Page 115 - Algae Anatomy, Biochemistry, and Biotechnology

98 Algae: Anatomy, Biochemistry, and Biotechnology

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gametes of Ulva; its area can range from about 0.3 to 10 mm . The globules range from 80 to
190 nm in diameter and their number varies from 30 to approximately 2000. The most common
organization consists of a single layer of closely packed globules lying between the outermost thy-
lakoid and the two-layered chloroplast membrane. Additional layers of globules can be present
underneath the first layer, individual layers subtended or not by a single thylakoid. In most
species the globules show a hexagonal packing, which enable the highest possible packing
density. In both Pandorina and Volvox colonies, the eyespot of cells in the anterior of the
colony are larger than those of the posterior, consisting of up to nine layers, marking the occurrence
of some degree of colony polarity. The photoreceptor of Chlamydomonas can be considered the
model of Type I photoreceptor. It consists of an extensive two-dimensional patch of photosensitive
proteins, identified as rhodopsin-like protein, localized in the plasma membrane overlying the
eyespot. The layered structure of the shading organelle in this type of photoreceptor works as a
quarter wave interference reflector that reflects the impinging light toward the photoreceptor, in
order to increase detectability of the light signal.

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 flagel-
lum of the motile stages of Xanthophyceae, Eustigmatophyceae, and Phaeophyceae. In the Xantho-
phyceae, 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.
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 photo-
receptor 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).
In the members of the Euglenophyta the eyespot consists of a loose collection of globules situ-
ated 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 sur-
rounding 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 orga-
nelle 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 struc-
˚
ture, with a monoclinic unit cell. Each layer has a height of about 70 A, which is the height of the
model cell membrane (Figure 2.69). Figure 2.70 shows the isolated Euglena photoreceptor–PFR
complex.