Re: How does the electron-transportation transport protons ?

Philipp:
     That is a good question.  Photosynthesis is an extremely complicated 
series of biochemical reactions utilizing light that we do not completely 
yet understand.  Only recently, were scientists able to visualize (Through 
x-ray crystallography) part of the ligh-harvesting complex of 
photosynthesis.  
     First of all, I will break your question up into two parts... the 
pH/charge gradient created by the proton pumping...   In this case, the pH 
change is created simply because you have more protons on one side of a 
membrane than the other... And the potential generated here, is utilized 
by the ATPase to synthesize ATP from AMP/ADP and ppi (inorganic 
phosphate).  This reaction is comparable to that performed in 
respiration,  as observed in the mitochondria, so I will not delve further 
into it now.
    Essentially electrons are converted into chemical energy through a 
multitude of reactions, but how are these electrons transduced?  In 
simplest terms, there are a series of chemicals in plants that are labile 
for accepting electrons and are able to 'release' them as well with 
different energetic properties.  These molecules, absorb a photon (Light) 
which in turn excites and removes an electron from an orbital.  This 
excitation is passed through the photosystem and in photosynthetic 
bacteria (The best understood photosynthetic system) the energy of the 
excited electron is traded off with the exchange of a proton across the 
membrane... A special protein called cytochrome b1 facilitates the 
pumping, and another component recycles the electrons, cytochrome b2.  So, 
essentially there is no net loss in electrons, as you move an electron one 
way (-1 negative charge) it is reduced in it's excited energy state by the 
oxidation and pushing of 4 protons across the membrane...    The actual 
chemistry of the electron is mediated by iron in the cytochrome, which as 
photosynthesis progresses continually is reduced and oxideized from Fe3+ 
to Fe2+ and back again as cytochrome b2 takes the electron (Fe2+ >> Fe3_) 
back to the PSI/II.
     The source of the hydrogen is from the breaking of water molecules, 
and the energy of the electron is used to reduce NADP+ to NADPH, which, 
comparable to NADH+ H+ in respirative organisms, is what is actually used 
to generate the ATP.  If you have had chemistry, the half reactions for 
this are:
O2 + 4e- + 4H+  <==> 2H2O   Eo'= +.815V
NADP+ + H+ +2e- <==> NADPH  Eo'= -.320V

2NADP+ + 2H2O <===> 2NADPH + O2 + 2H+ Eo' = -1.135V

     As for the transfer of electrons between photosystems, again, an iron 
containing molecule plays an important part, cytochrome f.  This 
cytochrome allows electron transport from PSII to PSI.  There are several 
herbicides that block this reaction, killing the plant, such as DCMU.
     The sum of photosynthesis is mediated by three main membrane bound 
proteins with these mobile electron carrying proteins around them, they 
are: PSII complex,--->cytochrome F complex--->PSI, and then the NADPH is 
generated through oxidation/reduction of FAD, and ferredoxin, and as you 
mentioned, the ATP is generated by the F1 Fo ATPase complex, which through 
all of that pH potential you mentioned, lets protons back through the 
membrane and makes ATP from ADP + Pi...
     There are dozens of other reactions, (oxidations/reductions) occuring 
here, but those are primarily the major players.  Remember, the take home 
message is that everything is ultimately recycled, there are limited 
quantities of most of the chemicals involved, (Particularly NADP+) and the 
electrons need to be 'taken back' to the beginning to regenerate the 
system.  Any good biochemistry text, such as Voet&Voet, Suzuki, or Strier, 
has lots of really good color drawings and explanations of the multitude 
of electron hand-off's that are occuring...  Good luck and have fun with 
your photons and protons...

-Matt-