Overall Reaction and Efficiency

The ATP and NADPH needed for the dark reactions are released into the stroma by the light reactions of photosynthesis. It takes two photons for every electron that passes through photosystems I and II and it takes two electrons to produce each NADPH molecule. Thus, four photons are needed to produce each NADPH. This, in turn, means 8 photons per O 2 molecule produced by the light reactions and that 48 photons are required to produce the 12 NADPH molecules consumed per glucose made in the dark reactions. Assuming the 12 NADPH produced by non-cyclic electron flow causes enough protons to be pumped across the thylakoid membrane to make the 18 ATP necessary for making a glucose, the overall reaction for photosynthesis is

> 6H 2O +6CO 2 + 48 photons -> C 6H 12 O 6 + 6O 2.

If cyclic electron flow is required to produce additional ATP via cyclic photophosphorylation, then more photons will be needed.

From these assumptions, the overall efficiency of photosynthesis is about 35% because the 48 moles of photons of light correspond to about 8000 kJ and the energy difference between a mole of hexoses and a mole each of CO 2 and H 2O is 2870 kJ. Thus, 35% = 2870/8000.

See also: Cyclic Electron Flow, Photosystem Summary, Calvin Cycle