<Li> The electron transfer back from the electron acceptor to the positively charged special pair is especially slow . The rate of an electron transfer reaction increases with its thermodynamic favorability up to a point and then decreases . The back transfer is so favourable that it takes place in the inverted region where electron - transfer rates become slower . </Li> <P> Thus, electron transfer proceeds efficiently from the first electron acceptor to the next, creating an electron transport chain that ends if it has reached NADPH . </P> <P> The photosynthesis process in chloroplasts begins when an electron of P680 of PSII attains a higher - energy level . This energy is used to reduce a chain of electron acceptors that have subsequently lowered redox - potentials . This chain of electron acceptors is known as an electron transport chain . When this chain reaches PS I, an electron is again excited, creating a high redox - potential . The electron transport chain of photosynthesis is often put in a diagram called the z - scheme, because the redox diagram from P680 to P700 resembles the letter z . </P> <P> The final product of PSII is plastoquinol, a mobile electron carrier in the membrane . Plastoquinol transfers the electron from PSII to the proton pump, cytochrome b6f . The ultimate electron donor of PSII is water . Cytochrome b6f proceeds the electron chain to PSI through plastocyanin molecules . PSI is able to continue the electron transfer in two different ways . It can transfer the electrons either to plastoquinol again, creating a cyclic electron flow, or to an enzyme called FNR (Ferredoxin--NADP (+) reductase), creating a non-cyclic electron flow . PSI releases FNR into the stroma, where it reduces NADP to NADPH . </P>

Where are the two places in the chloroplast that photosynthesis occurs