The 24 covalent bonds of glucose require a total of 2182 kcal to be broken. The six double bonds of oxygen require another 696 kcal. Thus a grand total of 2878 kcal is needed to break all the bonds of the reactants in cellular respiration.
Photosynthetic reduction of CO2 can be summarized by the equations:
2H2O → O2 + 4H+ + 4e- (3:6)
CO2 + 4H+ + 4e- → (CH2O) + H2O (3:7)
Four electrons are required to be transferred fromwater, through a redox span of 1.24 eV, to reduce one molecule of CO2. The energy required for the reduction of 1 mol of CO2 is therefore 4 mol 1.25 eV x 1.60 x 10-19 J eV-1 x 6.02 x 10-3 mol-1 = 47.77 x 104 J (115.10 kcal). Theoretically, the energy requirement could be satisfied by the capture of 4 mol of photons of PAR light, say a red photon of 700 nm, which have an energy content of 4 mol x 2.84 x 10-19 J x 6.02 10-3 mol-1 = 68.4 x 104 J (163.40 kcal). However, due to the thermodynamic losses during energy conversion, the fraction of absorbed photon energy converted into chemical energy seldom exceeds 0.35. Thus, eight moles of photons are required for the reduction of 1 mol of CO2 (8 mol x 2.84 x 10-19 J x 6.02 x 10-3 mol-1 = 136.82 x 104 J x 0.35 = 48.20 x 104 J (115.20 Kcal).Amole of glucose (formed by the addition of six CO2 molecules) requires 6 x 115.20 kcal equal to 691.20 kcal. This value is calculated by taking in account the balance of the energy of bonds previously described.
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