Page 128 - Algae Anatomy, Biochemistry, and Biotechnology

Anatomy 111

FIGURE 2.74 Dynamics of the light alignment of a model Euglena gracilis. The cell is represented by an
ellipsoid, the eyespot by a big spot, the photoreceptor by a small spot. The asterisk indicates the extension
of the flagellum and the tumbling of the cell.

CHLOROPLASTS
The photosynthetic compartment contains the pigments for absorbing light and channeling the
energy of the excited pigment molecules into a series of photochemical and enzymatic reactions.
These pigments are organized in proteic complexes embedded in the membrane of sac-like flat
compressed vesicles known as the thylakoids. These vesicles are about 24 nm thick, and enclose
a space, termed lumen, 10 nm wide.
In prokaryotes the thylakoids are free within the cytoplasm, while in eukaryotes they are
enclosed within bounding membranes to form the chloroplast. The colorless matrix of the chloro-
plast is known as the stroma. Inside the chloroplast, thylakoids are organized into two different
compartments, granal thylakoids, stacked into hollow disks termed grana; and stromal thylakoids
forming multiple connections between the grana. All thylakoids surfaces run parallel to the
plane of the maximum chloroplast cross-section. Chloroplasts contain nucleic acids and ribosomes.
DNA is naked, that is, not associated with proteins, and occurs in two configurations: scattered, but
not connected, small nucleoids or as a peripheral ring. Chloroplasts are semiautonomous organelles
that replicate their own DNA, and this replication is not linked to the division of the organelle. Syn-
thesis of RNA and proteins is possible inside the chloroplast, though they are not strictly