Biomedical Engineering Reference
In-Depth Information
In fact, over a single day, light intensity and temperature change dramati-
cally from dawn to midday and sunset, thus requiring regulation mechanisms
operating over a short time-span (minutes, hours) and thus cannot be ac-
tivated by the synthesis/degradation of photosynthetic components, whose
everyday operation would be energetically unsustainable. These short-term
processes will be described later.
3.3 State 1-State 2 Transitions
State transitions are a protective mechanism, which acts through a reversible
protein modification in a few minutes timescale [10, 11] to balance the energy
pressure between the two photosystems, physically displacing LHCII antenna
complexes from PSII to PSI [12].
Higher plants and green algae oxygenic photosynthesis works through the
synchronized action of two photosystems: PSII reaction center, whose maxi-
mum absorption peak is at 680 nm, and PSI, where it is at 700 nm.
The coordinated and energy collection by the two photosystems is a nec-
essary condition for the proceeding of photosynthesis and the generation of a
strongly reducing molecule, which is able to transfer its electron to
NADP
+
.
The kinetically limiting step in the electron transport chain between PSII
and PSI involves the oxidation of liposoluble hydrogen carrier plastoquinone
(
PQH
2
), which once reduced at the
Q
b
site PSII, diffuses in the thylakoid
membrane to cytochrome
b
6
f
complex. In turn, cytochrome
b
6
f
is main-
tained to an oxidized state by the activity of PSI, which transfers electrons to
NADP
+
. Thus, over-excitation of PSII leads to over reduction of PQ, leaving
PSII without a suitable electron acceptor. PSII undergoes charge recombi-
nation and closure of energy traps thus increasing Chl lifetime and singlet
oxygen production.
To avoid this problem, the
PQH
2
acts as a signal activating chloroplastic
protein kinase [13] to allow phosphorylation of LHCII antenna complex as-
sociated to PSII (State I). Phosphorylated LHCII undergoes conformational
change, disconnection from PSII, and migration to the stroma lamellae where
it associates to PSI (State II). This increases light harvesting, and thus elec-
tron transport capacity of PSI thus compensate for the uphill electron carriers
over-reduction. Consequence is the oxidation of PQH2 to PQ and downregu-
lation of the LHCII kinase. Dephosphorylation of LHCII and its reassociation
to PSII reaction centers is ensured by chloroplastic phosphatases. This mecha-
nism is photoprotective as it balances a potentially dangerous over-excitation
on PSII, source of ROS, and it regulates light-harvesting activity depending
on light intensity and light spectral quality changes consequent to rapid tran-
sitions from shade to full sunlight. In fact, the absorption spectrum of PSI is
red-shifted with respect to PSII.
