Beneath the mucus coating, there is the plasma membrane (Figure 2.25). This cell membrane is continuous and covers the ridges and grooves on the whole cell and can be considered the external surface of the cell. The protoplasmic face (PF) of the plasma membrane shows that the strips are covered with numerous peripheral membrane proteins of about 10 nm.
There is a consistent number and arrangement of microtubules associated with each pellicular strip, which are continuous with those that line the flagellar canal and extend into the region of the reservoir. Within the ridge in the region of the notch there are three to five, usually four, microtubules about 25 nm diameter running parallel along each strip. Two of these are always close together and are located immediately adjacent to the notch adhering to the membrane.
The lack of protein organization in the groove regions gives higher plasticity to these zones, and together with presence of parallel microtubules in the ridge regions gives the characteristic pellicular pattern to the surface of euglenoids.
The solid structure of the pellicle confers a very high degree of flexibility and resistance to the cells. Our experience with E. gracilis allow us to say that this alga possesses one of the strongest covering present in these microorganisms. A pressure of more than 2000 psi is necessary to break the pellicular structure of this alga.
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