Pellicle of Protozoa Sem - I Paper-101

Pell

Dr Biplob Kr Modak| 1

Pellicle of Protozoa Sem - I Paper-101

PELLICLE

The pellicle is comparatively thicker, tuff elastic and proteinous in nature. It helps in maintaining

general shape of the protozoans and performs the usual function.

The skeletal layers are secreted in still other protozoans in which their protoplasmic body

remains protected. These include cyst, theca, lorica and test or shell.

The protozoans body is usually enclosed only by the cell membrane. The rigidity or flexibility of

the body and its shape are largely determined by the cytoskeleton, which typically is located just

below the cell membrane. The cytoskeleton and cell membrane together form the pellicle, a short

of protozoan “body wall.”

The cytoskeleton often is composed of protein filaments/microfilaments (e.g. Actin),

microtubules, vesicles (such as alveoli), combination of all three.

1. Microfilaments (epiplasmic microfilaments):

This protein filaments may form a dense mesh in the outer most cytoplasm (e.g.- Euglena).

2. Microtubules:

These are conspicuous cytoskeletal structure, mostly found in flagellates, apicomplexan and

ciliates. They may arranged in different ways.

a. They can be arranged as a microtubular corset. e.g. Euglenoid flagellates.

b. The microtubules can originate on the flagellar basal bodies and radiate rear ward to the

opposite extremity of the cell as a short of axial skeleton (axostyle). Such microtubules

Body covering of protozoa:

The body envelop, in protozoans, may be either

plasmalemma or pellicle. In some species like Amoeba

proteous, the body envelope is a thin plasma membrane or

plasmalemma which is mucopolysaccharide in nature. It

helps in adhesion to the substratum and in the exchange of

various materials.

Pell



resemble the microtubules of the mitotic spindle which radiates from centrioles and form

the mitotic apparatus. e.g. Flagellates.

c. Bundle of microtubules may radiate from a centroplast (as MTOC) at the cell’s center

and then extend into and support a ray like projection of the cell’s surface (axopod). The

centroplast and its microtubules resemble the star like asters that form around centrioles

at the poles of the mitotic spindle. e.g. observed in spherical Radiolarian and Heliozoan.

[MTOC: A region around basal bodies and centriole called the microtubules organizing center,

controls the polymerization of microtubules, while the basal body is the template on which

developing axonemes are originated.]

3. Alvioli/Vesicles:-

The vesicles or alveoli occur immediately below the cell membrane in many protozoans, such as

Dianoflagllates, apicomplexans and ciliates. (These three groups are collectively called as

alveolata).

a. In ciliates, alveoli may be turgid and help to support the cell but they also stored Ca++,

which can be released to trigger cellular responses

b. In some Dianoflagellates, plates of cellulose secreted into the alveolar vesicles and form a

rigid endoskeleton.

Protozoan skeleton like those of metazoans can also be endo- or exoskeleton. A skeleton that

forms a more or less complete covering whether internal or external is called as test (cyst, lorica,

theca or shell).

Pell



The ciliate body is typically covered by a complex pellicle. Below the outer cell membrane, there

is a single layer of small membranous sac, the alveoli. Each of which is moderately to greatly

flattened. Cilia emerged from between adjacent alveoli. Alveoli have skeletal functions and also

stored Ca++ ions. Following an appropriate stimulation of the cell, these ions are released into the

cytoplasm, where they can initiate changes in ciliary beat or discharge of extrusomes.

Extrusomes: These are secretory bodies specialized for rapid releases at the surface of the cell.

These are following kinds-

A. Trichocyst: In Paramecium species and other ciliates, bottle-shaped extrusomes,

trichocyst, alternate to the alveoli is present. In the discharged state, the trichocyst is

perpendicular to the body surface. At discharge, the trichocyst rapidly ejects a long

striated thread like shaft, surrounded by a spike or barb. The shaft is non evident in the

undischarged state and probably polymerizes during discharge. Trichocyst appears to

function in defense against predators.

1. Beneath the pellicle and alternating with alveoli or basal bodies, are present numerous

pelicular, tiny, spindle-shaped or bottle shaped organelles, the trichocysts or trichites.

2. These are arranged perpendicularly to the body surface so that the ectoplasm looks

striated.

3. Each trichocyst is a minute organelles measuring 4µ X 2µ. It opens to the exterior

through a minute pore just above it in the pellicle.

4. The trichocyst contains a shaft with a terminal pointed tip, called the spike or barb,

covered by a cap.

5. If the animal is irritated, each trichocyst may be discharged to the exterior in only a

few milliseconds, as an elongated fine, needle-shaped thread on the body surface.

6. The true nature and function of the trichocysts are uncertain. It is believed that the

discharged thread serves as organs of adhesion, anchoring the Paramecium to a firm

substratum while it feeds on bacteria.

7. But, individual, which are discharged all the trichocysts cannot regenerate a new set

immediately.

Pell



Pell



B. Toxicyst: These are extrusomes found in pellicle of Dileptus and Didinium. A toxicyst

discharge a long thread with a bulbous base containing a toxin. Toxicysts are used for

defense and for capturing prey. They are commonly restricted to the parts of the ciliate

body that contact prey, such as around the cytosome in Didinium or the anterior body

region of Dileptus.

C. Mucocyst: These are arranged in rows like trichocyst and discharge a spray or network of

mucoid filaments. These may function in the formation of protective cysts or provide a

sticky surface for prey capture. They occur in many ciliates, including Didinium.

Most of the protozoans maintain a constant and characteristic body form. It is due to the

secretion of a definite covering called pellicle or periplast, around their body by the superficial

layer of the cytoplasm. It is underlined by the plasma membrane and is formed of proteins. The

pellicle maintains itself under mechanical stresses.

The pellicle of ciliates is a relatively thicker and more definite layer, often flexible but firm

enough to maintain the typical body form of the ciliates. It may be variously decorated or

sculptured with ridges, papillae, hexagonal depressed areas (Paramecium), elevated plates

(Coleps), nodules (Vorticella) or other markings.

A dense pellicle is often called cuticle. While pellicle is a living membrane, the cuticle is a dead

membrane applied closely to the surface of the body. It may be nitrogenous, as in Monocystis, or

of carbohydrate, as in many Phytomastigina.

Skeletal Structure of Protozoa:

In protozoa the surface of protoplasm is protected by many shorts of body covering like shell and

skeleton in addition to protoplasm and pellicle, which also determined to a great extent the form

of the body. The following kinds of skeletal structures or envelopes are generally observed in

case of protozoans.

Pell



a. Cyst: It is an external temporary sheath formed by free living and parasitic individuals

comfortably tide over the environmental hazards.

b. Theca: A special type of skeletal layer found in many dinoflagellates like Ceratium and

Glenodinium. It is a coat of closely-fitted hardened armour of cellulose, comparable to the thick

cell wall of higher plants. The theca in majority of dinoflagellates is differentiated into a number

of plates arranged into a definite pattern, but in some forms, it may be formed of two valves.

c. Lorica: It is coat of less-closely fitted armour of protozoans than the theca. The lorica is

usually vase-shaped or tubular having an opening for the emergence of the anterior part of

the animals or its appendages. The base of the lorica is either attached directly to the

substratum (in sessile individuals like Salpingoeca) or it may terminate in a stalk like

Monosiga. In colonial loricated protozoans, one lorica may be attached to anterior lorica

directly as in Dinobryon or one lorica may be attached to another lorica by a stalk as in

Poteriodendron.

d. Shell or Test: The shells/tests are wide spread among protozoans. These are loose armour

with one or more openings over the body of protozoans like Arcella, Difflugia, Euglypha etc.

these are following kinds:

1. In Arcella the shell is thin and composed of Pseudochitin and ventrally it has an

aperture from which 3 or 4 pseudopodia project out.

Pell



2. In Diffugia the shell is made of sand and other foreign substances like fragments

foraminiferan’s shell and sponge spicules. These foreign substances get embedded in

a secreted matrix by the animal, working like cement, to form the shell.

3. The foreminiferan’s shell are made of calcium carbonate, while shells found in some

rizopods like Euglypha are siliceous being made of silica.

Pell



4. The radiolarian’s shells are internal skeletal layer lying between ectoplasm and

endoplasm. It forms a central capsule which is composed of pseudochitin or silica or

strontium sulphate and secreted by the cytoplasm. The central capsule is perforated

by one to many pores through which the extra capsular cytoplasm extends out as fine

pseudopodia.

Pell



Pell



Documents

Pellicle of Protozoa Sem - I Paper-101