Golgi apparatus: Golgi apparatus was discovered by Golgi. This apparatus which was found virtually in all eukaryotic cells consists of stacks of flattened membrane bound sacs called cisternae. These cisternae together with associated vesicles are called Golgi complex. It is a complex system of interconnected tubules around central stacks. Cisternae stacks are continuously formed by fusion of vesicles which are probably derived by budding of SER. Their outer convex surface is forming face while inner concave surface is maturing face. The cisternae break up into from the latter. Whole stack consists of number of cisternae thought to be moving from outer to inner face. Golgi complex is concerned with cell secretions. Secretions are products formed within cell on ribosomes and then passed to outside through endoplasmic reticulum and Golgi apparatus. Secretions are converted into finished product and are packed inside membrane before export. For example in mammals pancreas secrets granules containing enzymes that help in digestion. Golgi complex has role in formation of these granules. Proteins or enzymes which have to be transported out of cell pass through the Golgi apparatus. The most important function of apparatus is to modify proteins and lipids by adding carbohydrates and converting them into glycoproteins or glycolipids.
Lysosomes: Lysosomes are cytoplasmic organelles and are different from others due to their morphology. These were isolated as a separate component for first time by De Duve. They are found in most eukaryotic cells. Any foreign object that gains entry into the cell is immediately engulfed by lysosome and is completely broken into simple digestible pieces. This process is known as phagocytosis. They are most abundant in those animal cells which exhibit phagocytic activity. They are bounded by single membrane and are simple sacs rich in acid phosphatase and several other hydrolytic enzymes. These enzymes are synthesized on RER and are further processed in Golgi apparatus. The processed enzymes are budded off as Golgi vesicle and are called as primary lysosomes. They contain those enzymes which can digest phagocytosed food particles. They are also involved in autophagy. During this process some old worn out parts of cell such as old mitochondria are digested. In this way materials of cell may be recycled and cell may be renewed. Their enzymes can also result in degeneration of cell as may occur during some developmental processes. Lysosomes also release enzymes for extra cellular digestion.
Several congenital diseases have been found to be due to accumulation within cell of substances such as glycogen or various glycolipids. These are also called storage diseases and are produced by mutation that effect one of the lysosomal enzymes involved in catabolism of certain substance. For example Tay-Sachs disease is because of absence of an enzyme that is involved in catabolism of lipids. Accumulation of lipids in brain cell lead to mental retardation and even death.
Peroxisome: De-Duve and coworkers isolated in 1965 particles from liver cells and other tissues which were enriched with some oxidative enzymes such as peroxidase, catalase, glycolic acid oxidase and some other enzymes. The name peroxisome was applied because this organelle is specifically involved in formation and decomposition of hydrogen peroxide in cell. These are single membrane enclosed cytoplasmic organelle found both in animal and plant cells. These are characterized by containing hydrogen peroxide producing oxidases and catalase. They have also been found in protozoa, yeast and many cell types of higher plants.
Glyoxysomes: Plants contain an organelle which in addition to glycolic acid oxidase and catalase also possess number of enzymes that are not found in animal cells. This organelle called glyoxysomes are most abundant in plant seedlings which rely upon stored fatty acids to provide them with energy and material to begin the formation of a new plant. One of primary activities in these germinating seedlings is conversion of stored fatty acid to carbohydrates. This is achieved through a cycle glyoxylate cycle the enzymes of which are located in glyoxysomes. In plants peroxisomes play important role in both catabolic and anabolic pathways. In seeds rich in lipids such as castor bean and soya beans glyoxysomes are sites for breakdown of fatty acids to succinate. This organelle is present only during a short period in germination of lipid rich seed and is absent in lipid poor seed such as pea.
Vacuoles: Although vacuoles are present both in animal and plant cells they are particularly large and abundant in plant cells often occupying major portion of cell volume and forcing remaining intracellular structures into thin peripheral layer. These vacuoles are bounded by single membrane and are formed by coalescence of smaller vacuoles during plant growth and development. They are serve to expand the plant cell without diluting its cytoplasm and also function as sites for storage of water and cell products or metabolic intermediates. Plant vacuole is major contributor to turgor that provides support for individual plant cell and contributes to rigidity of leaves and younger parts of plants.
Cytoskeleton: Cytosol contains cytoskeletal fabric formed of microtubules, microfilaments and intermediate filaments. Main proteins that are present in cytoskeleton are tubulin, actin, myosin, tropomyosin and others which are also found in muscles. Several cell organelles are derived from special assemblies of microtubules for examples cilia, flagella, basal bodies and centrioles. Movement of cyclosis and amoeboid movements are because of microfilaments whereas intermediate filaments are involved in determination of cell shape and integration of cellular compartments. Microtubules are long, unbranched and slender tubulin protein structures. One very important function of them is their role in assembly and disassembly of spindle structure during mitosis. Microfilaments are considerably more slender cylinders made up of contractile actin protein linked to inner face of plasma membrane. They are involved in internal cell motion. Intermediate filaments have diameter in between those of microtubules and microfilaments. They play role in maintenance of cell shape.
Centriole: Animal cells and cells of some microorganisms and lower plants contain two centrioles located near exterior surface of nucleus. In cross section each centriole consists of cylindrical array of nine microtubules. However each of nine microtubule is further composed of three tubules. The two centrioles are usually placed at right angle to each other. Just before cell divides its centrioles duplicate and one pair migrates to opposite side of nucleus. Spindle then forms between them. They are absent in higher plants. Centrioles play important role in the location of furrowing during cell division and in the formation of cilia.
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