may be endergonic but it is coupled with many other processes that can make the whole
thing spontaneous. The main process that is often used to explain the overall spontaneity
of photosynthesis is the efficiency of the reaction in terms of the light that is absorbed
and used to produce the products. Only a small percentage of the incoming light is absorbed
and the rest is dispersed to the surroundings to give an increase in entropy there. There
is a maximum possible theoretical efficiency of photosynthesis, above which not enough
energy is dispersed, so the combined processes of photosynthesis and dispersion stops
being exergonic. This has all sorts of implications to scientists who dream of a world
where all mankind's energy needs are supplied via photosynthesis.
To sum up, the "photosynthesis equation" in isolation should not happen. But when coupled
with the dispersion of energy due to the low efficiency of the capture of sunlight there
is a net increase in entropy so the whole process does happen.
I think it's the other way around. There is a decrease in entropy because something is
becoming more ordered. But it's about the net amount of energy. There is more energy
put into the system than comes out, so it's endergonic (not endothermic).
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