The electron on QA
-is then transferred to QB
-site. As already stated, plastoquinone
at the QB
-site differs from plastoquinone at the QA
-site in that it works as a two-electron acceptor
and becomes fully reduced and protonated after two photochemical turnovers of the reaction center.
The full reduction of plastoquinone at the QB
- site requires the addition of two electrons and two
protons. The reduced plastoquinone (plastoquinol, QB
) then unbinds from the reaction center
and diffuses in the hydrophobic core of the membrane, after which an oxidized plastoquinone
molecule finds its way to the QB
-binding site and the process is repeated. Because the QB
near the outer aqueous phase, the protons added to plastoquinone during its reduction are taken
from the outside of the membrane. Electrons are passed from QB
to a membrane-bound cytochrome b6f
, concomitant with the release of two protons to the luminal side of the membrane.
The cytochrome b6f
then transfers one electron to a mobile carrier in the thylakoid lumen, either
plastocyanin or cytochrome c6
. This mobile carrier serves an electron donor to PSI reaction
center, the P700
. Upon photon absorption by PSI a charge separation occurs with the electron
fed into a bound chain of redox sites; a chlorophyll a (A0), a quinone acceptor (A1) and then a
bound Fe–S cluster, and then two Fe–S cluster in ferredoxin, a soluble mobile carrier on the
stromal side. Two ferredoxin molecules can reduce NADP+
to NADPH, via the flavoprotein
oxidoreductase. NADPH is used as redox currency for many biosynthesis reactions
such as CO2
fixation. The energy conserved in a mole of NADPH is about 52.5 kcal/mol,
whereas in an ATP hole is 7.3 kcal/mol.
The photochemical reaction triggered by P700 is a redox process. In its ground state, P700 has a
redox potential of 0.45 eV and can take up an electron from a suitable donor, hence it can perform
an oxidizing action. In its excited state it possesses a redox potential of more than -1.0 eV and can
perform a reducing action donating an electron to an acceptor, and becoming P700+. The couple P700/P700+ is thus a light-dependent redox enzyme and possesses the capability to reduce the most electronnegative
redox system of the chloroplast, the ferredoxin-NADP+ oxidoreductase (redox
potential = -0.42 eV). In contrast, P700 in its ground state (redox potential = 0.45 eV) is not
able to oxidize, that is, to take electrons from water that has a higher redox potential (0.82 eV).
The transfer of electrons from water is driven by the P680 at PSII, which in its ground state has a
sufficiently positive redox potential (1.22 eV) to oxidize water. On its excited state, P680 at PSII
reaches a redox potential of about –0.60 eV that is enough to donate electron to a plastoquinone
(redox potential = 0 eV) and then via cytochrome b6f complex to P700+ at PSI so that it can
return to P700 and be excited once again. This reaction pathway is called the “Z-scheme of photosynthesis,”
because the redox diagram from P680 to P700 looks like a big “Z” (Figure 3.4).
|FIGURE 3.4 Schematic drawing of the Z-scheme of photosynthetic electron transport, with the positions of
the participants on the oxido-reduction scale.
From this scheme it is evident that only approximately one third of the energy absorbed by the
two primary electron donors P680
is turned into chemical form. A 680 nm photon has an
energy of 1.82 eV, a 700 nm photon has an energy of 1.77 eV (total = 3.59 eV) that is three times
more than sufficient to change the potential of an electron by 1.24 eV, from the redox potential of
the water (0.82 eV) to that of ferredoxin-NADP+
oxidoreductase (-0.42 eV).
It is worthwhile to emphasize that any photon that is absorbed by any chlorophyll molecule is
energetically equivalent to a red photon because the extra energy of an absorbed photon of shorter
wavelength (<680 nm) is lost during the quick fall to the red energy level that represents the lowest