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Anatomy
In the members of the Euglenophyceae, the eyespot consists of a loose collection of globules
situated on the dorsal side of the reservoir, the anterior invagination characteristic of these organisms. The globules vary in size (from 240 to 1200 nm) and number, can lie in a single layer, or be
bunched together. Individual globules may be membrane-bound, but there is never a membrane
surrounding the whole complex, and no association with any chloroplast component is present.
The position of the eyespot within the reservoir region can vary among the species, but it is
always in front of the photoreceptor situated on the long or emergent flagellum (Figure 2.72). As
example, the photoreceptor of Euglena gracilis will be described. This organelle is a 3D ordered
assemblage of stacked membranes formed by 2D crystals of photoreceptive proteins. The organelle is located inside the membrane of the locomotory flagellum, connected to its axoneme by the
paraxial rod. Figure 2.73a shows a portion of the photoreceptor at high magnification: the ordered
disposition of the membrane layers is evident, made easily recognizable by osmium tetroxide
staining of the phospholipid head groups. The periodicity is about 50 Å comparable to the distance between the hydrophilic head layers of a typical membrane bilayer. About 50 membrane
layers are present in the photoreceptor. Electron micrograph of a negatively stained 2D lamellae, obtained by ionically induced uncoupling of the 3D compact structure of the photoreceptor,
reveals stain excluding units protruding from the surface of the layer, arranged into a regular
mesh (Figure 2.73b). After Fourier analysis, this mesh shows the ordered patches formed by the
oligomers of the photoreceptive membrane spanning protein assembled in a hexagonal lattice as
visible in the gray scale contour plot (Figure 2.73c), which is a magnification of the central zone
of Figure 2.73b. The membrane layers composing the photoreceptor are characterized by in-plane
hydrophobic interactions, while their closely stacked disposition is due to the interlayer interactions between charged protein extramembrane domains and the membrane in adjacent layers
through charge density matching. About 10 6 photoreceptive proteins assembled in a hexagonal
lattice span the membrane layers. Because of these characteristics, the photoreceptor of Euglena
as a whole can be defined a “Type I” crystal, that is, an ordered assemblage of stacked membranes formed by 2D crystals of membrane proteins. The Euglena photoreceptor structure is an
example of a perfect device to absorb light with a maximal cross-section (inset in Figure 2.73a).
Figure 2.74 shows the isolated Euglena photoreceptor–PFR complex.
Type iii
This organization is present only in some dinoflagellates of the order Warnowiales, such as
Nematopsides sp. and Erytropsidinium sp. The photoreceptor system is very specialized and is
FIGURE 2.72 TEM image of the photoreceptive system of Euglena gracilis in transverse section, showing
the photoreceptor and the PFR inside the trailing flagellum, and the eyespot in front of them inside the cell.
Scale bar, 0.50 μm.
Anatomy
In the members of the Euglenophyceae, the eyespot consists of a loose collection of globules
situated on the dorsal side of the reservoir, the anterior invagination characteristic of these organisms. The globules vary in size (from 240 to 1200 nm) and number, can lie in a single layer, or be
bunched together. Individual globules may be membrane-bound, but there is never a membrane
surrounding the whole complex, and no association with any chloroplast component is present.
The position of the eyespot within the reservoir region can vary among the species, but it is
always in front of the photoreceptor situated on the long or emergent flagellum (Figure 2.72). As
example, the photoreceptor of Euglena gracilis will be described. This organelle is a 3D ordered
assemblage of stacked membranes formed by 2D crystals of photoreceptive proteins. The organelle is located inside the membrane of the locomotory flagellum, connected to its axoneme by the
paraxial rod. Figure 2.73a shows a portion of the photoreceptor at high magnification: the ordered
disposition of the membrane layers is evident, made easily recognizable by osmium tetroxide
staining of the phospholipid head groups. The periodicity is about 50 Å comparable to the distance between the hydrophilic head layers of a typical membrane bilayer. About 50 membrane
layers are present in the photoreceptor. Electron micrograph of a negatively stained 2D lamellae, obtained by ionically induced uncoupling of the 3D compact structure of the photoreceptor,
reveals stain excluding units protruding from the surface of the layer, arranged into a regular
mesh (Figure 2.73b). After Fourier analysis, this mesh shows the ordered patches formed by the
oligomers of the photoreceptive membrane spanning protein assembled in a hexagonal lattice as
visible in the gray scale contour plot (Figure 2.73c), which is a magnification of the central zone
of Figure 2.73b. The membrane layers composing the photoreceptor are characterized by in-plane
hydrophobic interactions, while their closely stacked disposition is due to the interlayer interactions between charged protein extramembrane domains and the membrane in adjacent layers
through charge density matching. About 10 6 photoreceptive proteins assembled in a hexagonal
lattice span the membrane layers. Because of these characteristics, the photoreceptor of Euglena
as a whole can be defined a “Type I” crystal, that is, an ordered assemblage of stacked membranes formed by 2D crystals of membrane proteins. The Euglena photoreceptor structure is an
example of a perfect device to absorb light with a maximal cross-section (inset in Figure 2.73a).
Figure 2.74 shows the isolated Euglena photoreceptor–PFR complex.
Type iii
This organization is present only in some dinoflagellates of the order Warnowiales, such as
Nematopsides sp. and Erytropsidinium sp. The photoreceptor system is very specialized and is
FIGURE 2.72 TEM image of the photoreceptive system of Euglena gracilis in transverse section, showing
the photoreceptor and the PFR inside the trailing flagellum, and the eyespot in front of them inside the cell.
Scale bar, 0.50 μm.