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| Non-Membranous Cell Organelles |
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| Following are the cell organelles that are not bound by the unit membrane. |
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| Ribosomes are granule-like structures found in both prokaryotic and eukaryotic cells. They are found distributed all over the cytoplasm. In a eukaryotic cell ribosomes occur freely in the cytoplasm, attached to the surface of branches of endoplasmic reticulum (RER), in the matrix of organelles like mitochondria and chloroplasts. It is also found attached to the nuclear membrane and inside the nucleolus. |
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| Ribosomes are of two types 70s type and 80s type. 70s ribosomes are found in prokaryotic cells while 80s ribosomes are found in eukaryotic cells. |
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| S stands for swedberg unit, which is a measure of particle size related to the speed at which the particles settle when subjected to centrifugation (S=1x10-13second). |
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| Each ribosome consists of two sub units. The sub units occur separately in the cytoplasm and join to form a ribosome particle only at the time of protein synthesis. The 70s ribosome has two sub units of 30s and 50s. The 80s ribosome has two subunits of 60s and 40s. The composition of each sub unit is indicated in the following diagram. |
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| fig. 16.10 - Ribosome and its Composition |
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| Many a times a number of ribosomes are found arranged in a series along a branch of EPR. Such strings of ribosomes, are called polyribosomes or polysomes. |
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| Ribosomes provide space for the synthesis of proteins in the cell. Hence, they are commonly described as protein factories of the cell. |
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| The cytoplasm contains a fine network of fibrous protein elements that form a framework for supporting the extensively branched membranes found in the relatively fluid cytoplasmic matrix. This framework is called cytoskeleton. It is found to be composed of three types of organelles microtubules, microfilaments and microbodies. |
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| fig. 16.11 - Structural Organization of Microtubule, Microfilament and Intermediate Filament |
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| Microtubules are hollow, unbranched, cylindrical tubules, which occur singly or in bundles. They radiate from the centrioles towards the periphery of a cell. The wall of a microtubule is composed of 13 parallel filaments called protofilaments. They are made up of a protein called tubulin. Microtubules provide support to other cell organelles, form the motile elements of cilia and flagella and are involved in the formation of spindle fibres during cell division. |
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| Microfilaments are solid, unbranched rod-like fibrils of varying length. They are most predominant in the muscle cells, where they are called as myofilaments. They are mainly composed of a globular protein called actin. Some of them are composed of another contractile protein called myosin. Apart from providing support, microfilaments are involved in intracellular movements including the streaming movements. |
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| Microbodies are minute, roughly spherical structures, which enclose a dense, granular matrix. It contains a variety of enzymes, which catalyse oxidation reactions not involved in respiration. The microbodies can be further distinguished into peroxysomes and glyoxysomes. Peroxysomes contain peroxide-synthesizing and peroxide-destroying enzymes. Glyoxysomes occur only in the cells of germinating seeds. |
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| Under the electron microscope the centrioles appear as two short, hollow, cylinders usually lying at right angles to each other. Each centriole is made up of nine microtubule triplets, which lie evenly spaced in a ring. There are no microtubules in the center (9+0 arrangement). |
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| The centrioles appear as two, darkly staining granules, usually above the nucleus in animal cells. They are generally absent in plant cells, except in motile cells. The centrioles lie in a small mass of specialised cytoplasm called centrosphere. It distinctly lacks other cell organelles. The centrioles and the centrosphere are together described as centrosome. |
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| fig. 16.12 - Structure of a Centriole |
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| Under the electron microscope the centrioles appear as two short, hollow, cylinders usually lying at right angles to each other. Each centriole is made up of nine microtubule triplets, which lie evenly spaced in a ring. There are no microtubules in the center (9+0 arrangement). |
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| Centrioles are involved in organizing the spindle fibres and astral rays during cell division in animal cells. They provide basal bodies from which arise cilia and flagella. |
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| The basal bodies, also called basal granules, are structures, which bear cilia and flagella. They exhibit a structure, which is exactly similar to a centriole. In fact, a centriole can become a basal body and a basal body can become a centriole by losing the cilium or flagellum. |
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| Cilia and Flagella |
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| Cilia and flagella are fine vibratile cytoplasmic processes arising from basal bodies. They are similar in their structure, chemical composition and function. Both serve to propel the organism or move a medium past a fixed cell. However, they differ in their size, number and mode of beating. |
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| Flagella occur in protozoans like Euglena and in several examples of algae. They also occur in choanocytes of sponges. |
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| fig. 16.13 - Structure of Cilium |
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| Cilia occur in protozoans like Paramecium and in flame cells of flatworms. They also occur in epithelial cells lining moist regions of the body. |
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| Flagella occur in protozoans like Euglena and in several examples of algae. They also occur in choancytes of sponges. |
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| The cilia and flagella are surrounded by a membrane. It encloses a matrix containing a supporting shaft or axoneme. The axoneme is composed of eleven microtubules (9+2 arrangement). |
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| The movement of cilia and flagella is brought about by sliding of the nine double microtubules past each other, using energy from ATP. |
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| Ciliary and Flagellar Movements |
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| The cilia normally exhibit a metachronous or isochronous rhythmic beat. In metachronous beat, cilia of a row beat one after the other while in isochronous rhythm all the cilia of a row beat simultaneously. |
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| fig. 16.14 - Ciliary and Flagellar Movements |
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| The beating of cilia occurs in two phases power or effective phase and recovery phase. During power phase cilia become straight and stiff and move against the medium such as water. This movement pushes water backwards and propels the organism forwards. In the recovery phase, cilia become limp and return to their original position in a curved state, offering minimum resistance to water. The whole movement can be compared to the rowing of a boat. ATP is utilized during both the phases. |
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| The beating of flagella is independent and involves an undulating movement. The waves of undulation pass from the base to the tip of the flagellum. Power and recovery strokes of the flagellum move the cell forwards. |
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