After reading this term paper you will learn about:- 1. Plasma Membrane 2. Cell Wall 3. Nucleus 4. Mitochondria 5. Endoplasmic Reticulum 6. Ribosome 7. Centriole 8. Golgi Complex 9. Plastids.
Term Paper on Eukaryotic Cell
Term Paper Contents:
- Term Paper on Plasma Membrane
- Term Paper on Cell Wall
- Term Paper on Nucleus
- Term Paper on Mitochondria
- Term Paper on Endoplasmic Reticulum
- Term Paper on Ribosome
- Term Paper on Centriole
- Term Paper on Golgi Complex
- Term Paper on Plastids
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1. Term Paper on Plasma Membrane:
In animal cells there is found an external covering is known as plasmalemma, cell membrane or plasma membrane. This membrane it provides is living ultra-thin, elastic, porous, and semipermeable. Primarily it provides mechanical support and external shape to the protoplasm.
It also helps to check the entry or exit of undesirable substances and it allows the transmission of necessary materials to and from the cells. Eukaryotic plasma membrane neither possess enzymes of aerobic metabolism and nor the small projections on the inner side of the plasma membrane.
The electron microscope has also revealed much about the plasma membrane that surrounds the cell. The membrane is now known not merely to enclose the cell but also to form the organelles within the cytoplasm. Organelles are specialized parts of a cell that have specific functions, just as organs have in higher forms. The plasma membrane therefore serves as a partition to subdivide the cell space into self-contained compartments in which biochemical reactions may take place for the living process.
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According to the electron microscope, the plasma membrane appears as two black lines with a space between and is about 70 to 100 A thick (an angstrom is 1/00,000,000 cm). Its structure suggests a lipid-protein membrane made up of two lipid monolayers between two protein monolayers.
Phospholipid molecules are long, with elongated hydrophobic organic compounds and a hydrophilic polar group at one end. Such molecules tend to be absorbed at an air-water or water-oil interface because the lipid end enters the oil or air and the polar end enters the water.
The molecules also tend to be tightly packed and oriented in parallel layers at interfaces. Many aspects of this phosphor-protein membrane are obscure. It serves to regulate the molecular traffic in and out of the cell. As far as, the molecular structure is concerned, it is composed of two, internal and external, protein layers and central bilayers of lipid molecules.
The specific protein and lipid components very generally from one type of membrane to another, but the basic structural and functional concepts are applicable to intracellular membranes as well as to plasma membranes. No. of models were suggested to explain the molecules structure of plasma membranes and the most, accepted models are discussed as below.
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Robertson’s Unit Membrane Model:
Unit membrane model was forward in 1953 by Robertson. The unit membrane is considered to be trilaminar with a bimolecular lipid layer between two protein layers. Two parallel outer dense osmiphilic layers of 20 Å which correspond to the two protein layers. The middle light coloured osmiophilic layer is about 35 Å its thickness.
Micellar Model:
A model of plasma membrane was postulated by Hilleir and Hoffman (1953). According to them biological membranes may have a non-lamellar pattern, consisting instead of a mosaic of globular subunits known as micelles, which have a lipid core and hydrophilic shell of polar groups. Lipid micelles are possible building-blocks for membranes since they tend towards spontaneous association.
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In this model of membrane structure the protein components of the membrane may form a monolayer on either side of the plane of lipid micelles. The spaces between the globular micelles are thought to form water-filled pores 0.4 nm (4Å) in diameter, lined partly by the polar groups of the micelles and partly by the polar groups of associate protein molecules.
Fluid Mosaic Model:
This model was suggested by Singer and Nicolson (1972). According to this model, the lipid molecules are arranged to form a rather continuous bilayer that forms the structural framework of plasma membrane. The protein molecules are arranged in two different manners.
Some proteins are located exclusively adjacent to the outer and inner surfaces of lipid bilayer and are called extrinsic proteins. Other proteins penetrate lipid bilayer partially or wholly and form integral or intrinsic proteins. The lipids and integral proteins of plasma membrane are amphipatic in nature.
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The term amphipaty was coined by Hartley, (1936) for those molecules which have both hydrophobic and hydrophilic groups. Therefore, the lipid molecules form a rather continuous bilayer. The integral proteins are intercalated in the lipid bilayer, with their polar regions protruding from the surface and non-polar regions embedded in the lipid bilayer.
Specialization of Plasma Membrane:
Microvilli:
These are minute structures which appear just like fine hairs. In the intestinal epithelium microvilli are very prominent and form a compact structure that appears under the light microscope as a striated border. These microvilli are 0.6 to 0.8 µm long and 0.1 µm in diameter and represent cytoplasmic processes from the plasma membrane.
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The outer surface of the microvilli remains covered by a coat of filamentous material composed of glycoprotein macro-molecules. Microvilli increase the effective surface of absorption. They are reported to be present in mesothelial cells, in the epithelial cells of the gall-bladder, uterus hepatic cells, kidney tubules, yolk sac etc.
Desmosomes:
Basically these are the functions which are found between the cells. Mainly these are cell junctions, i.e. desmosomes are found in the cells of simple columnar epithelium. These occur as specialized area along the contact surfaces. Under light microscope the desmosomes are seen as darkly stained bodies.
The thickenings are traversed by fine cytoplasmic fibrils called tonofibrils, which form a kind of loop in a wide are. These filaments stabilize the main function of tonofibrils is to provide stabilization to the junction. These fibrils also act as anchoring sites for the cytoplasmic structures. The plasma membranes of adjoining cells in the regions of desmosomes remain separated by an intercellular space. This space remains filled with intervening mucopolysaccharides and proteins coating material. It is concerned with cell adhesion.
Plasmodesmata:
When the cells are joined by bridges of cytoplasm passing between pores of cell wall or plasma membrane between the adjacent cells, then connections are called plasmodesmata. Usually these are simple but anastomosing plasmodesmata may also found.
They provide a mean for interaction between adjacent cells which are separated in other regions. Through them, the material can pass form cell to cell. At the basal surface of some epithelial cells, these are found connections similar to plasmodesmata but there are called hemi-desmosomes as they show half of the former.
Besides above mentioned specialization of plasma membrane, there are also found terminal bars, gap junctions, tight junction and septate junctions in different types of cells. Plasma membranes have selective permeability. This selective permeability helps them to maintain a proper balance of organic and inorganic substances on which life depends.
Mechanisms derived from chemical reactions within the cell (active transport) are often referred to as pumping devices. Thus by phagocytosis the membrane forms pockets in which solid food particles are engulfed. These pockets with the enclosed substance are pinched off and form a vesicle of food vacuoles within the cytoplasm.
Another pumping device is pinocytosis, or cell drinking, which is similar to phagocytosis except that drops of liquid are taken up discontinuously and sucked in to form vesicles within the cell. The concept has been broadened to include the uptake of dissolved substances as well as liquids. By this method it is though that ions, sugars, and proteins can be pumped into the cell.
The average diameter of a pinocytosis vacuole is about 1 to 2µ but often they coalesce. The formation of a pinocytosis vacuole takes about 30 minutes. Besides these methods of substance transfer across membranes, there are many instances that cannot be explained by any known method of permeability.
Osmosis is a special type of diffusion which involves the movement of water or other solvent molecules through a semipermeable or differentially permeable membrane from an area of high potential (pure solvent) to an area of low potential (more concentrated solution).
The entry of water into the cell from its medium is called endosmosis; the reverse process in which water leaves the cell is called exosmosis. An osmotic pressure is maintained by the salts present in cytoplasm. Passive Transport is straight diffusion of water, ions or molecules of various substances move through the plasma membrane from a region of higher concentration to the low concentration.
Transport of molecules takes place along concentration gradient so that no energy is required for diffusion. The diffusion of ions across the membranes is even more difficult because it depends not only on the concentration gradient but also on the electrical gradient present in the system.
Because active transport is a process that works against a concentration gradient, it is not surprising to learn that it requires the expenditure of energy. The process involves the use of carrier molecules within the cell membrane proper.
These carrier molecules apparently shuttle back and forth between the inner and outer cell membrane surfaces and either pick up or release a particular ion being regulated. The energy required for this process is obtained from adenosine triphosphate (ATP), which produced mainly by oxidative phosphorylation in mitochondria.
2. Term Paper on Cell Wall:
The presence of cell wall is the characteristic of plant cells. It represents the outermost bounding membrane, encircling the cytoplasm. Cell wall is absent in animals. In plants, this cell wall is hard, rigid and non-elastic, and made of a complex polysaccharide carbohydrate, the cellulose. The cell wall gives protection and support to the plasma membrane and the cytoplasm which lies beneath it.
The cell walls of some plant cells possess some pit-like or canal-like minute apertures by which the cells remain connected with each other. These canals are called plasmodesmata. In animal cells, the external covering is known as plasmalemma, cell membrane or plasma membrane.
It is living, ultra-thin, elastic, porous, and semipermeable. It primarily gives mechanical support and external shape to the protoplasm, checks the entry or exit of undesirable substances and it allows the transmission of necessary materials to and from the cells.
It consists of three layers— outer and inner layers are of proteins and the middle layer of lipids. The main function of the cell wall in plants is to provide mechanical strength. Being hydrophilic in nature it is capable of imbibing water and thus helps in the movement of water.
3. Term Paper on Nucleus:
Most conspicuous and lying usually in the centre of the cell is a rounded or avoid body. This body is known as nucleus. It is delimited from the cytoplasm by its extremely thin membrane, called karyotheca or nuclear membrane. This karyotheca consists of two membranes and also have some pores, called the nuclear pores. Both these membranes merge at the pores, openings of which allow the transfer of materials between nucleus and cytoplasm.
Inner membrane remains in contact with the chromatin fibres, while the outer membrane with the membranes of endoplasmic reticulum and plasma membrane. There is also present space between two nuclear membranes of the nuclear envelope which is known as perinuclear space.
The pores of the nuclear membrane are filled with electron-dense proteinaceous substance called the annular material. Beneath the nuclear membrane is the nucleoplasm or nuclear sap which represents uncondensed regions of chromatin; i.e., nucleoproteins where the chromosomes are largely dispersed.
In the nucleoplasm of nucleus, chromatin, nucleolus, endosomes, etc., are present. Nucleus controls all the vital activities of the cytoplasm and it carries the hereditary material the DNA. During cell division chromosomes become visible.
Within the nucleoplasm also occurs a conspicuous spherical body. This body is known as the nucleolus. It is very large in nerve cells; pancreatic cells. It plays an important role in the process of protein synthesis. The nucleoli are either single or multiple and usually acidophilic. The nucleus is a must for eukaryotic cells. In prokaryotic cell’s nucleus remains absent.
4. Term Paper on Mitochondria:
Mitochondrion (mitochondria = plural) is another most important organelle of the cell. These show their presence in all cells and can be detected by the light microscope. They show considerable diversity in shape, size, and number.
Many of them are rod-like and are about 0.2 to 5 µ in greatest diameter. About two thirds of their structure is protein and one third is lipid. According to the physiologic condition of the cell, mitochondria can alter their shape. They may be scattered more or less uniformly through the cytoplasm or they may be localized near cell surfaces and other regions whereas there is unusual metabolic activity.
Investigation of the fine structure of the mitochondrion with the electron microscope and centrifuge microscope reveals that it is a double membrane system that may be formed from detached pockets of the cell membrane. The inner layer of the double membrane is much folded and forms cristae.
There cristae may extend into the interior fluid or matrix. There are thus two structural systems—the membrane system and the homogenous fluid matrix. Both the membranes and the matrix contain many oxidative enzymes and coenzymes. The mitochondria contain DNA molecules and ribosomes and hence they synthesize certain proteins.
Mitochondria play a very significant role in cellular respiration and energy production. As the mitochondria play an important role in energy production so it is also known as ‘Power house of cell’. ATP is generated during this process by the oxiosomes of the mitochondria. Besides, the mitochondria help in the formation of yolk in a developing ovum.
According to Benda, Dulberg and Meves mitochondria also divide equally during the cytoplasmic division and perhaps play a part in inheritance.
Mitochondria also take part in gluconeogenesis, which is the conversion of non-carbohydrates to glucose from pyruvic acid. It is well known that pyruvic acid is converted into oxaloacetic acid in the presence of pyruvic acid carboxylase. This intermediate may escape mitochondria and become converted into phosphoenol pyruvic acid by phosphoenol pyruvic carboxyl kinase.
These are also present enzymes for oxidative deamination of amino acids in mitochondria. They are also able to oxidize fatty acids. Oxidation of fatty acid requires complete oxidation of ocetyl CoA in the Krebs’s cycle so that force CoA may be generates. Reversal of fatty acid oxidation leads to fatty acid synthesis. During starvation the mitochondria utilize fat to produce energy.
5. Term Paper on Endoplasmic Reticulum:
The focal point of investigation has been directed toward a complex membrane system called the endoplasmic reticulum. This system has been closely associated with the storage and transport of products of cellular mechanisms. The nuclear membrane is formed from parts of this membrane system, and it is so arranged that there is direct continuity between nucleus and cytoplasm by openings in the nuclear membrane.
The double-walled endoplasmic reticulum is a highly variable morphologic structure consisting of vesicles and tubules often it has the power to fragment and reform its structural features. There are two types of endoplasmic reticulum; these are rough and smooth-surfaced.
The rough surfaced type has on its outer surface small granules called microsomes, the dense granules are often called ribosomes because they contain ribonucleic acid. These ribosomes are important sites of protein synthesis. In some cases the granules can function without being attached to the membrane.
The endoplasmic reticulum is also important as an intracellular transport system for the product synthesized by the ribosomes. Among the substances transported by this system are the zymogens bodies that give rise to digestive enzymes and that are carried to the smooth-surfaced Golgi vesicles. Later these zymogen granules are discharged as enzymes through openings at the surface of the cell.
By this relationship of the endoplasmic reticulum to the surface membrane of the cell, the cell is a three-phase structure consisting of cytoplasm cavities of the endoplasmic reticulum, and membranes separating the other two phases. The concept also includes the idea that there is really a one-membrane unit from the cell surface membrane to the system of interior membranes.
The endoplasmic reticulum is found to be continuous with the plasma membrane and nuclear membrane. It forms the ultra-structural skeletal framework of the cytoplasm, giving mechanical support to the cytoplasm. It also acts as an intracellular circulatory system, through its various substances flow into and out of the cells. Synthesis of lipids, cholesterol glycerides, glycogen, etc., also occurs in the endoplasmic reticulum.
Smooth ER is also involved in the detoxification of many endogenous and exogenous compounds. Some specific instances of development more or less confirms the contention that the ER is important in the process of cell differentiation.
Not only this much, ER also plays role in coordinating the differentiation. Numerous enzymes mainly those involved in the metabolism of steroids, phospholipids and hormones are associated with the membranes of smooth endoplasmic reticulum. Electron microscopic studies suggest that the endoplasmic reticulum in plants play a special role in the inter connection of cells through the cytoplasmic strands called plasmodesmata. Most of cell organelles like Golgi complex, mitochondria, lysosomes, nuclear membrane and cell plate etc., are usually developed from endoplasmic reticulum.
6. Term Paper on Ribosome:
Ribosomes are rounded structures and remains occur either attached or scattered form. These are minute spherical structures found attached with the membrane of the endoplasmic reticulum and in the cytoplasm. These originate in the nucleolus and consist of mainly the ribonucleic acids (RNA) and protein.
In the electron microscopic studies negative staining reveals a cleft that divides the ribosomes into a larger sub-unit and a smaller sub-unit. In E. coli the larger particle is somewhat ‘cup’ shaped or dome shaped (140 to 169 Å) and smaller one forms a ‘cap’ (90-110 Å) that is applied to the surface of the other. In higher animals and plants it was shown that the ribosomes are attached to endoplasmic reticulum by the larger sub-units.
The ribosomes are of two basic types 70 S and 80 S ribosomes. The ‘S’ refers to Svedbere units. Actually it is the sedimentation coefficient which shows how fast cell organelle sediments in an ultracentrifuge.
It is well-known that ribosomes are factories of manufacturing of protein in a cell but actually one-ribosome cannot take part in the protein synthesis. Active units are no individual’s ribosome, but a group of these units, which is called as polyribosomes. Actually it is the polyribosomes, which are responsible for production of protein synthesis.
7. Term Paper on Centriole:
It is cell discovered by Van Benden in 1887. It is a clear zone around centrioles and also called microcentrum. It is found near the nucleus and includes a specialized portion of cytoplasm, called centrosphere. Its matrix is called as kinoplasm, in which two rounded bodies or centrioles are embedded.
Each centriole is open at both ends and each centriole consists of nine fibrillar units, and each fibrillar unit is found to contain three microtubules arranged in a circle. Both the centrioles are arranged at right angles to one another. The function of centrioles is to form the spindle of microtubules at the time of cell division. Plant cells lack centrioles, but the spindle is formed without their aid. It contains 9 marginal microtubules with no central tubules.
At the start of prophase the centrioles, as soon as they have reproduced, migrate toward opposite sides of the nucleus. At the same time portions of the cytoplasm are attracted to the regions of the centrioles and are transformed into fine get fibrils. Some of these fibrils run between the two centriole complexes to form a spindle, and some radiate out from each centriole or pole to form asters.
The whole structure is called a mitotic apparatus and it increases in size as the centrioles move farther apart. In higher animals the cells at mitosis have two large asters, one at each end of the spindle. At the center of each aster there is a spherical centrosome within which is the centriole. These asters are especially large at telophase.
Some plants do not have asters, but spindles are formed. During this process the nuclear membrane disappears, and the nucleolus disintegrates or becomes invisible. The mitotic apparatus is composed mainly of a single type of protein and some RNA nucleotides.
8. Term Paper on Golgi Complex:
It is the most important constituent of cytoplasm which is especially developed in high metabolically active cells. It is a stack of flattened parallelly arranged structures found in the association of endoplasmic reticulum. It is composed of many lamellae, tubules, vesicles, and vacuoles.
Its membrane is made of lipoproteins and supposed to be originated from the membranes of endoplasmic reticulum. In plant cells, the golgi complex is called dictyosome and it secretes necessary materials for the formation of cell wall at the time of cell division. It helps in the formation of acrosome of sperms, release of enzymes, hormones and other synthetic materials.
It is suggested that Golgi complex has different origin according to different scientists. According to Essner and Novikoft Golgi, cisternae arise from the ER. Bouch described the origin of Golgi from outer membrane of nuclear envelope in brown algae. It has also been suggested that Golgi complex may be originated by the division of pre-existing dictyosome.
9. Term Paper on Plastids:
These are characteristic of plant cells but these are also found in case of certain animal cells. They may be coloured green like chloroplasts, or colourless like leucoplasts. The leucoplasts store starch and lipids and are hence called amyloplasts and lipoplasts respectively.
The chloroplasts contain DNA, ribosomes and complete protein synthetic machinery. The chloroplasts held in photosynthesis and protein storage. The chloroplast has double outer membrane, a stroma pilled with many soluble enzymes, and a complex system of membrane-bound compartments.
Chloroplasts contain a kind of semiautonomous genetic system with its own DNA. They also contain ribosomes and RNA molecules, and are able to synthesize some of their own proteins.