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Name: Shervy
Status: Student
Grade:  9-12
Location: TX
Country: United States
Date: September 2006


Question:
Hi. I was wondering, how much energy is required to make one glucose molecule in photosynthesis?



Replies:
The formation of glucose C6H12O6(solid) from CO2(gas) and H2O(liquid) occurs by the reaction: 6CO2(gas) + 6H2O(liquid) = C6H12O6(solid) + 6O2(gas) There are tables listing the heat required to produce each of these compounds from their respective elements. All elements are by convention assigned a value of zero, so the oxygen (O2) does not contribute anything to the heat required. From the tables of heats (technically called the enthalpy) of formation one finds that the heat of this reaction is: (-1250 kJ/mol glucose), 6x(-393.5 kJ/mol CO2), and 6x(-285.8 kJ/mol H2O liquid).

Multiplying and taking the difference gives +2826 kJ/mol glucose. The usual "rule of thumb" is that reactions that have a positive heat of reaction are not favored to occur. So what is going on here? What has not been included here is the light energy from the Sun!! That energy input makes up the difference. Photosynthesis is a photochemical reaction. It requires light to occur.

Vince Calder



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