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Name: Nicole
Status: educator
Grade: other
Location: MI
Country: USA
Date: Winter 2009-2010

Question:
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?



Replies:
Nicole,

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 entropy).

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.

Best wishes,

Tom Collins


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).

Van Hoeck



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