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Photosystem I
-1.5
P700*
Photosystem II
−1.0
P680*
e -
−0.5
pheo
h v
NADP +
NADPH
PQ
PQ
0
h v
cyt
b-f
Pc
0.5
P700
OEC
2H 2 O
e -
1.0
P680
O 2
1.5
transmembrane proton potential
ATP
Figure 3.2. the coupled reaction centers in oxygenic photosynthesis, the so-called Z-scheme.
the trick in understanding this is to follow the electrons as explained in the text. Figure
kindly provided by raymond Cox.
To understand how oxygenic photosynthesis works, our simple goal
will be to follow electrons. The process is outlined in igure 3.2, and we
start with PSII. Energy from the antenna complex is transferred to a
special chlorophyll molecule called P680. This energy causes P680 to
develop an excited state, named P680*, which becomes a strong reduc-
tant (source of electrons). Like a child screaming for the bathroom,
P680* easily dispenses with an electron and hands it over to a chemical
compound known as a pheophytin, reducing the pheophytin in the
process. We will worry about what happens with the pheophytin in a
minute, but our immediate concern is the P680 chlorophyll that has just
lost an electron. This electron must be replaced or the whole process
will run down and stop.
In the remarkable innovation of oxygenic photosynthesis, the elec-
tron comes from water. This is a big deal because, as we know from ex-
perience, water is a stable substance; we don't worry about it bubbling
of oxygen, for example, when we take a bath. The beauty of P680 is that
 
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