| Method |
Comparing the operating costs of oxygenic photosynthesis, moving one electron from water to NADP+ with the coupled pumping of three protons across the photosynthetic membrane takes at least two absorbed photons given the relatively low photon yield of PSII. The energy stored in the products listed in the last sentence is 110 kJ + (3×18 kJ) or 164 kJ per mol electron moved (Raven 1984), BNID 105060. One mol 680 nm photon (PSII) contributes 176 kJ and one mol 700 nm photon (PS1) contributes 171 kJ, a total of 347 kJ. Even with 100% photochemical efficiency of both photosystems the overall efficiency of energy capture is 164/347 (i.e. 47%), BNID 105061 (Table link). For the mean wavelength of photosynthetically active radiation (i.e. 550 nm), the efficiency is 163/(2×218) or 37%. For an ion-pumping rhodopsin with an absorption maximum at 550 nm, the energy input is 218 kJ per mL photon. As the energy stored in the movement of a single mol of protons is only 18 kJ, the efficiency is only 8%, BNID 105056 |