Biomedical Engineering Reference
In-Depth Information
rate of any subsequent process; therefore, the system equilibrate at the first
singlet-excited state before anything else can happen.
Relaxation to ground state may occur through different pathways: (1)
internal conversion, where the excitation energy is dissipated as heat; (2)
fluorescence emission, where the energy is emitted as a photon; (3) energy
transfer, where excitation energy is transferred to a nearby acceptor molecule
(Forster energy transfer) [7]. In the photosynthetic apparatus, the most im-
portant is the latter: reiterated energy transfer between neighbor chlorophyll
molecules allows excitation energy to reach a particular site (reaction center),
where a redox reaction takes place, converting the excitation into chemical
energy.
For e
cient conversion of light energy into chemical energy, a highly e
-
cient energy transfer through neighboring pigments is essential. However, the
ideal conditions for high e
ciency in energy transfer is not always satisfied
in the photosynthetic apparatus; if the energy transfer rate is not competing
successfully with the other relaxation mechanisms, the excitation energy is
lost and a fluorescence emission or heat dissipation takes place.
Another important phenomenon must be taken into account. The lowest
singlet-excited state of chlorophylls can also undergo intersystem crossing to
the triplet state. This excited state cannot transfer its energy to the reaction
center and therefore cannot contribute to photochemistry. Worse than that,
chlorophyll triplet state can easily activate molecular oxygen to its singlet
state, producing a highly reactive and dangerous molecule.
The second class of pigments is represented by carotenoids, linear poly-
enes [8] involved in facing the problem of singlet oxygen production, besides
contributing to absorb light and transfer it to the nearby chlorophylls: they
can quench chlorophyll triplet-excited states and also scavenge singlet oxy-
gen. In both cases, this leads to the dissipation of the absorbed photon but
it prevents oxidative damage to the membrane and photosynthetic apparatus
components.
1.4 Photosynthetic Apparatus
Light absorption and energy conversion take place in specific and highly or-
ganized structures embedded in the thylakoid membrane. All together they
constitute the photosynthetic apparatus. There are four different complexes:
two Photosystems (PSI and PSII), the cytochrome b
6
f, and the F-ATPase.
These are composed of several protein subunits, cofactors, and pigments, while
additional electron carriers can move within the lipid bilayer of the mem-
brane or in the aqueous medium (Fig. 1.3). In the last years, high resolution
structures for the four complexes, even though from different species, have
become available, thus allowing a deeper comprehension of photosynthetic
mechanisms [9-13].
