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

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.

Vacuole

A vacuole is a large, fluid-filled storage sac inside a cell. Many types of cells contain vacuoles, but they are most prominent in plant cells. A vacuole can store materials that a cell needs. It also provides the necessary internal pressure for a plant to remain erect and not wilt.

A vacuole is an organelle or a specialized structure inside a cell that has a particular function. Its main purpose involves storage and transport of materials inside a cell. Vacuoles in animal cells can store a variety of substances, including lipids (a kind of fat) and carbohydrates. In plants, vacuoles are very prominent, and when seen under a microscope they appear as large, round, clear structures. Vacuoles are filled with a watery fluid called cell sap (made up of water, salts, and sugar). In some plant cells, a vacuole may occupy as much as 90 percent of a cell's total volume. When a vacuole takes up this much space in a cell, it presses the cytoplasm (jelly-like fluid in a cell) against the cell wall, which eventually remains stretched out under this force. As long as the vacuole remains full, the plant maintains its turgor pressure (like the air pressing against the inside wall of a balloon). When the vacuole loses this fluid and decreases the pressure, it collapses temporarily and the plant wilts. Maintaining this cellular pressure is the prime responsibility of the vacuole in a mature plant. When pressure is maintained, the entire plant is able to keep its crisp shape. When it does not, the whole plant wilts.

[See alsoCell; Organelle; Plant Anatomy ]

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.

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.