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| Membranous Cell Organelles |
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| Most of the organelles in the cell are bound by unit membranes similar to the cell membrane. |
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| Following are the membrane-bound organelles, which can be recognized in a generalised eukaryotic cell. |
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| It is a system of highly branching membranes, which almost fills the space inside the cell membrane. Some of the branches of endoplasmic reticulum are connected to the plasma membrane while some of them are connected to the nuclear membrane. An examination of endoplasmic reticulum under the electron microscope reveals two types of membranes: |
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 Smooth endoplasmic reticulum (SER) in which the surface of the tubules lacks granules. |
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| fig. 16.5 - Smooth Endoplasmic Reticulum |
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 Rough endoplasmic reticulum (RER) in which the surface of the tubules contain numerous granule-like structures called ribosomes. |
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| fig. 16.6 - Rough Endoplasmic Reticulum |
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| Endoplasmic reticulum is absent in some cells such as embryonic cells and mature red blood corpuscles. |
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| Endoplasmic reticulum serves the following functions. |
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 It facilitates the transport of materials inside the cell. |
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 It forms an internal supporting framework for the cell, called cytoskeleton. |
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 It provides surface for the synthesis of a variety of substances in the cell. SER provides surface for the synthesis of lipids while RER provides surface for the synthesis of proteins and carbohydrates. |
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 It provides a pathway for the distribution of nuclear material from one cell to another. |
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 Being highly coiled in nature, it provides an increased surface area for various enzyme catalyzed reactions. |
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| Mitochondria are tiny structures that occur in varying shapes cylindrical, spherical, oval, rod shaped etc. They are found distributed all over the cytoplasm. |
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| fig. 16.6 - Mitochondria |
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| Each mitochondrion is bound by a double membrane. The outer membrane is smooth and straight, while the inner membrane is thrown into numerous folds called cristae. Thus, the surface area of inner membrane is much higher than that of outer membrane. The cristae bear numerous, minute, regularly spaced particles called elementary particles or oxysomes. |
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| There may be any where between 1,00,000 to 10,00,000 oxysomes in a single mitochondrion! |
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| The oxysomes enclose enzymes that are involved in the release of ATP molecules form the oxidation of glucose. Rest of the inner membrane contains some coenzymes that are involved in this process. |
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| The inner membrane encloses a matrix, which is rich in proteins, lipids, ribosomes, RNA and one or two circular DNA molecules. |
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| The number of mitochondria in a cell varies considerably and depends on the mature of the cell. Cells with very high-energy requirements possess very large number of mitochondria. A human liver cell for example may have more than 1000 mitochondria. So also the flight muscle cells in insects. Mitochondria are absent in the mature human RBC. |
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| Mitochondria are able to change shape and even move to the areas where considerable activity is taking place. This is facilitated by cytoplasmic streaming movement. |
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| Mitochondria represent the sites of aerobic respiration in a cell. They use molecular oxygen to oxidize glucose into carbon dioxide and water. Oxidation releases energy in the form of molecules of adenosine triphosphate (ATP), which are stored and released in the cell. Hence, mitochondria are described as minute powerhouses of the cell. |
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| Golgi complex is represented by several structures like membranes, vesicles and granules. It appears as a central stack of flattened inter communicating sacs called cisternae in a parallel arrangement along with tubules and vesicles. The cisternae are continuously being formed at one end and bud off as vesicles at the other end of the stack. In plant cells cisternae are 15 to 20 in number and form discrete structures called dictyosomes. In animals they are 3 to 7 in number. |
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| fig. 16.8 - Golgi Complex |
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| Golgi complex takes its origin from the membranes of the SER. Golgi complex is absent in some cells such as mature RBC. They may become modified in some cells, such as the acrosome of sperms. |
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| The main function of Golgi complex is to take part in secretion. It modifies, sorts out and packages proteins and lipids received from endoplasmic reticulum. It is also known to be involved in the formation of lysosomes. |
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| Lysosomes are tiny, spherical sac-like structures evenly distributed in the cytoplasm. They are seen practically in all animal cells with the exceptions like mature mammalian RBC. Largely, they are absent in plant cells. |
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| The membrane that surrounds a lysosome, encloses a variety of lytic enzymes (Lysis=dissolve). This membrane prevents the enzymes from reaching cytoplasm. The substance that is to be hydrolyzed has to enter a lysosome. |
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| When a cell is injured or dead, the lysosome membrane ruptures spontaneously releasing the enzymes. |
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| The lysosomes are involved in several lytic processes in the cell such as: |
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 It brings about digestion of useful organic substances present in the cell (intracellular digestion). |
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 The old and worn out cells are destroyed and the component molecules are made available for the formation of new cells. |
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 At times the lysosomal enzymes are capable of destroying the entire cell. |
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| Lysosomes are commonly described as suicide bags or time bombs of the cell. |
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| Plastids are organelles that occur mostly in plant cells. They may also occur in some protozoans like Euglena. In embryonic cells they occur in the form of proplastids. |
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| Plastids are mainly of two types - leucoplasts and chromoplasts. |
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| Leucoplasts are colourless plastids. They do not enclose any colouring pigment. They occur in cells in those regions of a plant that are not exposed to sunlight. They are mainly involved in storage. |
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| Chromoplasts are plastids that contain various colouring pigments. They occur in cells in those regions that are exposed to sunlight. Some of the chromoplastids contain fat-soluble pigments that are yellow, red and orange in colour. The chief pigments are carotenoids. Chromoplasts impart bright colours to the flowers for pollination. Some of the chromoplasts exclusively contain a green coloured pigment called chlorophyll. Such plastids are called chloroplasts. |
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| Chloroplasts represent the most common type of plastids. They occur in varying shapes oval or spherical or disc shaped etc. They are found in very large numbers in cells that are actively taking part in photosynthesis. For example, the palisade parenchyma cells in the leaves of dicot plants contain as much as 500 to 600 chloroplasts per cell. |
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| fig. 16.9 - Chloroplast in Section |
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| Each chloroplast is bound by a double membrane. Both the membranes are smooth and straight. The inner membrane encloses a matrix called stroma. It contains proteins, lipids, sugars, ribosomes, RNA, few circular DNA molecules, pigment enzymes and metallic ions. The stroma also has a number of units called grana. Each granum is composed of many disc shaped structures called thylakoids placed one above the other. The grana are connected to one another through membranous extensions called intergrana lamellae or Fret membranes. The thylakoid membrane contains numerous photosynthetic units called quantasomes. |
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| Quantasomes are photosynthetic units in the chloroplasts, which can trap solar energy and convert it into chemical energy. A quantasome contains chlorophyll molecules associated with specific pigment systems called photo systems. Mainly two types of chlorophyll, chlorophyll-a and chlorophyll-b and two types of photosystems Photosystem-I and Photosystem-II are involved in the process of photosynthesis. |
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