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Vacuole

Vacuole

A vacuole is a characteristic type of organelle found in plant and fungi cells and many single-cell organisms. The single large vacuole of the cell is surrounded by a membrane, called the tonoplast, and filled with a solution of water, dissolved ions , sugars, amino acids , and other materials.

In plants, nicotine and other toxins are stored in vacuoles, since these are as toxic to the plant as they are to the herbivores they are meant to repel. The tart juice of the orange and other citrus fruits is stored in vacuoles, as are the bright pigments that give autumn leaves their color. The vacuole also serves as waste disposal and recycling center for worn-out organelles, such as mitochondria and chloroplasts, and in this function they are similar to lysosomes in animal cells. Expansion of the vacuole by water intake is the major driving force in plant cell growth, and is also the means for maintaining cell rigidity, or turgor . To increase turgor, the tonoplast will pump ions or other material into the vacuole, causing water to infiltrate by osmosis . In a mature cell, the vacuole may occupy as much as 90 percent of the cell volume, such that the rest of the cell contents are flattened against the cell membrane.

see also Anatomy of Plants; Cell Wall; Fungi; Protista; Secondary Metabolites in Plants; Water Movement in Plants

Richard Robinson

Bibliography

Raven, Peter, Ray F. Evert, and Susan E. Eichhorn. Biology of Plants, 6th ed. New York: W. H. Freeman and Company, 1999.

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Vacuoles

Vacuoles

Vacuoles are organelles of plant, fungal, and algal cells. They are part of the internal membrane system and are separated from the rest of the cytoplasm by a membrane called the tonoplast . A single large vacuole occupies more than 80 percent of the volume of most plant cells, mature fugal hyphae, and some algal cells. Many smaller vacuoles are found in expanding plant cells and in the tips of growing fungal hyphae . These vacuoles can be less than one micrometer in diameter. As the cell in which they reside matures, smaller vacuoles fuse to produce larger vacuoles.

Vacuoles are multifunctional organelles, and individual cells may contain more than one kind of vacuole, each kind having a different function. Vacuoles play crucial roles in cell expansion, serve as storage compartments for nutrients, and function as lytic organelles that contain digestive enzymes . Compounds contained within vacuoles also protect cells against environmental damage and deter attack by herbivores .

Vacuoles take up water through specialized membrane transporters called aquaporins. The hydrostatic pressure that develops within each cell, known as turgor pressure , is required for cell expansion and growth. Turgor pressure is carefully regulated in plants, fungi, and many algae by controlling rates of water and ion movement through the tonoplast. In fresh water algae and fungi lacking cell walls, contractile vacuoles fill with excess water from the cytosol and their contents are expelled from the cell through specialized pores.

The vacuole is an acidic organelle, and the pH of most vacuoles is around 5 to 6. Vacuole acidity is important for its lytic function since many vacuo-lar enzymes work most efficiently at or near pH 5. Acidification of vacuoles is brought about by transporters embedded in the tonoplast. These transporters use the energy stored in adenosine triphosphate (ATP ), or in some cases, pyrophosphate, to pump protons from the cytosol into the vacuole. In extreme cases, such as in the lemon fruit juice sac, the pH of the vacuole can be as low as 2.

Vacuoles store organic acids, carbohydrates, proteins, and minerals. Some of these compounds are important for human nutrition. These include proteins stored in the cotyledons of beans and peas or the grains of cereals; simple sugars such as sucrose found in many fruits, the stems of sugarcane and the roots of sugar beets; and minerals such as potassium. In the leaves and stems of forage grasses, vacuoles store complex polysaccha-rides that are the principal energy source for herbivores.

Many other compounds accumulate in vacuoles. These include the water-soluble anthocyanin pigments that give the blue or red color to red beets, grapes, and peonies. Anthocyanins are also contained in the vacuoles of leaves and stems and are important photoprotectants that absorb excess light. Alkaloids , enzyme inhibitors, and toxins are contained in some vacuoles. Although these compounds may deter herbivory, some have been used to produce medicines. Aspirin and morphine are two examples. Waste products and xenobiotics , including herbicides, are often shuttled into vacuoles by specialized membrane transporters. Once in the vacuole, these compounds are digested or detoxified.

see also Anthocyanins; Cells.

Paul Bethke

Russell Jones

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"Vacuoles." Plant Sciences. . Encyclopedia.com. 19 Aug. 2017 <http://www.encyclopedia.com>.

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vacuole

vac·u·ole / ˈvakyoōˌōl/ • n. Biol. a space or vesicle within the cytoplasm of a cell, enclosed by a membrane and typically containing fluid. ∎  a small cavity or space in tissue, esp. in nervous tissue as the result of disease. DERIVATIVES: vac·u·o·lar / ˌvakyoōˈōlər; ˈvakyoōələr/ adj.vac·u·o·la·tion / ˌvakyoōəˈlāshən/ n.

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vacuole

vacuole A space within the cytoplasm of a living cell that is filled with air, water or other liquid, cell sap, or food particles. In plant cells there is usually one large vacuole bounded by a single-layered membrane (tonoplast or vacuole membrane); animal cells usually have several small vacuoles. See also contractile vacuole.

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vacuole

vacuole A membrane-bound sac that is found in many cells, normally acting as a storage organ of various types. A large central vacuole is a particular feature of many plant cells, where it can occupy 80–90 per cent of the total cell volume.

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vacuole

vacuole Membrane-bound, fluid-filled cavity within the cytoplasm of a cell. Vacuoles perform various functions including the discharge of wastes from cell metabolism.

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vacuole

vacuole (vak-yoo-ohl) n. a space within the cytoplasm of a cell, formed by infolding of the cell membrane, that contains material taken in by the cell.

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vacuole

vacuole A membrane-bound sac found in many cells; vacuoles normally act as storage organs of various types.

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vacuole membrane

vacuole membrane (tonoplast) See vacuole.

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