Type II


The photoreceptor consists of a multilayered membrane structure of photoreceptive protein. The eyespot is outwardly concave and is located close to this structure.

In Heterokontophyta, complex photoreceptors, consisting of layered electron-dense material organized in a rounded, wedge-shaped, or T-shaped organelle are present inside the smooth flagellum of the motile stages of Xanthophyceae, Eustigmatophyceae, and Phaeophyceae. In the Xanthophyceae, the eyespot consists of a single layer of about 40 globules located at one side of the anterior end of the chloroplast. It is contained by the outermost thylakoid of the chloroplast, bounded by the chloroplast envelope and its associated endoplasmic reticulum. The cell membrane above the eyespot forms a depression through which the posterior smooth flagellum passes. The eyespot depression accommodates the photoreceptor.

In the Eustigmatophyceae, the prominent eyespot occupies nearly the whole anterior part of the cell, adjacent to the flagellar insertion. It consists of a somewhat irregular collection of globules situated in a slight bulge of the zoospore, but not enclosed by a membrane. The anterior hairy flagellum bears a photoreceptor swelling which fits alongside the eyespot. In the Phaeophyceae, the eyespot is situated in the posterior part of the cell, inside a strongly reduced chloroplast, and behind a depression of the cell surface through which the posterior flagellum runs. The eyespot appears concave in shape and prominent, containing a single layer of about 60 globules. The photoreceptor swelling is localized at level of the eyespot.


Transmission electron microscopy image of the photoreceptive system of Ochromonas danica in longitudinal section, showing the photoreceptor inside the trailing flagellum. (Bar: 0.30 µm.)" />
FIGURE 2.67 Transmission electron microscopy image of the photoreceptive system of Ochromonas danica in longitudinal section, showing the photoreceptor inside the trailing flagellum. (Bar: 0.30 µm.)
The flagellate species of the Chrysophyceae possess eyespot within a chloroplast and closely associated with a flagellum. The eyespot anatomy is similar to the Xanthophyceae, while the photoreceptor consisting of a 3D assemblage of rhodopsin-like proteins, with a presumptive regular organization, is found in association with the smooth flagellum directly above the eyespot depression (Figure 2.67).
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FIGURE 2.67 Transmission electron microscopy image of the photoreceptive system of Ochromonas danica in longitudinal section, showing the photoreceptor inside the trailing flagellum. (Bar: 0.30 µm.)


In the members of the Euglenophyta 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 2.68). This organelle is a three-dimensional assemblage consisting of a stack of more than 100 membrane protein layers with a regular organization. First-order crystallographic analysis suggests a crystalline structure, with a monoclinic unit cell. Each layer has a height of about 70Å, which is the height of the model cell membrane (Figure 2.69). Figure 2.70 shows the isolated Euglena photoreceptor–PFR complex.

Transmission electron microscopy 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. (Bar: 0.50 µm.)
FIGURE 2.68 Transmission electron microscopy 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. (Bar: 0.50 µm.)
Transmission electron microscopy image of the photoreceptor of Euglena gracilis in skew section, showing the regular layer organization of this structure. (Bar: 0.20 µm.)
FIGURE 2.69 Transmission electron microscopy image of the photoreceptor of Euglena gracilis in skew section, showing the regular layer organization of this structure. (Bar: 0.20 µm.)
Scanning electron microscopy image of the isolated photoreceptor–PFR complex of Euglena gracilis. (Bar: 0.50 µm.)
FIGURE 2.70 Scanning electron microscopy image of the isolated photoreceptor–PFR complex of Euglena gracilis. (Bar: 0.50 µm.)

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