Geoscience Reference
In-Depth Information
after it loses its electron, it becomes a very strong oxidant (an electron
acceptor, the strongest known in nature in fact), which means that it can
take electrons from water, producing oxygen as a byproduct. This all
sounds easier than it really is. Getting the electrons to flow from water
to P680, with the formation of oxygen, requires a carefully orchestrated
biochemical ballet. This dance is conducted by the so-called oxygen-
evolving complex (OEC), which at its core contains a four-membered
Mn (manganese) cluster. We will hear more about this shortly.
But irst, back to the pheophytin. hen it obtains its electron from
P680*, it does so quickly (within 3 picoseconds in fact; that's 3 trillionths
of second!); this is critical or the electron would recombine again with
the oxidized P680, which, as we saw above, is a strong oxidant and is
very good at extracting electrons. If it did so, the whole process would
be cut short and stop. Ultimately, the cell needs to put the electron in
the pheophytin into a soluble electron carrier known as NADP(H).
7
Packed in NADP(H), the electron can be used to conduct all kinds of
biochemistry for the cell. However, the pheophytin is barely able (barely
reducing enough in chemical terms) to put its electron into NADP(H),
and if it did so now, the cell would gain very little from the photosyn-
thetic process.
Instead, the electron is used to do work. In what happens next, we
can imagine the electron sliding downhill, like a go-cart on a fine sum-
mer day. It rides from the pheophytin to a quinone molecule and fur-
ther downhill to a series of other proteins. The cell takes advantage
of this carefree ride, and as the electron rolls downhill, ATP is formed.
8
ATP is the energy currency of the cell, and in desperately unromantic
terms, one could make the case that the purpose of life is to make ATP.
Anyway, after the electron makes it to the bottom of the hill, there isn't
much energy left in the protein holding it. There is no way it can make
NADP(H) and no way it can do much else for the cell. So, the electron
is handed over to PSI (photosystem I). Here, there is another chloro-
phyll molecule known as P700, whose oxidizing form lies in wait for
this electron. As P700 and the electron unite, energy from the antenna
complex (
hv
) propels P700 to an excited and reduced state, P700*,
which is, in fact, a much stronger reductant than the excited P680* we
just discussed. The go-cart ride begins again, but because the electron
starts from a higher level (a more reduced state), it remains sufficiently
