A vacuole () is a membrane-bound organelle which is present in plant and fungal cells and some protist, animal, and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic molecules including enzymes in solution, though in certain cases they may contain solids which have been engulfed. Vacuoles are formed by the fusion of multiple membrane vesicles and are effectively just larger forms of these. The organelle has no basic shape or size; its structure varies according to the requirements of the cell.
Discovery Antonie van Leeuwenhoek described the plant vacuole in 1676. Contractile vacuoles ("stars") were first observed by Spallanzani (1776) in protozoa, although they were mistaken for respiratory organs. Dujardin (1841) named these "stars" as vacuoles. In 1842, Schleiden applied the term for plant cells, to distinguish the structure with cell sap from the rest of the protoplasm. In 1885, de Vries named the vacuole membrane as tonoplast. Christian de Duve discovered mammalian lysosomes using biochemical methods in the mid-1970s. De Duve named lysosomes based on their biochemical properties (from the Greek lysis – digestive and soma – body). Their physical form was confirmed shortly afterward by electron microscopy. Because the lysosome shares many properties with vacuoles across taxonomical kingdoms, the notion that vacuoles and lysosomes are distinctly different organelles is more historical than functional.
Function The function and significance of vacuoles varies greatly according to the type of cell in which they are present, having much greater prominence in the cells of plants, fungi and certain protists than those of animals and bacteria. In general, the functions of the vacuole include:
Isolating materials that might be harmful or a threat to the cell Containing waste products Containing water in plant cells Maintaining internal hydrostatic pressure or turgor within the cell Maintaining an acidic internal pH Containing small molecules Exporting unwanted substances from the cell Allowing plants to support structures such as leaves and flowers due to the pressure of the central vacuole By increasing in size, allowing the germinating plant or its organs (such as leaves) to grow very quickly and through using up mostly just water. In seeds, storing proteins needed for germination (these are kept in 'protein bodies', which are modified vacuoles). Vacuoles also play a major role in autophagy, maintaining a balance between biogenesis (production) and degradation (or turnover), of many substances and cell structures in certain organisms. They also aid in the lysis and recycling of misfolded proteins that have begun to build up within the cell. Thomas Boller and others proposed that the vacuole participates in the destruction of invading bacteria and Robert B. Mellor proposed organ-specific forms have a role in 'housing' symbiotic bacteria. In protists, vacuoles have the additional function of storing food which has been absorbed by the organism and assisting in the digestive and waste management process for the cell. In animal cells, vacuoles perform mostly subordinate roles, assisting in larger processes of exocytosis and endocytosis. Animal vacuoles are smaller than their plant counterparts but also usually greater in number. There are also animal cells that do not have any vacuoles. Exocytosis is the extrusion process of proteins and lipids from the cell. These materials are absorbed into secretory granules within the Golgi apparatus before being transported to the cell membrane and secreted into the extracellular environment. In this capacity, vacuoles are simply storage vesicles which allow for the containment, transport and disposal of selected proteins and lipids to the extracellular environment of the cell. Endocytosis is the reverse of exocytosis and can occur in a variety of forms. Phagocytosis ("cell eating") is the process by which bacteria, dead tissue, or other bits of material visible under the microscope are engulfed by cells. The material makes contact with the cell membrane, which then invaginates. The invagination is pinched off, leaving the engulfed material in the membrane-enclosed vacuole and the cell membrane intact. Pinocytosis ("cell drinking") is essentially the same process, the difference being that the substances ingested are in solution and not visible under the microscope. Phagocytosis and pinocytosis are both undertaken in association with lysosomes which complete the breakdown of the material which has been engulfed. Salmonella is able to survive and reproduce in the vacuoles of several mammal species after being engulfed. The vacuole probably evolved several times independently, even within the Viridiplantae.
Types
Central
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