Diversity in Cell Size & Shape Academic Script. Paper No. & Title: Cell Biology. Topic No. & Title: Topic - 2 Diversity in Cell Size & Shape

Transcription

1 Diversity in Cell Size & Shape Academic Script Course Name: Paper No. & Title: Zoology Z-102 B Cell Biology Topic No. & Title: Topic - 2 Diversity in Cell Size & Shape

2 INTRODUCTION Cell is the basic unit of life. Whether it is a prokaryotic cell or a eukaryotic cell or the cells of the same animal, a great variation is seen in their size & shape, as-well-as structure. These differences form the basis of biodiversity. PROKARYOTIC CELLS The prokaryotic cells are the most primitive type of cells. The DNA/RNA of a prokaryotic cell is not enclosed by a nuclear membrane but is surrounded directly by the cytoplasm. Absence of a nucleus differentiates the prokaryotic cells from the eukaryotic cells. DIVERSITY IN PROKARYOTIC CELLS 1. PPLO (Pleuropneumonia Like Organism) ( μ) This is the first picture I am showing you. This is perhaps the smallest and simplest prokaryotic cell that is existing on earth. This is the picture of PPLO (Pleuropneumonia Like Organism). The best examples that have been thoroughly been studied about the PPLO are the Mycoplasma. There are two different genus of the Mycoplasma. Whatever they may be but this Mycoplasma are usually found in the respiratory tract of the vertebrate animals or they are also found in the cells of the respiratory tract or they are also found in the exudes of the respiratory tract. These PPLO, the Mycoplasma, they cause

3 different type of respiratory diseases in these vertebrate animals. Look at the picture properly and you will also note that unlike the plant cells even the prokaryotic plant cells, PPLO does not have any cell wall. It has got a cell membrane, it has got some cytoplasm. In the centre it has got a very diffused type of DNA that is found there which is surrounded by plenty of ribosomes. Besides these there is nothing else found in the cell. There is a small bleb like structure found at the top of the cell. But so far its function is not properly yet known. 2. Bacteria (1.25 μ diameter) ( μ length) The second picture shows us the typical bacterium. They have a cell wall and hence they are considered to be belonging to the plant kingdom. However there are many microbiologist who do not want bacteria to be classified as plant cells. At the same time, botanists since they classify plant cells as those having cells having a cell wall, they consider bacteria to be part of the plant kingdom. Friends, whatever may be the case, bacteria generally are around about 1-10µ in its size. Depending upon its shape, the bacteria can be classified into different types. Spherical shaped (coccus) These type of bacteria that you see are spherical shaped bacteria and hence they are classified as coccus type of bacteria. Rod/cylindrical shaped (bacillus) This second type of bacteria you see they are rod shaped, they are cylindrical shaped and hence are classified as bacillus type of bacteria.

4 Spiral shaped (spirillum/spirochete) And these third type of bacteria that you see now are spiral shaped and hence they are classified as spirillum or spirochete bacteria. Whatever may be the shape, the bacteria are prokaryotic cells because even in the bacteria you can very easily find out that they do not have a well defined nucleus and their nucleic acid is in direct contact with the surrounding cytoplasm. They have however a cell wall alongwith the cell membrane. 3. Blue-green algae This picture that now I am showing you is of a typical blue-green algae. Blue-green algae may be unicellular or they may be found in colonies. But all the cells are prokaryotic in nature. Blue-green algae is a group of special type of prokaryotic cells by themselves. Though they are algae, they are not eukaryotic in nature and their properties are absolutely different from all the other algae present on earth. EUKARYOTIC CELLS The eukaryotic cells are very much larger and developed than the prokaryotic cells. The DNA in the eukaryotic cells is enclosed within a nuclear membrane and hence remains separated from the cytoplasm. There is one major difference between the plant & animal eukaryotic cells.

5 Unlike the plant cells, the animal cells do not have any cell wall. DIVERSITY IN THE SHAPE OF THE ANIMAL EUKARYOTIC CELLS The shape of the eukaryotic cells varies greatly. Friends, let us look at some examples of the eukaryotic cells how they differ in their shape. The most commonly occurring eukaryotic cells are those which are irregular in shape. They constitute major portion of the Protozoan cells. 1. Irregular shaped (Ceratium) This is the picture of a Ceratium. It is a protozoan. Try to classify its shape and you will not be able to do so. It does not have any particular shape. It is neither round, it is neither spherical nor square nor triangle, absolutely no shape. That is why we say it is having a irregular shape. 2. Changing shaped (Amoeba, WBC) There are other cells which keep on changing their shape. They can also be classified as irregular. But the difference between the irregular eukaryotic cells and those cells which keep on changing their shape is that the irregular cells have a definite irregular shape which does not change whereas those which keep on changing they do not have a definite shape because the shape keeps on changing. For example, Amoeba. Friends, I am sure there is not a single of you who have not have heard or seen about Amoeba. Amoeba due to its pseudopodia keeps on locomoting around and that why it keeps on changing its shape.

6 Similarly the WBC, the White Blood Cells also known as the leucocytes which are found in the blood in all vertebrate animals they also are amoeboid in nature. They also keep on changing their shape and they can actually swim against the flow of the blood in the blood vessels and that is why because they have amoeboid movement they can swim against the flow of the blood stream. 3. Cuboidal (Epithelial cells of glands) Then you have the cuboidal cells. Practically in all the vertebrate animals and even in the invertebrate animals, if you look at any of the secretory glands, all these secretory glands, they are lined internally by cuboidal cells. In other words, you can say that all cells which are found in secretory glands they are mostly cuboidal cells. These cuboidal cells may be large cuboidal, they may be small cuboidal they may be slightly flattened cuboidal, but practically they are all cuboidal in shape and they are secretory in nature. 4. Columnar (Absorptive epithelial cells) Opposite to the cuboidal cells you have got other types of cells which are columnar cells. They are like tall columns that why they are classified as columnar cells which line the small intestine. These cells are columnar in shape. Friends, if you recall what you have studied in perhaps in your 11 th or 12 th standard or maybe somewhere earlier when you had studied about the digestive system of a vertebrate animal and when you had studied the histology of the small intestine and when you had studied the innermost lining of the small intestine, you must have studied in its physiology that this internal lining which is made

7 up of the columnar cells is meant for the absorption of the digested food. Hence, you will see that whenever any sort of absorption, not only of the digested food, absorption of even of electrolytes, absorption of other salt materials, absorption of other organic molecules anywhere not even in small intestine, it maybe the kidney, it may be anywhere else, you will see that all the columnar cells are absorptive in nature. And hence, as I have already mentioned somewhere earlier, that these cells differ in their shape because they have got different functions to carry out. So you see that all cells which are columnar they are absorptive in nature. All cuboidal cells are secretory in nature. 5. Flattened (Squamous epithelial cells) You have got flattened cells also. Cells which are absolutely flattened in shape, they look like the stones that are found in the pavement that is why they are also known as the pavement cells sometimes. But technically and scientifically they are known as squamous epithelial cells. Why do these cells become flattened? These cells are mainly found on those surfaces which are constantly under pressure. This pressure might be due to some solid object or even due to some liquid material. Let me give you an example. Where do we find these squamous epithelial cells? On the surface of the skin or maybe this palm. The palms of our hands, all the cells that are found here, they are all squamous. Usually what do we use our palms for?

8 We use our palms for catching hold of some object or maybe putting our hands on the surface of the table or just even to pat someone. So a pressure is exerted there and hence these cells become flattened. Similarly if we look at the endothelium of any blood vessel or even a capillary, which is a blood vessel, you will see that this endothelium is the innermost lining of the blood vessel that is made up of the squamous epithelial cells. The blood which is flowing through the blood vessel goes on continuously exerting pressure on these cells and that is why these cells are flattened. They are squamous epithelial in nature. They are squamous in shape and hence they have to perform a particular function and that function is to bear the pressure. 6. Cylindrical (Striated myocytes) Similarly you have got cylindrical shaped cells. All the striated muscles The striated muscles are also called skeletal muscles. All these skeletal muscles which are found in the body of the vertebrate or invertebrate animals, they are long, fibrous and cylindrical. Why are they long? The reason is that these muscle cells they have to be attached at one end of the bone to the other end of the bone. Their function is for the contraction of that particular organ on which they are situated. For example, look at my hand. I can bend my hand in this way for the very simple reason that I have got a muscle that is attached at this end uptill right this end. All the muscle fibres that are there, they are elongated. They are all cylindrical in shape.

9 They are all fibrous in shape and all these muscles they contract together. If these muscles would not have been elongated in nature then this contraction would not have been possible. That is the reason why they are elongated fibrous in nature. 7. Spindle shaped (Smooth myocytes) At the same time we have got smooth muscles also which are non-skeletal in nature. Smooth muscles are found wherever we have got tubular shaped organs. For example, our alimentary canal or the ureter or the nephrons, we will have smooth muscles present in all these tubular structures. These smooth muscles are spindle shaped, they are not as long as the skeletal muscles, they have got a very definite length, and the shape is definite that is spindle shaped. They do not help in the contraction lengthwise but help in the contraction transversely. Why does the food that we put in our mouth pass very easily right from our mouth upto our stomach and further beyond to the intestine and then out of our body? For the very simple reason that groups of these smooth muscles contract rhythmically one after another in a wave like motion and that is the reason why we see that food passes right from one end of the alimentary canal to the other end of the canal and this is the reason why these smooth muscles are spindle shaped. They work in groups. They do not work like the striated muscles. 8. Polygonal (Liver cells) Then we have got polygonal shaped cells also.

10 Such type of cells are generally found in those type of organs which are solidly packed. The most common example is the liver. If we look at the hepatocytes, i.e. the liver cells, they are all almost polygonal in shape. They may be cuboidal but that is considered to be polygonal as far as the hepatocytes are concerned. Why do they assume such a shape? For the very simple reason that these cells, if they have to be packed very close by then they cannot be spherical shaped, they cannot be triangular cells, they cannot assume any other shape. If they assume any other shape, then there would be lot of intercellular space which is not acceptable for such a type of organ. If they are polygonal then all the cells can very easily be packed very nearby to each other having the least amount of intercellular space and these cells can work together unlike other organs where cells can work independently. In liver the hepatocytes cannot work independently. They have to work in groups because liver is a detoxifying organ in the body and the cells cannot work independently and if they have to work together they have to be positioned very close to each other and that is the reason why they assume a polygonal shape. 9. Tree shaped (Multipolar neurons) Then you have tree shaped cells. If you look at multipolar neurons, i.e. neurons that have more than two poles, then you will see that they have got plenty of branches of the dendrites. They will be having only one axon but the dendrite will branch out profusely and if you look at that particular cell under a microscope, you will feel as

11 if you are looking at a tree with a trunk and plenty of branches and the foliage all over it. Why do they assume such a shape? Because their function is to capture impulses from hundreds and thousands of different other neurons and transmit those impulses along the axon. If there had been only one dendrite or two dendrites, then the number of impulses which they can capture would be very less and the transmission of the impulse would have been very slow and that is the reason why they have assumed such a type of shape. 10. Oval/spherical shaped (Ova) Then you have got oval shaped cells which are also called sometimes spherical shaped. All the eggs of different animals (eggs are also known as ova). All the ova of all different type of animals, whether vertebrate or invertebrate animals, they are all oval in shape. Why? Their function is to be fertilized. If they had been some other shaped then the sperms that come in contact with this particular cell will not have an even surface to penetrate. If it is oval shaped then the sperms can enter the ova from any side. That is the reason why it is oval shaped. And this ovum when it is produced from the ovary, it cannot remain for a very long time in the system of the female. It has to be shunted out within two-three days and that is the reason if it is some other shape, it cannot be rolled inside and moved forward. 11. Discoid (RBC) You have also got discoid cells.

12 Look at this picture of the erythrocytes and you will see that all the erythrocytes are disc shaped, whether they are the erythrocytes of the frog, or the fishes or whether the human beings or the dogs or monkeys or donkeys or any other mammal, they are all disc shaped. When these erythrocytes have to move through the capillaries, the diameter of the capillaries is so narrow that all the erythrocytes cannot move together at the same time. These erythrocytes have to move in a single file, one erythrocyte behind you have another one and that behind another one and so on. If they are having any other shape, they are spherical shape or having a polygonal shape then they cannot move in a single file. Since they are flattened they can stand one behind other and move in a whole group and the flow of the blood can be very easy. Spear shaped cells (human sperms) You have also got spear shaped cells. (Spear i.e. javelin that we use in our sports). Look at the human sperms. You will see that they are spear shaped. The anterior most part, the head region of the sperm is pointed and that helps in penetration of the sperm inside the ovum during fertilization. So friends, you see that there is a big diversity in the shape of the cells and nature has provided this diversity in shape so that they can assume different functions in the body of the same animal. The animal does not need the same shaped cells. If it has got the same shaped cells throughout the entire body then the animal would have been a

13 very simple animal and there would have been no different type of functions that can be carried out by the different cells and hence nature has provided different shapes to the different cells. The diversity in the shape of the various cells in a multicellular animal is usually a clear reflection of the function they carry out in the organism. DIVERSITY IN THE SIZE OF THE EUKARYOTIC ANIMAL CELLS Now let us look at the diversity in the size of the eukaryotic cells. In one single sentence, if I tell you, the size of the cell can vary right from atleast about one foot in length to right upto a few microns. 1. Dinosaur egg (about 300mm long) Look at this picture. Can you identify what it is? Some of you I am sure must have identified it. This is a picture of an egg of a dinosaur. The minimum size of the egg of any dinosaur will be atleast 1 foot, i.e. almost 300mm. 1mm=1000µ. So if you have got 300mm, that is equal to 300,000µ in size!! Compared to that you have got very tiny cells which are hardly very few microns in size. 2. Human sperm (60µ long) Look at the size of a spermatozoon. The spermatozoa are usually about 20 to 50 or maybe 60µ in length and compared these eggs are single cells.

14 Even the egg of the dinosaur that you see is one single cell though it is a very large structure. 3. Ostrich egg (about mm long) Now look at this picture. This is the picture of an egg of a Ostrich. The size of the ostrich egg can be anything between or slightly more millimetre in diameter, again a very larger size. 4. Hen egg (about 80mm long) Compared to that, the egg of a hen is still smaller. It can be about 80mm in length. 5. Human egg (1-2mm diameter) Compared to that, if you look at the human egg, it is an extremely small structure. The size can be around 1-2mm. This human egg even if it is lying just below your nose, you might not even be able to identify it. It is such a tiny structure. So you just compare the size of the different eggs of different animals. Look at the eggs of the dinosaurs which are extinct today but are found as fossils at plenty of places all over the world and look at the size of the human egg. Such a vast diversity. There can be so many different reasons. One of the most obvious reasons a biologist can think about in the diversity in the size of the eggs (the same type of cell but of different animals) can be evolution. As evolution had taken place, the size of all animals had shrunk and in order to adapt themselves to the changing environment so many

15 changes occurred in the whole body of the animal and this is one of the adaptations. 6. Mammalian WBC (7-20µ) This is the picture of a mammalian erythrocytes. The shape you have already seen is discoid. The size is just about 7.5µ in diameter. Since these erythrocytes are so tiny in size, if you have studied somewhere in your school and if you remember that on single cu.mm. of blood contains atleast about 4-5 million RBC/erythrocytes. 1cu.mm. is this much. You may not even be able to find the space between my two fingers. That much amount of blood can contain atleast 4-5 million erythrocytes. This is possible because the size of the erythrocytes is very small. If you compare this size with the function then you will understand that the main function of erythrocytes is to transport oxygen. If the size of the erythrocytes had been very large, then very less oxygen could have been transported. It is because they are very tiny they can transport more amount of oxygen at the same time. This is the wonder of nature. 7. Amoeba ( µ) Look at the Amoeba. Amoeba can be between µ in size. Amoeba is a fast dividing cell and inspite of a large structure, it can live very peacefully by dividing itself very quickly. 8. Muscle cell (upto about 3-5 ft. long) Look at this picture of a muscle, a typical striated muscle cell. Can you imagine how long a muscle can be? Let me tell you and you will be amazed that a muscle cell can easily be atleast about 3-5 ft. long.

16 They can be tiny also. They can also be even just 1 cm long but they can be as long as 3-5 ft. and in certain cases even 5 ft. long. They can such long cells, one single cell I am talking about. 9. Neuron (upto about 3-5 ft. long) All the neurons that are lying parallel to the muscle cells can also assume such a long length. CONCLUSION Hence, it can be noted that, in a cellular organism, since there are various functions to be performed by different tissues, the cells of these different tissues will exhibit a great diversity in their shape & size.