After reading this term paper you will learn about:- 1. Introduction to Cell 2. Historical Aspects of Cell 3. Cell Theory 4. Shape and Size of Cells 5. Methods of Studying Cells 6. Flux of Cells 7. Compartments of Cells.
Term Paper on Cell
Term Paper Contents:
- Term Paper on the Introduction to Cell
- Term Paper on the Historical Aspects of Cell
- Term Paper on the Cell Theory
- Term Paper on the Shape and Size of Cells
- Term Paper on the Methods of Studying Cells
- Term Paper on the Flux of Cells
- Term Paper on the Compartments of Cells
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Term Paper # 1. Introduction to Cell:
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Cell is an essentially functional unit (performing all the vital activities of life), comprising the protoplasm limited by a membrane and containing one or more nuclei at some time in its life. A living cell has capacity to liberate energy for its life activities. The cell is responsive to its environment, and synthesizes cell substances for growth and cell division.
Before a science is properly established as such, it must have- (a) One or more basic concepts, (b) A body of observational and experimental data, and (c) A series of working hypotheses. By the turn of the century, cytology was well equipped with regard to these requirements. The basic concept, of course, was the notion that the cell represented the unit of structure, function, and reproduction. Although the idea that the cell represents the basic unit of living forms and functions is still often referred to as the cell theory, it has long passed the status of theory and should be known as the cell concept or doctrine.
Perhaps no principle of biology is more accepted or is considered more important. It is virtually the chief cornerstone for biologic study and understanding. There must have been many precellular forms in the long evolution of the cell because the properties of life did not arise all at once. Many intermediate forms represent continuity from the nonliving to the living. In this way, the cell representing a combination of all the vital phenomena may be considered the basic unit of life.
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Term Paper # 2. Historical Aspects of Cell:
Like all other all-important concepts, the cell concept also has an extensive background of development. The English scientist R. Hook is often credited with seeing and naming the first cells when the observed the small boxlike cavities in the surface of cork and leaves.
The classic microscopist A. van Leeuwenhoek, the Dutch lens-maker, described many kinds of cells in addition to his famous protozoan discoveries. M. Malpighi, the Italian microscopist who described capillary circulation among many other discoveries no doubt observed cellular units.
The French biologist R. Dutrochet gave some basic ideas about cells in 1824. In 1931, R. Brown discovered and described the nuclei of cells. J. Purkinje (1839) not only described cells as being the structural elements of plants and animals but also coined the term protoplasm for the living substance of cells. M.J. Schleiden and T. Schwann, German biologists, are often given credit for the cell theory formulation (1838) because of their rather extensive descriptions and diagrams, although they had erroneous ideas about how cells originate.
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In 1858, R. Virchow stressed the role of the cell in disease, or pathology and stated that all cells came from preexisting cells. M. Schultze (1864) gave a clear-cut concept of protoplasmic relations to cells and its essential unity in all organisms.
It may be said in summarizing the main features of the cell doctrine that all plants and animals are composed of cells or cell products, that a basic unity exists in their physical construction, and that all cells come from preexisting cells. These cells lead to the fundamental idea that the total processes and activities of life can be interpreted on the basis of the cellular components of the organism.
This statement does not contradict the belief that the whole organism behaves as a unit in its development and in its integration of cell activities or that the action of cells is determined in accordance with the physiologic behaviour of the organism at all stages of its existence. This limitation of the cell doctrine is often expressed as the “inadequacy of the cell theory.”
Term Paper # 3. Cell Theory:
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In 1838, M. J. Schleiden (1804-1881), a German botanist, observed nucleoli and said that cells are the units of structure in plants, thus formulating his famous cell theory. In 1839, a German zoologist, T. Schwann (1810-1882), extended this view of cell concept to animals, i.e., all organisms are composed essentially of cells.
This cell theory as postulated by Schleiden was primitive in its considerations and was modified according to advanced research in Cytology. Although Schleiden and Schwann are universally recognized as the founders of cell theory, but its significance was earlier realized by Leeuwenhoek, Grew, Malpigte and others. Wolff (1759) clearly demonstrated the “spheres” and “vesicles” composing the various parts of body.
Later, Mirbel, Sprengel and Treviranus concluded the existence of cells and Oken (1805) foreshadowed the concept of cell theory. Later, Meyen, Von Mohl and Raspail clearly defined the cells and thus formulated the cell theory, which was developed by Schleiden and Schwann. The immediate followers of cell theory were Remak, Nageli and Kollikers, etc., who demonstrated the cell division as basis of genetic continuity.
Recently, organismal theory has been introduced, according to which an organism is regarded as a protoplasmic unit which is incompletely divided into small centres, the cells, for the performance of various biological activities.
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It is just a modified interpretation of cell theory. Laurence Picken, in “The organization of cells” (1960), accepts cell as an organism. “At its own level of organization it is a unity, and it remains a unity, though without analytical mental-equipment we conceive it more easily as a plurality of discriminated organelles.”
According to Rudolf Virchow (1858), each animal is composed of a sum of vital units bearing complete characteristics of life. He confirmed the cell’s unique role as a vessel of living matter, arising from pre-existing cells, i.e., omnis cellulae cellula. Cell is usually defined as a structural and functional unit of body.
More often, it was defined generally as a mass of protoplasm containing a nucleus. But this definition was also erroneous as red blood corpuscles of mammals have no nuclei. So this definition was modified that cell is a mass of protoplasm which contains one or more nuclei at some time in its life.
Thus, “Cell is an essentially functional unit (performing all the vital activities of life), comprising the protoplasm limited by a membrane and containing one more nuclei at some time in its life.”
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According to A.G. Loewy and P. Siekevitz (1963), cell is a “unit of biological activity delimited by a semipermeable membrane and capable of self-reproduction in a medium free of other living systems”. This definition does not apply to the viruses which lack a definite semipermeable membrane and nucleus.
Modern Concept of Cell Theory:
The evolutionary pattern of life from the unicellular to the multicellular forms of life has increased in complexity as new qualities arise at each level. The various functions of the life process carried on at the unicellular stages tend to be allotted to specialized cells in multicellular organisms.
Functional specialization is accompanied by structural specialization or division of labour and the hierarchy of tissues, organs, and organ systems arise as a consequence in the evolutionary development of life. Although each cell is integrated with the functioning of the body as a whole, it retains the capacity to act independently of the others.
One cell of a group divide, secrete, or die, while adjacent cells may be in a different physiologic state. Previously, the structure of the cell was thought to be as two-phase system, consisting of nucleus and cytoplasm.
It was realized that the two phases functioned as an integrated whole and contained within them various organoids such as nucleoli, mitochondria and paraplasm or Golgi complex embedded in a semifluid medium. Thus, cell appeared as a heterogeneous system comprising elements of different shapes and sizes.
Typically, a cell is a semifluid mass of microscopic dimensions, completely enclosed within a thin, differentially permeable plasma membrane usually a cell consists of two distinct regions—the nucleus and cytoplasm. Cells which lack nuclei are known as prokaryotic cell while those with nuclei called eukaryotic. The nucleus remains enclosed in a membrane which is known as nuclear membrane.
The nucleus contains the chromatin and one nucleolus or more. Within the cytoplasm remains present many organelles, such as mitochondria, Golgi complex, centrioles and endoplasmic reticulum. In addition to these organelles, plant cells also contain plastids or chloroplast.
Above said the structure of the cell was described to be as two-phase system. It was realized that the two phases functioned as an integrated whole and contained within then various organoids.
Thus, cell appeared as a heterogeneous system comprising elements of different shapes and sizes. Electron microscopy has changed this conception drastically and revealed that the basic components of the cell are essentially the forms of a limited number of basic components.
These basic structural units of the cell are:
(i) The membranes
(ii) The microtubules or fibres, and
(iii) The granules.
Term Paper # 4. Shape and Size of Cells:
Cells may be of different shapes and sizes. Their different shapes are mostly correlated with their particular function. Although many cells, because of surface tension forces, will assume a spherical shape when freed from restraining influences, there are others that retain their shape under most conditions because of their characteristic cytoskeleton, or framework.
These cells may attain shapes as rounded, cylindrical, irregular, triangular and tubular. They may be columnar; flat, spherical stellate or long and thin. The shape of cells depends mainly on functional adaptations and partly on the surface tension and viscosity of the protoplasm, the mechanical action exerted by the adjoining cells and the rigidity of the cell membrane. The microtubules also take the shape of the cell.
Cells vary greatly in size. Some of the smallest animal cells are certain parasites that may be 1 µ (1/25,000 inch) or less in diameter. At the other extreme we have the fertilized eggs of birds, some of which, including the extracellular material, are several inches in diameter. A red blood corpuscle in man has a diameter of about 7.5 µ.
The longest cells are nerve cells because the fibers, which are parts of the cells, may be up to several feet long. Some striped muscle cells or fibers are several inches long. For example, the ostrich egg cell is 176 mm in diameter, thus visible to the naked eye. But this is an exception; the great majority of cells are visible only with a microscope. The smallest animal cells have a diameter of 4 µm. The nerve cell found in mammals may reach a length of 3 or 3.5 feet.
Shape, like size, is highly variable, ranging from spherical to columnar and including amorphous types which have no specific geometrical formula. Most of this variation can be assigned more or less adequately to extraneous factors such as mechanical pressure and surface tension.
A naked protoplast left to itself tends to approach the spherical as an ideal rarely realized in nature except to a certain extent in the case of gamete mother cells and possibly certain blood cells. That there is a close correlation between shape and function is generally admitted, but again the question has received relatively little attention.
A few such associations appear obvious, as, for example, the varying shape of an amoeba with motility, the spindle shape of a smooth muscle cell with unidirectional contraction and elongation, and the columnar shape of a vascular element in plants with transport of sap. How such shapes become established is, however, an unsolved problem bound up with the whole question of growth and differentiation.
Term Paper # 5. Methods of Studying Cells:
Since cells are small and mostly invisible, it follows that the microscope has been the tool of choice in studying them. But the microscope alone was unable to fulfill its function without the aid of staining methods. Fortunately the discovery and development of the aniline dyes by W.H.
Perkin and others gave the investigators of the last half of the nineteenth century the opportunity to work out the details of cellular structures and cell division within the limits of the light microscope. It was at this time that cytology, the study of cells, developed into a flourishing science—a study that has greatly broadened under the impact of the electron microscope.
Various techniques have constantly widened in every generation of investigators. Among these advances are the careful histologic techniques of fixing the tissues to preserve them as naturally as possible, the art of preparing and slicing tissues with microtome and proper staining methods for differential staining of cell constituents, or the selective affinity of the different cell components for the various stains.
More precise physicochemical methods for locating specific entities within cells and for identifying them are constantly being sought. Ultraviolet light is employed because different chemical substances absorb rays of characteristic wavelengths.
Some of the histochemical techniques for demonstrating inorganic or organic substances in cells and tissues are:
(i) The periodic acid-Schiff (PAS) reaction for showing carbohydrates;
(ii) The fluorescent antibody method that injects antibodies conjugated with a fluorescent substance into an animal, determining where the antibodies are localized in the cells; and
(iii) Injecting tagged atoms that have been labelled with a radioactive isotope (tritium HP), iodine 131, and many others, and then photographing the desired specimen of tissue on a special photographic emulsion plate that will record the beta or other particles from the radioactive isotope.
Term Paper # 6. Flux of Cells:
In many organisms, certain tissues continually shed their cells because of wear and tear or other causes. The epidermis of the skin, the lining of the alimentary canal, and the blood- forming tissues lose large numbers of cells daily. There must be a constant replacement of the cells that are lost, for there is no net loss or gain in the overall picture.
In main it has been estimated that the number of cells shed daily is about 1% to 2% of all the body cells. Due to contact with the environment the organism have to face a constant physical and chemical force. Mechanical rubbing wears away the outer cells of the skin, and emotional stresses destroy many cells.
Food in the alimentary canal rubs off lining cells, the restricted life, cycle of blood corpuscles must involve a renewal of enormous numbers of replacements, and during active sex life many millions of sperm are produced each day. This loss is made up by a chain reaction of binary fission or mitosis.
At birth the child has about 2,000 billion cells. This immense number has come from a single fertilized egg (zygote). Such a number of cells could be attained by a chain reaction in which the cell generations had divided about 42 times, with each cell dividing once about every 6 to 7 days.
In about five more cell generations by the chain reaction, the cells have increased to 60,000 billion at maturity (in an individual of 170 pounds). However, not all cells divide at the same rate and some cells (nervous and muscular), as we have seen, stop dividing altogether at birth. The growth of an organism is not merely an increase in number of cells but it also involves some molecular reproduction or increase in cell size.
The life span of different cells varies with the tissue, the animal and the conditions of existence. Nerve cells and muscle cells, to some extent, persist throughout the life of the higher animal. Red blood corpuscles live about 120 days. The normal process of metamorphosis found in many animals involves a great loss of cells. Many cells are removed in the shaping of organs during morphogenesis.
Term Paper # 7. Compartments of Cells:
The compartments of a cell are typically called organelles. These organelles are the source of metabolic functions that occur in each cell. Mitochondria and chloroplasts are the powerhouse of the cells, providing energy. Chloroplasts are found particularly in plants, while mitochondria are found within the cells of animals and humans.
ER or endoplasmic reticulum is the transportation network for particles targeted to specific destinations or to join with specific proteins. Golgi apparatus is similar to the wrapping department at a store during the holidays.
They package various proteins and lipids after being synthesized by the cell. Inside a cell, digestion is also a part of daily operations. Lysosomes provide the acids for digestion, which means digesting old cell parts, as well as viruses and bacteria when they invade a cell. In many ways, these provide a cleaning and security service all in one at the cellular level.
Vacuoles are the waste storage facility, until the cell is able to send the waste out of the cell itself. For plants, vacuoles are also serve a water storage function at the cellular level.
When plants stalks are standing upright, part of that structure definition comes from these water containers. Finally, there are ribosomes. These are complex combinations of both RNA and various proteins. Consider this a factory where various proteins are synthesized into amino acids under the direction of RNA.