vacuole(redirected from vacuolar)
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An intracellular compartment, bounded by a single membrane bilayer, which functions as a primary site of protein and metabolite degradation and recycling in animals, but serves additional complex functions in fungi and plants (see illustration). Scientists who study vacuoles also define them as the terminal product of the secretory pathway. The secretory pathway functions to transport protein and metabolite-containing membrane vesicles from sites of synthesis or uptake to the vacuole. See Cell membranes, Cell metabolism, Golgi apparatus, Secretion
In animals, a lytic vacuole known as the lysosome typically functions to process macromolecules. Such macromolecules can be targeted to the lysosome from sites of synthesis. For example, proteins that assemble incorrectly in the endoplasmic reticulum (ER) can be degraded in the lysosome and their constituent amino acids recycled. Proteins that can serve as nutrients are also targeted to the lysosome from the cell surface. An important process for the recycling of cytoplasm in eukaryotic cells is autophagy, in which molecules or organelles are encapsulated in membrane vesicles that fuse with the lysosome. See Endocytosis, Endoplasmic reticulum, Lysosome
In the mammalian immune system, macrophages and neutrophils take up particles and pathogens in the process of phagocytosis, during which the pathogen is eventually digested in the lysosome. A number of diseases in humans can be caused when intracellular pathogens evade destruction in the lysosome. See Phagocytosis
In fungi, vacuoles can serve functions not found in animals. Besides a lytic function, they serve in the storage of ions as well as amino acids for protein synthesis. In yeast, vacuoles can also function in the destruction and recycling of cellular organelles, such as peroxisomes, which help protect the cell from toxic oxygen-containing molecules. The process of peroxisome digestion by vacuoles is known as pexophagy.
The most complex vacuoles are found in plants. Some contain hydrolytic enzymes and store ions similar to those found in lysosomes, whereas others serve a role in storing pigments which impart color to flowers to attract pollinators. Specialized ER-derived vacuoles in plant seeds, known as protein bodies, function in the storage of proteins called prolamines that are common in the endosperm of cereals. Upon germination, the proteins are degraded and used as a source of amino acids and nitrogen for the growing plant. Toxins, such as alkaloids, are stored in vacuoles in parts of the plant, such as the leaves, which are subject to frequent herbivory. Scientists have learned that plants produce and store in their vacuoles a vast array of unique chemicals which may, in addition to their natural functions, have medicinal value. See Plant cell
Another unique function that vacuoles serve in plants is in cell growth. As a consequence of the accumulation of ions, metabolites, and water, plant vacuoles are under considerable internal osmotic pressure. The vacuolar membrane in plants, known as the tonoplast, as well as the cell itself would burst under this pressure if not for the rigid wall that surrounds the cells. The resulting turgor pressure provides mechanical stability to plant stems. Loss of osmotic pressure in the vacuole due to a lack of water results in plant wilting. The osmotic pressure of the vacuoles also provides the driving force that allows plants to grow by enlarging their cell volume. Enzymes reduce the rigidity of the cell wall, which permits cell expansion under the force of turgor. This is a fundamental process in plants and explains why vacuoles can occupy as much as 95% of the volume of some cells. See Cell walls (plant)