Biomedical Engineering Reference
In-Depth Information
Antenna:
xanthophylls
Core
complex:
β
Antenna:
xanthophylls
-carotene
Core
complex:
β
A
-carotene
B
Fig. 3.2.
Location of carotenoid species in both PSI (
a
) and PSII (
b
) supramolecular
complexes. (
a
) PSI image by [58], based on PSI crystal structure by [59]; (
b
) PSII
image by [60]
whose lifetime is
200 ns, while the activation of heat dissipation of excess en-
ergy (NPQ) requires less than a minute. In fact, carotenoids are involved in
many of the mechanisms that allows survival of the photosynthetic cell when
light intensity and photosynthetic electron transport rate undergo sudden
changes, thus providing a more e
cient response in conditions of repetitive
excess light exposure, such as under sun flecks deriving from the overcast-
ing of the leaf canopy; particularly, those mechanisms that are localized in
the hydrophobic bilayer of the photosynthetic membrane, hosting reaction
centers, and antenna proteins. Additional mechanisms located on the soluble
phase of the chloroplast stroma are also of great importance for photoprotec-
tion and are based on the activity of superoxide dismutase, ascorbate perox-
ydase [20]. Their role has been the object of excellent reviews and will not be
discussed here.
The study of xanthophyll function in plants and algae has been carried on
in the most recent years through an integrative approach, using genetics, phys-
iology, biochemistry, and molecular biology [21]. First of all, it can be asked
why plants do actually need xanthophylls at all. In fact all the main photopro-
tective functions of xanthophylls (namely, triplet quenching and ROS scaveng-
ing) are well-known properties of
β
-carotene [22]. Thus, the extreme conser-
vation of carotenoid composition in plant species has little or no explanation
on the basis of published chemical properties of these molecules. Nevertheless,
the fact that all plant species and to a large extent green algae contain, be-
∼
