|
A contractile vacuole (abbreviation: CV) is a sub-cellular structure (organelle) involved in osmoregulation. It is found predominantly in protists and in unicellular algae. It was previously known as pulsatile or pulsating vacuole. ==Overview== The contractile vacuole acts to regulate quantity of water in a cell. In freshwater environments the concentration of solutes inside the cell is higher than outside the cell (i.e., the environment is hypotonic). Under these conditions water flows from the environment into the cell by osmosis. The contractile vacuole acts as part of a protective mechanism that prevents the cell from absorbing too much water and possibly lysing (rupturing) through excessive internal pressure. The contractile vacuole is a specialised type of vacuole; vacuoles are found in most cells but not all vacuoles are contractile vacuoles. The contractile vacuole, as its name suggests, expels water out of the cell by contracting. The growth (water gathering) and contraction (water expulsion) of the contractile vacuole are periodical. One cycle takes several seconds, depending on the species and the environment's osmolarity. The stage in which water flows into the CV is called diastole. The contraction of the contractile vacuole and the expulsion of water out of the cell is called systole. Water always flows first from outside the cell into the cytoplasm, and is only then moved from the cytoplasm into the contractile vacuole for expulsion. Species that possess a contractile vacuole typically always use the organelle, even at very hypertonic (high concentration of solutes) environments, since the cell tends to adjust its cytoplasm to become even more hyperosmotic than the environment. The amount of water expelled from the cell and the rate of contraction are related to the osmolarity of the environment. In hyperosmotic environments less water will be expelled and the contraction cycle will be longer. The most well understood contractile vacuole belong to the protists ''Paramecium'', ''Amoeba'', ''Dictyostelium'' and ''Trypanosoma'', and to a lesser extent the green alga ''Chlamydomonas''. Not all species that possess a contractile vacuole are freshwater organisms; some marine, soil microorganisms and parasites also have a contractile vacuole. The contractile vacuole is predominant in species that do not have a cell wall, but there are exceptions (notably ''Chlamydomonas'') which do possess a cell wall. Through Evolution, the contractile vacuole has typically been lost in multicellular organisms, but it still exists in the unicellular stage of several multicellular fungi, as well as in several types of cells in sponges (amoebocytes, pinacocytes, and choanocytes). The number of contractile vacuoles per cell varies, depending on the species. ''Amoeba'' have one, ''Dictyostelium discoideum'', ''Paramecium aurelia'' and ''Chlamydomonas reinhardtii'' have two, and giant amoeba, such as ''Chaos carolinensis'', have many. The number of contractile vacuoles in each species is mostly constant, and is therefore used for species characterization in systematics. The contractile vacuole has several structures attached to it in most cells, such as membrane folds, tubules, water tracts and small vesicles. These structures have been termed the spongiome; the contractile vacuole together with the spongiome is sometimes called the contractile vacuole complex (CVC). The spongiome serves several functions in water transport into the contractile vacuole and in localization and docking of the contractile vacuole within the cell. ''Paramecium'' and ''Amoeba'' possess large contractile vacuoles (average diameter of 13 and 45 µm, respectively), which are relatively comfortable to isolate, manipulate and assay. The smallest known contractile vacuoles belong to ''Chlamydomonas'', with a diameter of 1.5 µm. In ''Paramecium'', which has one of the most complex contractile vacuoles, the vacuole is surrounded by several canals, which absorb water by osmosis from the cytoplasm. After the canals fill with water, the water is pumped into the vacuole. When the vacuole is full, it expels the water through a pore in the cytoplasm which can be opened and closed. Other protists, such as ''Amoeba'', have CVs that move to the surface of the cell when full and undergo exocytosis. In ''Amoeba'' contractile vacuoles collect excretory waste, such as ammonia, from the intracellular fluid by both diffusion and active transport. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「contractile vacuole」の詳細全文を読む スポンサード リンク
|