Path of electrons in photosynthesis.
Electrons, from the oxidation of water, inside the grana disks (thylakoids) are transferred across the membrane to the outside, to the Calvin cycle or the Hill oxidant. The span in redox level may be 2.3 V. Part of the system II chlorophyll is on the inside of the membrane and part on the outside. An electron trap is embedded in the membrane. Alternately, an excited chlorophyll on the inside gives an electron to the trap, and an excited chlorophyll on the outside gives a hole to the trap. Two quanta move an electron from inside to outside. The charging of this condenser drives the redox levels on the inside positive and those on the outside negative. The final voltages depend upon the electron flow and a carotene diode. A voltage of 0.3 is involved. Delayed light is the exact reverse of the light reaction. System I makes ATP.
Arnold
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Biology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830.