Cell Vacuole Essay
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Cell Vacuole Essay Cell vacuoles are present in both animal cells and plant cells. Those found in animal cells are usually relatively small such as phagocytic vacuoles, food vacuoles, autophagic vacuoles and contractile vacuoles. Plants, however, have much larger cell vacuoles which are usually the largest structure in a plant cell often dominating the inside of the cell, crowding other organelles to the cell wall. The vacuole is bound by a very active membrane called the tonoplast. There are many different types of vacuoles, each with their own specialised functions. Here are examples of some common types of cell vacuoles: * Plant sap vacuole - present only in plants * Contractile vacuole * Food vacuole * Lysosomes * Pinocytic vesicles As mentioned earlier, the plant cell vacuole is surrounded by a membrane, the tonoplast. This membrane separates the cytoplasm from the vacuole and controls the exchange of substances between the vacuole and the cytoplasm. An important feature of plant vacuoles is its osmotic properties. These are responsible for a variety of essential processes that occur in plants including support by turgidity to water transport throughout the plant. It is to water transport via plant vacuoles to which we first turn to. The movement of water into and out of cells is based on the principle of osmosis where there is net movement of water molecules from an area of high water potential to an area of low water potential across a selectively permeable membrane.
These compounds are usually waste products and secondary products of metabolism which are accumulated in the vacuole e.g. crystals of calcium oxalate have sometimes been observed in certain plant cell vacuoles. Secondary products of metabolism such as alkaloids and tannins are mainly responsible for the deterring of animals. Some waste materials produced and stored by plants are of use to humans. For example, latex, a milky liquid, from rubber tress contains chemicals needed for rubber synthesis. Latex from opium poppies contain alkaloids such as morphine from which heroin can be obtained. The importance of the plant cell vacuole and its association with osmosis has already by illustrated with the role of water transport. Nevertheless, osmosis also plays a vital role in the growth of the plant via its interaction with the cell vacuole. This process is called vacuolation whereby the cells elongate as their vacuoles absorb water. This is a very efficient method of growth as the cells become larger with minimal investment in new cytoplasm. As well as water transport and growth, the vacuole also provides support for the plant using the principle of osmosis. When the solution outside the plant cell has a higher water potential than the cell vacuole or a plant is well watered then water will enter the cell by osmosis.
Besides helping to feed the cell, lysosomes play an important custodial role when cell components wear out. Lysosomal enzymes can degrade membranes, ribosomes, proteins and a variety of other components and the subunits can then be returned to the cytoplasm for reuse. In white blood cells they are used to digest bacteria. The head of a sperm contains a special lysosome which is used for digesting a path to the ovum. Sometimes, within injured or old cells, lysosomes also break open and free their enzymes, literally digesting the cell from the inside out. Another function of vacuoles, similar to phagocytosis, is pinocytosis. A good example of this is in capillaries where the membranes of cells that line capillaries pinch off vacuoles filled with yellowish, protein - rich blood serum, carry the fluid across the cytoplasm of the cell as a pinocytic vesicle, then expel it from the capillary. The discharge of the fluid vascuolar contents is often called reverse pinocytosis. by coupling pinocytosis and reverse pinocytosis, capillary cells can transport serum from the blood vessel to the surrounding tissue space without exposing the serum components to the cytoplasm, where it might be chemically damaged. Vacuoles play vital roles not just in plant but also in many other types of cells. Their functions range from providing a plant with support to digesting endocytotic materials. The diversity of their structure means that they are suited to an extensive and varied range of tasks. Sean Fang 12SD
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