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Term Paper on Euglena

Term Paper Contents:

  1. Term Paper on the Habits and Habitat of Euglena
  2. Term Paper on the Culture of Euglena
  3. Term Paper on the Morphology of Euglena
  4. Term Paper on the Physiology of Euglena
  5. Term Paper on the Excretion of Euglena
  6. Term Paper on the Behaviour of Euglena
  7. Term Paper on the Reproduction in Euglena
  8. Term Paper on the Encystment of Euglena
  9. Term Paper on the Position of Euglena

Term Paper # 1. Habits and Habitat Euglena:

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Euglena is a simple free-living flagellate. It is found in stagnant water containing nitrogenous substances. Sometimes, their number increases so much that they form a green layer on the surface of the water. Many species of Euglena are found but Euglena viridis is more common, hence, is generally studied in the class.

Euglena viridis is formed of two Greek and one Latin word. Gr. Eu-True and Glene-Eye pupil and Latin – Virids-Green. This means a green-coloured animal which possesses an eye-like photoreceptive structure.

Term Paper # 2. Culture of Euglena:

Euglena may be cultured in the laboratory easily by the following method. Boil some cow or horse dung in distilled water in a jar and allow it to cool for two days. Then put some weeds from a pond containing Euglenae into the jar and place the jar near the well-lighted window. In a few days Euglenae will appear in this nitrogenous infusion.

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Term Paper # 3. Morphology of Euglena:

1. Shape and Size:

Euglena viridis is green, slender, elongated and spindle-shaped in appearance. Its anterior end is bluntly rounded, the middle part is wider and posterior end is pointed.

Euglena viridis is about 40-60 µm in length and 14-20 µm in breadth at the thickest part of the body.

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2. Pellicle:

A very fine membrane called pellicle is present around the body. It has spiral striations. These striations are short in Euglena spirogyra. Although the pellicle is tough and strong but it is very thin and elastic due to which, if needed, Euglena can change the shape of its body. According to Chadefaud, the pellicle is two layered.

The outer thin layer is known as the epiculticle and the inner thick layer is called the cuticle. Pellicle consists of a number of thickened longitudinal strips and micro-fibrils (microtubules) articulating with each other. It consists of parallel helical striations or sculpturing which can be observed by electron microscopy.

These striations extend counter-clockwise from anterior to posterior ends of body and involve longitudinal rigid strips which are interlocked with each other along their margins. When minute fibrils, called myonemes, occurring just beneath the pellicle in the cytoplasm contract, they cause the strips of pellicle to slide over one another and affect a change in body shape of Euglena.

3. Cytostome and Cytopharynx:

Near the anterior end is a funnel-like depression, called cell-mouth or cytostome, which leads into a short, tubular cell-gullet or cytopharynx. It is often mistakenly called gullet. Cytopharynx passes into an enlarged, spherical permanent cavity or vesicle in the protoplasm, which is called reservoir.

4. Flagella:

At the anterior blunt end of the body is seen a single, long and delicate, protoplasmic process, the flagellum, which bifurcates and terminates into two belpharoplasts. Manter and Millier (1959) are of the opinions that the flagellum arises from a small granule, the blepharoplast. Hollande (1942), Pringsheim (1948) held that, in addition to a long flagellum, there is also found a short flagellum in the neck of reservoir, which extends out of the cytostome.

Some protozoologists and authors viz., Villee Walker and Smith (1963) are of the opinion that the flagellum is formed by fusion of two flagella. Kozloof (1972) describes, “There are two unequal flagella. The short flagellum does not emerge from the reservoir and its tip may be applied to the longer flagellum in such a way that there may seem to be just one flagellum with two roots”.

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Flagellum is a contractile axial filament enclosed in a protoplasmic sheath which arises from blepharoplasts. Whip­-like lashing movements of the flagellum do not move the animal from behind, but draw it forward. Hence it is also called tactellum. We already know that there are actually two flagella. One flagellum is long, root. However, after recent researches it is almost certain Euglena the flagellum is mentioned as possessing a bifurcated while the other is short.

The short flagellum is secondarily associated. On one of the roots of the flagellum there is a swelling known as photoreceptor, which is sensitive to light. Kozloff (1972) describes, “Adjacent to the reservoir is a bright red mass, the stigma”. This consists of small granules of carotenoid pigment embedded in a colourless stoma. The actual photoreceptor appears to be the swelling noted in the longer flagellum at about the stigma.

The flagella and their basal granules are fibrillar structures; each flagellum contains 11 (eleven) longitudinally arranged microtubular fibres which are arranged in a specific pattern of two central singlet fibres and nine peripheral doublet fibres (9 + 2). Only the 9 peripheral fibres extend into the basal granule.

The distal part of the long flagellum bears along the one margin a row of short, hair-like, contractile fibrils or processes, called mastigonemes. These mastigonemes are suggested to arise from two of the nine peripheral fibres.

5. Reservoir:

The tip of the anterior end contains a small funnel-like opening, the cell mouth or cytostome. It does not serve for food capture. From the cytostome a characteristic, short, tubular or flask-shaped cell gullet or cytopharynx leads into the body. It expands basally into a large permanent spherical vesicle or reservoir. The name “gullet” is a misnomer because Euglena does not ingest solid food, nutrition.

6. Stigma:

Near the reservoir, and on the side opposite to that of the contractile vacuole, is also found a small but conspicuous discoid red spot, called the eye-spot or stigma. It contains a pigment, called haematochrome, which is sensitive to light. The pigment is found in the form of fine granules embedded in a matrix. According to Leedale (1966), the stigma is composed of oil droplets containing a red pigment, the carotenoid. The stigma and the paraflagellar body, together form a photoreceptor apparatus.

7. Cytoplasm and Organelles:

The cytoplasm of Euglena is differentiated into a thin, clear and dense outer layer of ectoplasm and a more fluid and granular inner (central) layer of endoplasm. Endoplasm contains a variety of cellular organelles such as stigma, chloroplasts (chromatophores), mitochondria, nucleus, contractile vacuole and also usual cellular organelles such as endoplasmic reticulum, Golgi apparatus, ribosomes, etc.

(i) Nucleus:

A single, large, spherical, vesicular nucleus lies in the middle or toward posterior end of body. It contains chromosomes (in the form of chromatin) and a few large endosomes or nucleoli.

(ii) Contractile Vacuole:

A single large contractile vacuole lies near the reservoir on one side (opposite to stigma). It is surrounded by several minute accessory contractile vacuoles which fuse together to form the large vacuole. The contractile vacuole discharges its watery contents into the reservoir; from there fluid escapes through cytopharynx and cytostome.

8. Chromatophores:

Many green coloured chloroplasts of different shapes are found in the cytoplasm. Some of these are disc-shaped or oval or star-like and some are band-like. In Euglena viridis, a group of star-shaped thin chloroplasts is found in the middle of the body. In the centre of each chloroplast is present a structure called pyrenoid which is surrounded by a shell of paramylum. Paramylum is a glycogen-like carbohydrate (β-1, 3 glycan). It is also found in the cytoplasm as large granules. Their shape varies in different species.

9. Paramylon:

The endoplasm contains numerous scattered small refractile granules of paramlon or paramylum. It is a polysaccharide (β-1, 3 glucan) similar to starch but it is not coloured blue with iodine solution. The paramylon bodies are light bluish-green in contrast to bright grass-green chloroplasts.

10. Other Endoplasmic Inclusion:

The endoplasmic reticulum is formed by interconnecting minute vesicles and tubules. The Golgi bodies consist of several elongated flattened sacs with rounded vesicles pinching off from their ends. The mitochondria are more abundant near the reservoir and bear tubular cristae. The ribosomes occur scattered as well as on the endoplasmic reticulum and inside the chloroplasts.

In some species, a fine fibril, the rhizoplast, connects the blepharoplast with a granule near the nucleus, connected with a chromatin body near the nuclear membrane. It suggests a miniature neurometer organelle that may function in relation to flagellar movement.

Term Paper # 4. Physiology of Euglena:

1. Locomotion:

There are two methods of locomotion in Euglena:

(i) Flagellar Movement:

Euglena moves by the lashing of flagellum and by the movement of the whole body. When Euglena swims, the flagellum trails obliquely to the rear and then is thrown into waves. As such, it is evident that movement of the flagellum is not only by the lashing of whip. When flagellum is removed from the body, it can continue lashing provided that blepharoplast remains attached.

When Euglena moves forward, it rotates on its axis and also revolves. These movements are due to the reaction of the body to the force produced by the heating of flagellum. Euglena, thus, moves forward through the water. Flagellar movements of Euglena are explained by three main views.

Butschli explained this type of movement by an action of screw causing a propelling action, pulling the animal forward. Metzmer concluded that a simple flagellar beat in a circle causes sufficient current to pull the animal forward.

Ulehla (1911) and Krijaman (1925) stated that the movement of flagellum is sidewise, consisting of an effective down-stroke and a relaxed recovery-stroke in which flagellum is brought forward again. Thus, the animal moves forward and rotates on its longitudinal axis.

According to Butschi, the flagellum undergoes a series of lateral movements and in doing so a pressure is exerted on the water at right angles to its surface. This pressure creates two forces- one directed parallel and the other at right angles to the main axis of the body. The parallel force will drive the animal forward and the force acting at right angles would rotate the animal on its own axis.

According to Gray a series of waves pass from one end of the flagellum to the other. These waves create two types of forces, one in the direction of the movement and the other in the circular direction with the main axis of the body. The former will drive the animal forward and the latter would rotate the animal.

According to Lowndes the flagellum is directed backwards during locomotion. According to him, a series of spiral waves pass successively from the base to the tip of the backwardly directed flagellum at about 12 per second with increasing velocity and amplitude. The waves proceed along the flagellum in a spiral manner and cause the body of the Euglena to rotate once in a second.

Paddle Strokes:

During rapid locomotion, the flagellum performs sideways lashes or paddle strokes. Each stroke consists of an effective stroke, backwards with the flagellum held out rigidly. In recovery stroke, flagellum is relaxed, strongly curved so as to offer the least resistance to water, and brought forward again.

The movement of the flagellum involves contraction of its nine peripheral fibres. The energy for contraction is supplied by the ATP (adenosine triphosphate) present in the mitochondria in the blepharoplasts. The function of the mastigonemes found on the flagellum remains unknown.

(ii) Euglenoid Movement:

This is a characteristic slow and limited movement of Euglena called metaboly or euglenoid movement. Upon a solid substratum, Euglena slowly wriggles like a worm by means of peristalsis, i.e., waves of contraction and expansion sweeping over the body from anterior to posterior ends and animal moves forward. Peristalsis is performed by the help of myonemes which are located just beneath the pellicle.

2. Nutrition:

The mode of nutrition in Euglena is mixotrophic i.e., no simple mode of nutrition is sufficient for maintaining life in Euglena.

Following types of nutrition is seen:

(a) Holophytic or Autotrophic:

This is the main method of nutrition in Euglena. Chlorophyll is found in the chromatophores present in the body of Euglena. Euglena manufactures its own food by photosynthesis with the help of chlorophyll in the presence of sunlight, carbon-di-oxide and water. The chlorophyll breaks carbon-di-oxide into carbon and oxygen. This carbon combines with water to from a carbohydrate called paramylum, the excess of which is stored in pyrenoid bodies.

(b) Holozoic or Heterotrophic (Animal-Like):

Some species of Euglena are able to ingest solid particles, which pass down the gullet into the body and are assimilated. Movements of the flagellum create a whirlpool by which minute fragments are forced to enter into the cytostome. This kind of nutrition is similar to that of typical animals and is called halozoic.

Several zoologists doubt this method of nutrition but others are more or less certain about it. Hall (1939) established that Euglena lacks no solid food but maintains its existence by holophytic and saprozoic types of nutrition. Kozloff (1972) mentioned that “Euglena is not known to ingest particulate organic material but can utilize organic nutrients in solution”.

(c) Saprophytic or Saprozoic:

It feeds on decaying organic substances like fungi. Euglena, if it is kept in the solution of carbohydrate and nitrogenous compound in the dark, persists and multiplies rapidly but chlorophyll is lost. This shows that Euglena continues its existence by saprophytic nutrition by which the nutritive materials are dissolved into water and are absorbed.

3. Respiration:

Euglena respires through us general body surface. Oxygen dissolved in the water diffuses in through the pellicle and carbon dioxide liberates to outside. Enzymes present in the mitochondria catalyze oxidation reactions with the aid of oxygen.

The liberated energy is entrapped in the high energy phosphate bonds of ATP, which supplies energy for metabolic activities. It is likely that some of the CO2 resulting as a by­product in the catabolic activities of the body is utilized for photosynthesis and the liberated O2 for respiration.

4. Osmoregulation:

The water content of body is regulated by the contractile vacuole which periodically empties into the reservoir. This is called osmoregulation. During the diastole, the contractile vacuole grows in spurts because smaller accessory vacuoles, formed in its vicinity, coalesce with it one after another.

After its maximum growth, the contractile vacuole undergoes systole by fusing with the lining of the reservoir and bursting into it. To recall, water tends to enter the animal body by endosmosis.

Term Paper # 5. Excretion of Euglena:

The nitrogenous waste products (ammonia) resulting from catabolism pass out by diffusion through the general surface of the body. Some excretory substance may be emptied by the contractile vacuole into the reservoir. According to recent investigations of Chadefaud, the contractile vacuole is surrounded by specialized, granular and excretory cytoplasm.

Several small accessory vacuoles appear in the excretory cytoplasm and later fuse together to form a large contractile vacuole. Periodically, the vacuole reaches its maximum size (diastole) and then it bursts (systole), so as to discharge its contents into the reservoir and hence to the outside.

It is believed that the wall of the contractile vacuole in contact with the reservoir is very unstable and easily bursts. With the removal of the excess of water some of the dissolved nitrogenous excretory substances also get rid of.

Term Paper # 6. Behaviour of Euglena:

If the forwardly moving Euglena rotates around in axis reaches an unfavourable area, it soon turns and comes back into the favourable area. Thus it tries to escape from the unfavourable conditions. Similarly, it is attracted towards the favourably intensive light but it tries to move away very bright light.

This sense of light is gained by Euglena with the help of its eye-spot. It is especially noteworthy since Euglena depends for food mostly on photosynthesis for which the sense of light is necessary to the animal.

If Euglenas are kept near the light in a small dish they soon move into the lighted area, however, if the dish is removed to a darkened place, they again scatter all over the dish. Euglena also reacts to heat and chemical stimuli by which all its activities are slowed down or it tries to swim away from these stimuli.

(i) Reaction to Light:

Euglena shows positive photo-taxis, avoiding strong light but turning and swimming towards a moderately intense light such as that from a window. Like most motile and free living unicellular organisms containing chlorophyll, it orients itself parallel to a beam of ordinary light and swims towards the source of the illumination.

In a dish containing culture, most of the individuals are found to aggregate on the side towards the light. If the dish is placed in direct sunlight and one half of it is covered, the animals avoid the region of direct sunlight as well as the shade but gather in between the two in a small band.

(ii) Shock Reaction:

A swimming Euglena shows a shock-reaction where the direction of illumination is changed. It has been found that the paraflagellar body is more photosensitive than any other part of the body. The stigma is so placed that if Euglena is illuminated from the side, its paraflagellar body or photoreceptor is shaded by stigma, once in every normal rotation.

Whenever it happens, the photoreceptor becomes excited and effects flagellar action in such a way that the organism turns towards the new light source. First the body bends downwards, then slightly straightens up in each subsequent rotation with a gradual curving towards the new light source.

While moving in the direction of light the photoreceptor does not become shaded by the stigma, so that animal continues to follow the same course. Light being essential for photosynthesis, or carbon assimilation by means of chlorophyll, this specialization bringing the animal into light is of distinct advantage in its nutrition.

Term Paper # 7. Reproduction in Euglena:

Only asexual reproduction occurs in Euglena.

It takes place by the following two methods:

i. Binary fission.

ii. Multiple fission and palmella stage.

i. Binary Fission:

Under favourable ecological conditions such as food availability, optimum light, temperature and water, an active Euglena reproduces by longitudinal binary fission. It is initiated by division of basal bodies. Nucleus is divided by peculiar mitosis, in which, the mitotic spindle is formed inside for intact nucleus, i.e., nuclear envelope and endosome or nucleolus are not disintegrated during mitosis.

Karyokinesis (nuclear division) is followed by cytokinesis (cytoplasmic division) which involves duplication of flagella, stigma, contractile vacuole, chloroplasts, mitochondria, etc. During later stages reservoir, cytopharynx and cytostome are duplicated by a longitudinal furrow. During final step of cytokinesis, a longitudinal furrow appears at anterior end of body and proceeds backwards, dividing the parent body into two daughter euglenae.

ii. Multiple Fission and Palmella Stage:

In unfavourable conditions, multiple fission takes place in Euglena. It secretes a cyst around itself. The walls of the cyst may be thick or thin. The animal divides to form 16 to 32 daughter euglenae within the cyst. Sometimes the flagellum of Euglena disappears and its body contracts to become a round alga-like structure. All the activities become slow in this condition and the animal divides by fission. In this way many small Euglenae are formed from a large Euglena.

These euglenae appear as green algae on the surface of water. This condition is known as palmella stage which occurs regularly in some species of Euglena. Later flagella grow in these daughter euglenae which become free and develop into adults.

There is no clear evidence of sexual reproduction in Euglena although Biechler has described sexual reproduction by syngamy is some species.

Term Paper # 8. Encystment of Euglena:

During unfavourable conditions, Euglena undergoes encystment. For encystment, first Euglena becomes inactive, loses its flagella and secretes a thick rounded, red-coloured (red colour is due to haematochrome pigment) protective cyst. The cyst is secreted by the muciferous bodies lying below the pellicle of Euglena.

Encysted euglenae stand a fair chance of wide dispersal. On return of favourable conditions, encysted euglenae emerge out and resume normal active life.

Term Paper # 9. Position of Euglena:

Euglena shows many characters of plants such as chromatophores with chlorophyll and holophytic nutrition.

However, it is regarded as an animal due to the following facts:

(i) It lacks a cellulose cell wall.

(ii) Its pellicle is made of proteins like other protozoan cell (i.e., ciliates).

(iii) Presence of stigma and paraflagellar body, the photosensitive structure.

(iv) Presence of contractile vacuole.

(v) Reproduction by binary fission.

Thus, Euglena is a small microscopic living being in which there is no marked morphological differentiation of structure by which it can be distinguished clearly.

As Euglena shows affinities with plants as well as with animals, its position is considered to be controversial. Some biologists suggest that we should regard Euglena as the intermediate stage in the evolution of plants and animals.

Euglena shows following plant characters:

(a) Photosynthesis occurs in the presence of sun light.

(b) Pyrenoid bodies make their presence.

(c) Nutrition is holophytic type.

(d) Palmela stage exist.

(e) Chromatophore and chlorophyll are present Euglena shows following animal characters:

(i) Reproduction takes place by binary fission.

(ii) Contractile vacuole is present.

(iii) Pellicle is made up of protein.

(iv) Nutrition is heterophytic, partial or complete.

(v) It also uses amino acids pentoses or polypeptides as sources of nitrogen.

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