Euglena
Q.7 Give an illustrated account of the structure and life history of euglena. (2005)
Related Questions -
Q. Write note on structure of Euglena. (2010)
Q. Write short note on Life cycle of Euglena. (2014)
Q. Write the structure of Euglena. (2015)
Ans. Structure: -
Size and Shape: -
Euglena is a small microscopic organism measuring about 60 microns in length. The body is slender, elongated and spindle shaped. The anterior end is rounded or blunt ,the middle part is somewhat wider and the posterior end is pointed. From the anterior end arises a whip like flagellum which is seen moving when the Euglena is progressing forward. Euglena has following structures -
(i) Pellicle: -
The outer covering of Euglena is called pellicle. It lies beneath the plasma membrane .It consists of thin, firm, elastic strips. These strips fuse at both the ends of the cell body and each bears a groove along one edge and a ridge along the other. These striations are called myonemes. The pellicle is made up of a protein of fibrous elastic nature.
(ii) Reservoir: -
At the blunt anterior end of the body ,there is an invagination forming a flask shaped cavity. This Cavity consists of a wide reservoir or flagellar sac which leads through a canal, the cell gullet or cytopharynx to outside. Its external opening is called the cell mouth or cytostome.
(iii) Flagellum: -
At the anterior end of the body a single thread like flagellum arises through the cytostome.
Flagellum is paired structure, the other being smaller and confined within the reservoir. The two flagella originate from the two tiny granules the blepharoplasts or kinetosomes which lie embedded in the cytoplasm near the base of the reservoir. Each flagella consists of 9+2 arrangement of fibres. Each central fibre is single whereas the peripheral fibres are paired i.e.each is made up of two sub fibres.
(iv) Cytoplasm: -
It is divisible into two parts -
(a) Ectoplasm-a dense, clear and peripheral zone.
(b) Endoplasm-a fluid like, granular and central zone or the endoplasm. It consists of a number of inclusions which are as follows -
(1) Chloroplast or Chromatophores: -
These are large bodies which contain the green pigments chlorophyll a and b along with beta carotene. In E.viridis the chloroplast are elongated and appear to be radiating from the center of the body so as to form a star shaped grouping. In the center of the star shaped configuration there is probably a single proteinaceous pyrenoid surrounded by a number of small granules of paramylon.
The chloroplast contain groups of chlorophyll bearing lamellae or thylakoids which are arranged in groups each with three lamellae. These are placed in the matrix or stroma also containing ribosomes and fat globules.
(2) Paramylon: -
It is a poly saccharide similar to starch but not identical with it as it is not coloured with iodine solution. It is the reserve food material and is synthesized from the immediate product of photosynthesis and stored around the pyrenoid.
(3) Contractile Vacuole: -
It is surrounded by several smaller accessory vacuoles which probably
fuse together to form the larger vacuole. These vacuoles play a role in the discharge of water along with some waste products of metabolism to outside via the reservoir,cytopharynx and cytostome.
(4) Stigma: -
Lying near the reservoir at the level of the paraflagellar body on opposite side to that of contractile vacuole is a discoid orange–red body. The stigma which is also called the red spot or eye spot.
(5) Other Cytoplasmic Structures: -
For e.g. golgi bodies, the endoplasmic reticulum, mitochondria with tubular cristae, the ribosomes etc. are the other structures present.
(6) Nucleus: -
A single large,spherical or oval nucleus lies near the center of the endoplasm. The nuclear membrane is a double membrane. The nucleoplasm contains several nucleoli and a large number of granular and thread like chromosomes. Since these nucleoli persist during binary fission they are also called as endosomes.
LIFE HISTORY OF EUGLENA: -
Asexual Reproduction: -
It multiplies asexually by binary fission or multiple fission.
(1) Binary Fission: -
Euglena divides by a simple longitudinal binary fission,during active periods,under unfavourable conditions of water, temperature and food availability. The division is always symmetrogenic i.e.the parental euglena divides into two daughter individuals, where one is the plane mirror image of the other. First the nucleus divides into two by mitosis which is followed by the division of the cytoplasm (cytokinesis). Here the nuclear membrane remains persisted which is the unusual feature.It takes place as follows -
(i) Prophase: -
All the nucleoli (endosomes) fuse together into a single nucleolar body and each chromosome splits longitudinally into two daughter chromosomes or chromatids.
(ii) Metaphase: -
In this stage the paired chromatids come to lie in a longitudinal plane. The microtubules are present in the nucleus but they do not form a spindle.
(iii) Anaphase: -
It is marked by the segregation of paired chromatids and their movement towards their respective poles.
(iv) Telophase: -
In this stage, constriction in the nuclear membrane deepens and finally separates the nucleus into two daughter nuclei. Simultaneously the nucleolar body also undergoes division by simple splitting and takes place in the daughter nucleus of its own side.
Next follows the cytokinesis resulting into two daughter euglenae.
(2) Multiple Fission And Palmella Stage: -
Under inactive periods Euglena undergoes multiple fission in an encysted condition. The movement ceases altogether,the flagellum is thrown off and the euglena becomes rounded and embedded in an extensive thick and mucilaginous coat which is secreted by the muciferous bodies. Encystment is usually followed by repeated longitudinal binary fissions with the formation of several daughter individulas (16 or 32), embedded within a mucilaginous mass. Later these daughter individuals acquire flagella and escape to grow into adult euglenae.
(3) Encystment: -
It take place as a measure to tide over unfavorable conditions such as the lack of food and oxygen, draught, excessive heat etc. A cyst wall is secreted in the form of a thick, spherical, yellowish brown and gelatinous covering, composed of a spherical carbohydrate. When the conditions become favourable the animal becomes active and emerges from the cyst to resume its normal free-swimming life.
As already stated above the individual may undergo a single or several divisions resulting in two or many new individuals (palmella satge).
Q.8. Describe the physiology and Nutrition of Euglena. (2015)
Related Question-
Q. Describe the different types of nutrition found in Euglena. (2007, 09, 17)
Q. Describe nutrition in Euglena. (2019)
Ans. 1. Physiology: -
Locomotion: -
Euglena performs two different kinds of movements; (i) flagellar and (ii) euglenoid movement.
(i) Flagellar movement: -
The long highly contractile flagellum acts as a locomotory organ. It remains directed obliquely backwards towards the side bearing stigma. It undergoes spiral undulations which are passed from the base to the tip. The active lashing movements of flagellum produce waves of motion which induces the body forward with a spiral rotation or gyration along the axis.
(ii) Euglenoid movements: -
The pellicle being flexible permits the organism to perform worm-like wriggling movements of the body. A peristaltic expansion passes along the entire length of the body from the anterior to the posterior end and the animal progresses very slowly. This is known as euglenoid movement. The contraction of body is brought about by the contraction of the myonemes of ectoplasm.
2. Nutrition: -
Euglena exhibits mixotrophic type of nutrition as it feeds in more than one ways :
1. Holophytic or autotrophic nutrition: -
It is the chief mode of nutrition in Euglena. In bright sunlight Euglena synthesises its food from CO2 and water with the aid of chlorophyll (photosynthesis as found in plants). The synthesised food is a variety of starch, the paramylum, which is stored in the pyrenoid bodies or is found scattered as paramylum bodies.
2. Saporophytic or saprozoic nutrition: -
In the absence of sunlight, Euglena lives by the saprophytic or saprozoic method, which means the products or decaying organic substances dissolved in surrounding water are absorbed through its general body surface. It seems that Euglena secrets digestive enzymes that are typically animal-like in nature.
3. Holozoic nutrtion: -
Some species of euglena are recorded to feed on solid food particles (animal like) but is occurrence is doubtful.
4. Respiration: -
Euglena respires with the of free oxygen dissolved in water, which diffuses in through the pellicle. This oxygen brings about oxidation reactions. As a result of oxidation reactions, water and CO2 are formed as by products. In sun light, it is likely that this CO2 is utilized for photosynthesis, but in dark it is liberated to outside by diffusion through the general body surface.
5. Osmoregulation: -
Being a water animal the water is continually enters into its body by osmosis and the excess water of the cytoplasm is continually expelled outside the body through contractile vacuole. The water regulation process is known as osmoregulation this process is completed in two stages i.e., (a) diastole in which the contractile vacuole is filled with water and (b) syotole when the water of contractile vacuole is discharged into reservoir.
6. Excretion: -
Ammonia, the nitrgenous waste product, resulting from catabolism, passes out by diffusion through the general surface of the body. Excretory substances may also be emptied by the contractile vacuole into the reservoir.
Q.9. Give reasons for keeping Euglena in animal kingdom.
Ans. Euglena is studied as an animal as well as a plant. It is more an animal than a plant.
Following characters are responsible for keeping euglena in animal kingdom :
(a) Absence of a cellulose cell-wal overlying the plasma membrane.
(b) Presence of centrioles forming the blepharoplasts or kinetosomes.
(c) Reserve food is paramylon which is not a true starch.
(d) Presence of paraflagellar body, a sensory (photoreceptive) organelle.
(e) Moves from place to place like an animal.
(f) Responds to various stimuli like an animal.
(g) Pinocytosis and probably holozoic nutrition takes place.
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