The electron on Q
A-is then transferred to Q
B-site. As already stated, plastoquinone
at the Q
B-site differs from plastoquinone at the Q
A-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 Q
B- site requires the addition of two electrons and two
protons. The reduced plastoquinone (plastoquinol, Q
BH
2) 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 Q
B-binding site and the process is repeated. Because the Q
B-site is
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 Q
BH
2 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 P
700. 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
ferredoxin-NADP
+ oxidoreductase. NADPH is used as redox currency for many biosynthesis reactions
such as CO
2 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 P
680 and P
700 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
excited level.
