UNICELLULAR ORGANISMS: CELL STRUCTURE

Cells

If a thin slice of a plant is cut with a razor and examined under a microscope, it is seen to have a honeycomb structure ; it is divided into a great many small compartments termed cells, which are separated by partitions termed cell walls. The cell structure of plants was first discovered by Hooke in 1667, who examined sections of cork and other plant material with a primitive microscope. Hooke believed the cells to be hollow and thought that the cell wall was the most important part. In time, however, it was realized that all living cells have contents and that these are the most important part of the cell. Animals as well as plants are made up of cells, and in large organisms the number of cells runs into many millions. In such multicellular organisms there are many types of cell which differ in function and also in shape and size. Although most animals and plants are multicellular, there are some which appear to consist of one cell only and are therefore termed unicellular. Such simple organisms are sometimes described as a-cellular, since their bodies are not divided into separate units. The smallest cells are those of bacteria, which are unicellular organisms so minute that they can barely be seen with the microscope. The yolks of the eggs of birds are the largest cells, but most cells have a diameter between 0.01 and 0.001 in.

The Internal Structure of a Plant Cell

The young cells which are found at the apex of the root or shoot of a plant are roughly cuboid in shape. The wall of the cell is formed of a substance called cellulose, which is secreted by the cell contents and is permeable to water. Within the cell is a thick, jelly-like substance termed the cytoplasm, which consists of a complicated mixture of chemical com pounds together with a large proportion of water. The cytoplasm is colourless and similar in appearance to the uncooked white of an egg.

Just beneath the cell wall there is a thin, limiting membrane at the surface of the cytoplasm. This membrane plays an important part in the life of the cell, for it controls the entrance of substances into the cytoplasm. Within the cytoplasm is a spherical or oval body termed the nucleus, which frequently cannot be seen without the aid of stains.

Besides the nucleus the cytoplasm contains many small granules. In many plant cells there are small green granules, the chloroplasts, which contain the green pigment chlorophyll. The nucleus and cytoplasm together make up the living material of the cell, to which the general name protoplasm is given. The nucleus plays an extremely important part in the life of the cell and controls and regulates most of the living processes of the protoplasm. The cytoplasm cannot live without the nucleus, for if a cell is cut into two halves, only one of which contains the nucleus, then the nucleated part continues to live, but the part without the nucleus dies. The nucleus is also concerned with the transmission and production of inherited characteristics.

As the cell grows it gradually undergoes changes in shape and structure which depend upon the type of tissue to which it belongs. It is only in the young cell that the cytoplasm entirely fills the space within the cell wall and in older living cells there are one or more spaces called vacuoles which contain a liquid, the cell sap. This consists of water containing sugars and other substances in solution. Each vacuole is surrounded by a thin limiting layer of cytoplasm which is similar to the memhrane at the outer surface of the cytoplasm. In some plant cells the wall is formed of cellulose throughout life, but in others it becomes altered by impregnation with various substances.

The Internal Structure of Animal Cells

In general structure animal cells are similar to those of plants, but their protoplasm is denser and contains numerous minute vacuoles. Further, animal cells do not possess a thick cellulose cell wall but are usually enclosed by a thin limiting membrane. Chloroplasts are never present.

Practical Work on Cells

Mount a moss leaf in water on a slide beneath a coverslip and examine with the low and high powers of the microscope. Note the cells and the numerous chloroplasts.

Scrape the inside of the cheek with a blunt instrument and mount the scrapings in a little saliva. Examine with the low and high powers, and note the oval flattened cells. Run a little weak aqueous eosin solution under the coverslip and note the staining effect on the nucleus and the cytoplasm.

Examine a prepared longitudinal section through the root tip of a flowering plant.

Also examine cells obtained as follows :— Tear a piece of epidermis from an iris or daffodil leaf, or from the inner surface of an onion scale and mount it in water. Place a small piece of ripe apple, privet berry or the red part of a rose hip in a drop of water on a slide, and then squash it gently with a coverslip. Examine a leaf of Canadian pond-weed mounted in water. Notice that it resembles that of the moss in general structure except for black lines here and there between the cells due to air bubbles enclosed in spaces between the cells. In some of the narrower cells in the midrib, notice the chloroplasts moving along inside the cell walls like barges in a stream as they are carried along by the circulation of the protoplasm.

AMOEBA

Structure

The simplest members of the animal kingdom are unicellular and are included in the group called the Protozoa. Amoeba proteus is a Protozoon, and is to be found in the mud at the bottom of ponds and ditches. It can just be seen with the naked eye as a whitish speck and it reaches a maximum length of about TOO in. It is an irregular, jelly-like mass of protoplasm, in which can be distinguished a clear outer layer called the ectoplasm, and a more granular, greyish interior, the endoplasm. The nucleus is a rounded body lying in the endoplasm. It is surrounded by a delicate membrane, and has the appearance of a denser patch of endoplasm. Besides the granules there are large particles of different shapes and sizes in the endoplasm. These are the remains of the microscopic organisms on which Amoeba feeds, and each food particle is surrounded by a bubble of liquid termed a food vacuole. Frequently the food particles have a greenish or yellowish colour, and are the remains of minute plants called diatoms.

Movement

The name Amoeba is derived from the Greek amoibe, which means change, and if Amoeba is ex amined under the microscope the shape is seen to be continually changing.

There are streaming movements among the granules of the endoplasm, and lobes and projections of the protoplasm are pushed out in various directions.

These lobes are called pseudopodia or raise feet, and each begins as a mobile projection of ectoplasm into which the granular endoplasm flows. By the con- tinual formation of pseudopodia in one direction and the withdrawal of those behind, the Amoeba moves slowly along.

Respiration

Amoeba has no special respiratory organ, but absorbs oxygen in solution from the surrounding water through the whole surface of the ectoplasm, whence it diffuses to all parts of the endoplasm, helped by its streaming movements. In a similar way carbon dioxide passes from the inside of the Amoeba through the ectoplasm to the water outside.

Excretion

In the endoplasm there is a clear round bubble of liquid, the contractile vacuole, which slowly enlarges until it reaches a certain size, when its contents are squirted out through the ectoplasm to the outside. The small bubble then reappears, enlarges and repeats this process. It is probable that the liquid excreted thus is mainly water containing a little ammonia.

Nutrition

Amoeba lives on microscopic organisms or upon the decayed remains of larger plants. If an Amoeba comes into contact with a diatom, for example, pseudopodia are thrown around the plant and unite beyond it so as to engulf the food into the body of the Amoeba.

A bubble of water, the food vacuole, is taken in with the food particle which is slowly digested by secretions poured into the food vacuole by the surrounding cytoplasm. Amoeba has no anus, but can get rid of undigested material by passing it through to the outside at any point on the ectoplasm.

Irritability

Amoeba has no sense organs, but the protoplasm as a whole is sensitive to certain conditions of the environment. If a little dilute acid or alkali is run under a coverslip beneath which an Amoeba has been mounted in water, the animal will slowly move away from the reagent. Amoeba is attracted by food particles, but will avoid taking in sand grains if these lie in its path. If Amoeba is touched with a needle or if a weak electric current is passed through the water in which it lies, it quickly withdraws its pseudopodia and becomes a rounded blob.

Growth and Reproduction

As the food is digested it is built up into fresh protoplasm, and the Amoeba grows until it reaches the maximum size of about TOO in., and then it divides into two. The nucleus lengthens and becomes constricted into two halves, and at the same time the protoplasm becomes dumb-bell shaped and eventually breaks into two parts, each containing a nucleus. By this process of division, called Binary Fission, the original Amoeba gives rise to two smaller Amoeba; which grow and will divide in their turn. In this way, unless Amoeba is eaten or killed, no part of it ever dies, for the organism lives on in the form of its offspring.

Encystment

If the ditch or pond dries up, then Amoeba draws in all its pseudopodia, assumes a rounded shape and secretes a thin protective covering called a cyst. Within the cyst the protoplasm is protected from drought, and remains inactive until moist conditions prevail, when the cyst is burst and Amoeba takes up active life again. The encysted Amoeba is light and can be blown about like a particle of dust. In this way it may be dispersed to some new habitat.

Spore Formation

Sometimes when Amoeba is encysted the nucleus divides into small portions, round each of which some cytoplasm collects. These small nucleated masses of cytoplasm are called spores, and when the cyst breaks each spore takes on the form of a young Amoeba and flows away.

Practical Work on Amoeba

Examine Amoebas in a watch glass with a lens. Note that the largest Amoeba can just be seen with the naked eye as small greyish blobs.

Mount Amoeba in water on a slide with a coverslip and examine under the low and high powers of the microscope. Make drawings at intervals under the low power to show the different stages in the process of movement. Under the high power notice the streaming movements of the granular cytoplasm.

Run a little iodine under the coverslip of a slide of a mounted Amoeba. This can be done by placing a drop of iodine on the slide just in contact with the edge of the coverslip, and then drawing water from beneath the other side of the coverslip by means of a piece of blotting-paper. This will cause the iodine to be drawn under the coverslip. The iodine kills the Amoeba and stains the nucleus a slightly darker colour than the cytoplasm. It is difficult to see the nucleus in an unstained specimen.

PROTOCOCCUS

This is one of the simplest of plants, consisting of small spherical cells. It is found growing on the surface of damp wooden fencing or bark of trees, usually on the side facing north. Like Amoeba, it is unicellular, but differs from. it in the following ways :—

It has a cell wall of cellulose which prevents any change of shape by the production of pseudopodia.

It possesses a large, irregularly shaped chloroplast which appears to fill the cell, colouring the plant green.

While possessing a nucleus, it forms no food vacuoles or con- tractile vacuoles.

Nutrition

This is typical of all green plants. It absorbs water containing in solution carbon dioxide from the air and mineral salts from dust blown on to the surface on which it is growing. In light it carries out a process called photosynthesis, using light energy to construct its food from these simple substances. Lying in the chloroplast is a small body called the pyrenoid, around which starch produced by photosynthesis may be stored for future use. This plant, therefore, feeds on liquid food and not on solid food as does Amoeba, and being bathed in the raw materials from which its food is formed it does not need to move in search of it or to display irritability ; nor does it excrete waste substances since these it can reconvert into food.

Growth and Reproduction

As in Amoeba, growth to a certain size is followed by binary fission. The nucleus divides first, followed by the division of the cytoplasm and chloroplast. A new cell wall then forms, splitting the cell in two. For a time the products of division may hang together, but separation ultimately takes place.

Dormancy

In dry weather Protococcus becomes dormant and semi-desiccated. Protected by its cell wall, it does not need to encyst.

Practical Work on Protococcus

Scrape some of the green powder from the bark of a tree and mount it in water. Examine under the high power of the microscope and note the spherical green cells of this plant, some occurring in clumps as the result of repeated binary fission.

PARAMECIUM

Structure

Paramecium is another Protozoon common in muddy ponds and stagnant ditches and puddles. Like Amoeba it is just visible as a whitish speck about TOO in. long, but it differs in having a definite shape, which is somewhat like that of a slipper, flattened and elongated, with a pointed posterior and a more rounded anterior end, so that Paramecium is often called the Slipper Animalcule. On the ventral side there is a shallow oral groove, leading into a short tube, the cytopharynx, which ends in the internal cytoplasm by an opening called the cytostome. The cytoplasm shows two layers, a dense outer layer, the ectoplasm, and a more liquid interior, the endopiasm. The ectoplasm is covered by a thin skin, the pellicle, which also lines the cytopharynx. In the endopiasm there are two nuclei of different sizes. The smaller of these, the micronucleus, lies closely touching the larger or meganucleus.

The surface of Paramecium is covered by a great many tiny, hair-like processes of cytoplasm called cilia, which are arranged in regular lines running in a slight spiral. In the ectoplasm there are numerous organs known as trichocysts, which look like little rods lying at right angles to the surface of the animal. If Paramecium is irritated a long thread is shot out of each trichocyst. It has been suggested that these threads have a stunning effect on any small animal that they hit, but it now seems more likely that they serve rather as anchors which fix Paramecium to some object.

Movement

The cilia of Paramecium wave backwards and forwards rhythmically and, acting like minute oars, cause the organism to move through the water with the blunt end first. Partly because the body has a slight twist at the anterior end and also because the cilia are moved in oblique waves, the animal swims along a spiral course and rotates as it moves along. It can reverse the action of the cilia for short periods in order to go backwards.

Feeding

Cilia line the cytopharynx, and are arranged in a number of longitudinal rows which are sometimes called undulating membranes. The lashing of these cilia causes a current of water to be swirled down the cytopharynx. In this way bacteria and other small organisms are carried down the cytopharynx and, when sufficient have been collected, they enter the cndoplasm in a drop of water, thus forming a food vacuole. The streaming of the endoplasm carries the food vacuole round the body until the food is digested. Then the residue is ejected at a definite anal pore behind the oral groove, where the ectoplasm and pellicle are less firm than elsewhere.

Respiration

As in the case of Amoeba, there is no respiratory organ and the respiratory exchange takes place over the whole surface of the Paramecium through the pellicle.

Excretion

Paramecium has two contractile vacuoles situated in the endoplasm, one towards the posterior and the other towards the anterior end. Each consists of a bubble of liquid surrounded by several radiating, spindle-shaped channels. Water from the surrounding endoplasm gradually drains into these channels, which then contract and disappear, discharging their contents into the central space to form a bubble of liquid. Finally the protoplasm round the central drop contracts and causes the contents of the vacuole to be squirted through the cuticle to the exterior. Then the radiating channels reappear and the process is repeated. As in the case of Amoeba, the liquid excreted is mainly water containing a little ammonia. The main function, however, of the contractile vacuole is that of osmoregulation.

Reproduction

Paramecium usually reproduces by the asexual method of binary fission. The mcganucleus and micronucleus become dumb-bell shaped and each separates into two, one half passing to each end of the animal. Then the protoplasm becomes divided into two behind the gullet in a transverse plane, so that while both the daughter-cells contain a contractile vacuole, only one possesses a gullet and the other develops one. Both the daughter-cells grow rapidly and, if there is plenty of food, may re-divide in ten hours ’ time.

Conjugation

If a single Paramecium is isolated in water containing plenty of food it will divide and its descendants will multiply by binary fission until there are thousands. There comes a time, however, when the individuals begin to show a slowing down in activity. If at this stage members of a different race are introduced into the stock, the process of conjugation takes place and increased activity results. The essential feature of this conjugation is the exchange of nuclear materials between two individuals. The individuals ultimately produced after conjugation are normal in structure and also resistant to the bad conditions, although if not subcultured the whole colony will eventually die out notwithstanding the benefits apparently conferred by conjugation.

The process begins during the late hours of night and lasts until the next afternoon. Two individuals termed conjugants come together so that their ventral surfaces are in contact, their mouths and gullets disappear and the pellicle at the point of contact dissolves so that their endoplasm is continuous. Each meganucleus divides into many comma-shaped parts which later disappear. Each micronucleus grows larger and divides twice. Three of the products disappear, whilst the remaining one divides unequally to give a small male pronucleus and a larger female pronucleus. Each male pronucleus crosses over into the other conjugant and fuses with the female pronucleus there, forming a zygote nucleus. The conjugants separate. This marks the end of conjugation. Immediately afterwards the mouth and gullet reform, and the zygote nucleus divides three times to give eight nuclei, four of which enlarge, whilst each organism divides into two so that each daughter-cell has two large and two small nuclei. After a day or two these divide again, each daughter-cell now possessing one large and one small nucleus which function as mega- and micro-nucleus respectively—in fact, these daughter-cells behave like the ordinary Paramecia produced by ordinary fission.

Conjugation in Paramecium.

We note here that the process is brought about by abnormal conditions, but each zygote does not itself grow into a normal Paramecium after stage, since immediately after this stage it possesses eight nuclei, but is a rejuvenated individual which soon divides to give rise to normal individuals. It would seem that the protoplasm of the micronucleus supplies something lacking in the protoplasm of the other conjugant, so that normal fission can occur afterwards. Thus the process itself is seen to be essentially a rejuvenating and not a reproductive one, rejuvenation being effected by the exchange of micronuclear material. Some authorities claim that the essential feature of it is that of producing a new meganucleus from micronuclear material. Just before the micronuclei change over we may regard each conjugant as an hermaphrodite, the female pronucleus with all the cytoplasm forming a female gamete and the male pronucleus functioning as a male gamete. From this point of view the whole animal splits up into gametes, so that it cannot be said to possess a soma or mortal body.

Irritability

Paramecium is sensitive to contact with foreign bodies and to chemical substances in the water. It displays an ’avoiding reaction ’when it comes up against an obstacle in its path by going backwards for a short distance, turning a little to one side, and then going ahead. If it does not clear the obstacle at the first attempt, it repeats the process until it sucoeeds.

Practical Work on Paramecium

Mount Paramecia in a drop of water beneath a coverslip, and observe their rapid movements. Then place a little teased-out cotton wool beneath the coverslip in order to slow down the Paramecia by entangling them between the fibres of the wool. Examine under both powers. Another method of slowing down the Paramecia is to add a drop or two of a thick Carragheen solution to the mounted drop of water containing them.

Run a little dilute iodine solution beneath the coverslip of a slide of mounted Paramecia ; or run a drop of methyl green in acetic acid beneath the coverslip. With the high power examine the specimens thus stained for the nucleus and discharged trichocysts.

Sorry, comments are closed for this post.