The Energy Relationships in Cellular Respiration and Photosynthesis: the Balance Sheet

The respiration (or burning) of a mole of glucose releases 686 kcal of energy. This value represents the difference between the energy needed to break the bonds of the reactants (glucose and oxygen) and the energy liberated when the bonds of the products (H2O and CO2) form.

Average bond energies, kcal/mole
C-H 98
O-H 110
C-C 80
C-O 78
H-H 103
C-N 65
O=O 116 (2 x 58)
C=O 187* (2 x 93.5)
C=C 145 (2 x 72.5)
(* as found in CO2)
Conversely, the photosynthesis of a mole of glucose requires the input of 686 kcal of energy.

The reasons:

The diagram shows the details.

The overall equation for each process is the same; only the direction of the arrow differs. (The actual equation is:

C6H12O6 + 6O2 + 6H2O 12H2O + 6 CO2

but we shall ignore the six molecules of water that occur on each side as they "cancel out".)

The structural formulas are shown as well as the average bond energies for each bond involved.

Cellular Respiration

As you can see,

Thus a grand total of 2878 kcal is needed to break all the bonds of the reactants in cellular respiration.

As for the products,

Thus a grand total of 3564 kcal is released as all the bonds of the products form.

Subtracting this from the 2878 kcal needed to break the bonds of the reactants, we arrive at -686 kcal, the free energy change of the oxidation of a mole of glucose. This value holds true whether we oxidize glucose quickly by burning it or in the orderly process of cellular respiration.

The minus sign indicates that free energy has been removed from the system.
Link to discussion of free energy.


The details of the energy budget are just the same. The only difference is that now it takes 3564 kcal to break the bonds of the reactants and only 2878 kcal are released in forming glucose and oxygen. So we express this change in free energy (+686 kcal) with a plus sign to indicate that energy has been added to the system. The energy came from the sun and now is stored in the form of bond energy that can power the needs of all life.
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27 December 2010