Principles of Biochemistry, Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
Photosynthesis
General equation of plant photosynthesis

Standard Free energy change in the Synthesis of glucose from СО₂ and Н₂О

Class="center">6СО₂ + 6Н₂О → С₆Н₁₂О₆ + 6О₂

is + 686 kcal/mol (recall that The oxidation of glucose, which is the reverse of this process, is accompanied by a standard free energy decrease of 686 kcal/mol). Let us now compare this energy requirement with The amount of energy provided by the light reactions of plant Photosynthesis. As we already know, for The transfer of one electron from Н₂О to NADP⁺, two quanta of light must be absorbed, one for each photosystem. The formation of one molecule of О₂ is coupled with the transfer of four electrons, i.e., it requires the absorption of eight quanta. To evolve six molecules of О₂ in accordance with the equation above, 6 ∙ 8 = 48 quanta must be absorbed and utilized. Since the energy of one "mole" of quanta ranges from 72 kcal at 400 nm to approximately 41 kcal at 700 nm (Table 23-1), under standard conditions green Cells require anywhere from 48∙41 = 1968 to 48∙72 = 3456 kcal to form one mole of glucose, which "costs" 686 kcal, depending on the wavelength of the absorbed light.