Re: how ATP formation occurs during cyclic photophosphorylation ?

In the normal Z-scheme of Robin Hill & Fay Bendall (Hill, R., and Bendall, 
F. (1960). Nature 186, 136–137. - I have had the pleasure of meeting Fay 
Bendall while working with her husband Derek), electrons from Photosystem 
II (PSII) are transfered to PSI via the cytochrome b6f complex and 
plastocyanin (PC). My research was into the interaction between cyt f and 
PC. PSI transfers an electron to NADP via ferredoxin.

In cyclic photophosphorylation, the electron from PSI is recycled through 
ferredoxin to the cytochrome b6f complex. This means that electrons from 
PSII are not required, but NADP is not reduced to NADPH.

Since PSII obtains electrons from the oxidation of H2O, cyclic 
phosphorylation switches off the indirect reduction of NADP by H2O while 
still cycling electrons through cyt b6f. It is as electrons cycle through 
b6f that protons are transfered across the thylakoid membrane, 
establishing the proton gradient that powers ATP synthase.

Bendall, D.S. and Manasse, R.S. (1995) Cyclic photophosphorylation and 
electron transport. Biochim. Biophys. Acta, 1229, 23-38.