Both the surface structure of the Cryptophyta and that of the Euglenophyta can be grouped under
this type. The main diagnostic feature of the members of the Cryptophyta is their distinctive kind of
cell surface, colloquially termed Periplast. Examples are Chroomonas (Figure 2.20) and Cryptomonas;
in these algae the covering consists of outer and inner components, present on both sides of the membrane, variable in their composition. The inner component comprises protein and may consist
of fibril material, a single sheet or multiple plates having various shapes, hexagonal, rectangular,
oval, or round. The outer component may have plates, heptagonal scales, mucilage, or a combination
of any of these. The pattern of these plates can be observed on the cell surface when
viewed with SEM and freeze-fracture TEM, but it is not obvious in light microscopy view.
|FIGURE 2.20 Periplast of Chroomonas sp.
Euglenophyta possess an unusual membrane complex called the pellicle, consisting of the plasma
membrane overlying an electron-opaque semicontinuous proteic layer made up of overlapping strips.
These strips or striae that can be described as long ribbons that usually arise in the flagellar pocket and
extend from the cell apex to the posterior. Each strip is curved at both its edges, and in transverse
section it shows a notch, an arched or slightly concave ridge, a convex groove, and a heel region
where adjacent strips interlock and articulate. The strips can be arranged helically or longitudinally;
the first arrangement, very elastic, is present in the “plastic euglenids” (e.g., Euglena, Peranema, and
Distigma), either heterotrophic or phototrophic, where the strips are more than 16. Their relational
sliding over one another along the articulation edges permits the cells to undergo “euglenoid movement”
or “metaboly.” This movement is a sort of peristaltic movement consisting of a cytoplasmic
dilation forming at the front of the cell and passing to the rear.
The return movement of the cytoplasm
is brought about without dilation. The more rigid longitudinal arrangement is present in the “aplastic
euglenids” (e.g., Petalomonas, Pleotia, and Entosiphon), all heterotrophic, where the strips are
usually less than 12. These euglenids are nor capable of metaboly.
The ultrastructure of the pellicular complex shows three different structural levels
- The plasma membrane with its mucilage coating (first level)
- An electron-opaque layer organized in ridges and grooves (second level)
- The microtubular system (third level)
|FIGURE 2.21 Transmission electron microscopy image of the surface of Euglena gracilis in transverse
section, showing the three different structural levels of the pellicle. Arrows point to the first level (mucus
coating); a square bracket localizes the second level (ridges and grooves); arrowheads point the third level
(microtubules). (Bar: 0.10 µm.)