Biomedical Engineering Reference
In-Depth Information
To carry out these reactions, in particular those of the light phase, plants
need a specific apparatus, which is in part shared by other photosynthetic
organisms like cyanobacteria and green algae. The photosynthetic apparatus
is located within the cells in dedicated organelles called chloroplasts. The
origins of chloroplasts is most probably due to an event of endosymbiosis of a
cyanobacterium by an eukaryotic cell [3].
1.2 Chloroplast Structure
Chloroplasts are characterized by two different membrane systems: a double
membrane envelope, which encloses a soluble fraction called stroma; within
the stroma are located thylakoids, an extended and morphologically complex
membrane system carrying the photosynthetic apparatus. This inner membra-
nous system defines an internal space called thylakoid lumen. The membrane
can be divided in two main regions: the
grana
region, where the membranes
form stacked structures of flat vesicles and the
stroma lamellae
, which create
connections between stacked vesicles, insuring continuity to the lumenal space
(Fig. 1.1). Thylakoid membranes are composed principally of glycerol-lipids,
in particular monogalactosyldiacyl-glycerol and digalactosyldiacyl-glycerol,
which distinguish photosynthetic membranes from all the others [4].
The prokaryotic origin of chloroplast is supported by the presence of a
circular genome, which encodes almost 90-120 sequences, while many others
(
3,400) or more proteins, predicted as chloroplastic, are encoded by nuclear
genes [5].
∼
Fig. 1.1.
Chloroplast structure. Localization in a typical mesophyll cell, schematic
representation and electron micrograph, showing thylakoid membranes divided in
grana and stroma lamellae
