Euglena viridis

Euglena viridis

Classification

Phylum- Protozoa

Class- Mastigophora

Genus- Euglena

Species- E.Viridis

Euglena viridis (Gr., eu, true+glene, eye-ball or eye-pupil +L., viridis, green), 

as the name implies, is a green organism with an eye-like photoreceptive structure.

Euglena is a phytoflagellate as it possesses both the chloroplasts as well as the flagella. 

It is autotrophic in sunlight, but becomes heterotrophic in the dark. 

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HABITAT

Euglena viridis is a solitary and free-living freshwater flagellate. It occurs in freshwater ponds, pools, ditches and slowly-running

streams. It is found in abundance, where there is a considerable amount of vegetation. 

Ponds in well-maintained gardens, containing decaying nitrogenous organic matter, such as faeces of animals, leaves, twigs, etc.

STRUCTURE

1. Size and shape

E. viridis is a small microscopic organism measuring about 60μ (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.

2. Pellicle

The body of Euglena is enveloped by organized structures of the cell surface, the pellicle. It lies beneath the plasma membrane. 

The pellicle is made up of a protein of fibrous elastic nature. The whole complex is a part of the living cell .

3. 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. The reservoir is lined by plasma membrane without pellicle beneath it.

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4. Flagellum

At the anterior end of the body, a single and thread-like flagellum arises through the cytostome.

The flagellum is not just one but is paired, the other being smaller aud confind within the reservoir. 

The two flagella originate from two tiny granules, the blepharoplasts or kinetosomes which lie embedded in the cytoplasm near the base of the reservoir. 

The long flagellum bears a lateral swelling, the para flagellar body near its base, within the reservoir. 

This body acts as a photoreceptor and probably contains lactoflavin as sensitizer.

Each flagellum consists of 2 central and 9 peri pheral fibres. 

Each central fibre 15 single, whereas the peripheral fibres are paired, that is, each is made of two sub-fibres. 

The central fibres are enclosed in an inner membranous sheath. Of cach peripheral fibre, one of the two sub-fibres bears a double row of short projections or, arms, all pointing in the same direction. 

The whole flagellum is enveloped by an outer sheath which is continuous with the plasma membrane. 

In the space between the peripheral and central fibres lie 9 secondary fibres which are somewhat inconspicuous. 

Alongside this boundle of fibrils runs a rod-like structure. 

On the long axis of the flagellum is found a unilateral row of hair-like contractile processes called mastigonemes. 

5. Cytoplasm

The pellicle encloses the cytoplasm which is divisible into two zones a dense, clear and pheriperal zone or the ecto plasm and a fluid-like, granular and central zone or the endoplasm. 

The endoplasm contains a number of inclusions, which are as follows:

(a) Chloroplasts or chromatophores

These are large bodies which contain the green pigments, chlorophyll a and b, along with B-Carotene. 

In E. viridis, the chloroplasts are elongated and appear to be radiating from the centre of the body so as to form a star-shaped grouping. 

In the centre of the star-shaped configuration there is probably a single proteinaceous pyrenoid, surrounded by a number of small granules of paramylon.

The chloroplasts contain groups of chloro phyll 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.

 Each chloroplast is bounded by a triple membrane envelop .

(b) Paramylon

The endoplasm contains several granules of paramylon or paramylum which is a polysaccharide (B-1, 3 glucan) similar to starch but not identical with it, as it is not coloured blue with iodine solution. 

It is the reserve food material and is synthesized from the immediate product of photosynthesis and stored around the pyrenoid.

(c) Contractile apparatus

Associated with the reservoir occurs a dense osmoregulatory zone. It includes a large central 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.

(d) Stigma

Lying near the reservoir, at the level of the paraflagellar body, on opposite side to that of the contractile vacuole, is a discoid orange-red body, the stigma, which is also called the eye-spot or red-spot. 

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(e) Other cytoplasmic structures

The cytoplasm contains other subcellular organelles as well. 

The Golgi bodies are piles of large flattened sacs with minute vesicles pinching off from them. 

The endoplasmic reticulum is in the form of small interconnecting tubules and vesicles.

The mitochondria are with tubular cristae and are near the reservoir. 

The more in number ribosomes occur scattered freely in the endoplasm and also on the endoplasmic reticulum and in the chloroplasts.

6. Nucleus 

A single, large, spherical or oval, vesicular nucleus lies near the centre of endoplasm, usually towards the posterior end of the body. 

The nuclear membrane is a double membrane which is perforated by pores. 

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 referred to as endosomes.

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