What Is The Function Of The Contractile Vacuole – The contractile vacuole function in the cell is a form of regulation of water levels. This organelle is found mainly in single-celled organisms that do not have a cell wall. Basically, the function of the contractile vacuole is to expand and contract in order to control the water levels inside and outside the cell. Contractile vacuoles (CV) are unlike any other type of vacuoles in animal or plant cells that store food or water.
A contractile vacuole (CV) is a sub-cellular organelle in an organism involved in osmoregulation. This structure was formerly known as a pulsatile or pulsating vacuole. It is a special type of vacuole found in most living organisms—a single cell without a cell wall. Contractile vacuoles can be found widely in the Protista kingdom for example amoeba and unicellular algae.
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What Is The Function Of The Contractile Vacuole
As the name suggests, the contractile vacuole expands and contracts to pump water out of the cell through a process known as osmoregulation. Osmoregulation involves the control of osmotic pressure. Contractile vacuoles occur in freshwater protists but are more common in the entire Protista kingdom. However, not all species with a contractile vacuole live in freshwater environments, some are aquatic and terrestrial.
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As abundant as contractile vacuoles are in organisms without cell walls, there are few exceptions. The CV through evolutionary processes was largely eliminated in many organisms. However, this organelle is still present in the unicellular stage of many multicellular fungi and in several cell types in sponges, such as pinacocytes, amoebocytes, and choanocytes.
A contractile vacuole can be defined in biology as a specialized vacuole in eukaryotic cells such as protozoa that is involved in osmoregulation. This organelle allows the flow of water from the cytoplasm of the cell and then by opening the narrow neck, it lets the water out. In other words, a contractile vacuole is a structure that collects excess water and removes it from protists or other organisms.
Contractile vacuoles regulate organelles. These organelles are usually spherical or star-like and can be found in freshwater protozoa and lower metazoans, such as sponges and hydras. The function of the contractile vacuole is to collect excess water from the protoplasm and periodically pour it into the surrounding medium. Also, the contractile vacuole functions in the release of nitrogenous wastes. For example, sponges have a contractile vacuole that absorbs waste.
A contractile vacuole is not like other types of vacuoles. In general, vacuoles are membrane-bound sacs that appear in various forms. These organelles are found in protists, plant cells, fungi, and some animal cells. However, contractile vacuoles should not be confused with fat that stores food or water. Contractile vacuoles differ from other types of vacuoles because they can contract and expand to control the amount of water in the cell.
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Some types of vacuoles have specific functions in the cells they are found in but the contractile vacuole is most commonly seen in protists and unicellular algae. In freshwater habitats, the concentration of solutes in the cell is usually higher than the concentration of solutes outside the cell. Because of this situation, water flows from the environment into the cell through osmosis. Under such conditions, the contractile vacuole acts as a defense against cell proliferation (or bursting) from excess water. In the cell, the contractile vacuole is used to expel excess water from the cell by contraction.
Also, depending on the type, the filling and draining cycle with CV can last from seconds to minutes. The contractile vacuole in the amoeba changes position according to the movement of the organism. In most ciliates, contractile vacuoles follow a specific path through the cell. Whereas, the contractile vacuole in euglena and other flagellates remains stationary.
Water usually flows into the cytoplasm first from the outside of the organism’s cell and moves only into the CV from the cytoplasm of expulsion. Species with a CV always use the organelle even in a highly hypertonic (high concentration of solutes) environment. This is because the cell normally adjusts its cytoplasm to be more hyperosmotic than the environment. The rate of CV expansion and the amount of water expelled from the cell depends on the osmolarity of the environment. For example, in a hyperosmotic environment, the amount of water expelled will be less and circulation will be longer.
The structure of the contractile vacuole is unlike other types of vacuoles that store food or water. When seen with an electron microscope, the contractile vacuole structure is seen surrounded by a system of membranous vesicles and (or) tubules.
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In most cells, several structures such as membrane folds, tubules, water tracts, and small vesicles are attached to the CV. These structures are known as spongiomes. Contractile vacuoles may also be connected by canals and pores to the outside. The CV together with the spongiome form the contractile vacuole complex (CVC).
If there is too much water, the contractile vacuole is used to expel the excess water from the cell by contraction. In a cell, a fixed amount of solute (other substances) vs solvent (water) is needed. This means that the water in the cell needs to be balanced with the other substances in the cell.
A cell works through osmosis and water moves across a semi-permeable membrane. The contractile vacuole, therefore, functions to pump water out when there is too much water. This protects the cell because if there is too much water in the cell, the cell swells and swells until it bursts and dies.
The contractile vacuole serves to keep this water balance in check and growth (accumulation of water), as well as contraction (expulsion of water) of the CV, occurs periodically. A single cycle can take several seconds. This depends on the quality and osmolarity of the environment. Contractive vacuoles expand when fluid enters, and the phase at which fluid flows into the CV is called diastole. When the CV is full of water, it contracts and takes water and waste out of the cell. This stage of contraction and expulsion of fluid from the cell by the CV is called systole.
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The role of fluid in CV has been unclear for years but there have been several discoveries since the 1990s that have increased the understanding of the process. Theoretically, water can cross the CV membrane by osmosis, but only if the inner contractile vacuole is hyperosmotic to the cytoplasm. This only applies if there is a higher solute concentration within the CV than in the cytoplasm. Also, the pumping of protons either into or out of contractile vacuoles can cause different ions to enter the CVs. For example, some proton pumps act as cation exchangers, where a proton is pumped out of the CV and at the same time, a cation is pumped into the CV.
In some cases, protons pumped into the contractile vacuole drag anions with them (e.g carbonate) to balance the pH. This ion flow in the CV causes an increase in the osmolarity of the CV and due to this, water enters the vacuole by osmosis. In some species, water has been shown to enter the contractile vacuole through aquaporins. Moreover, acidocalcisomes have been suggested to work together with CV in response to osmotic stress.
Acidocalcisomes are acidic calcium-storage organelles found in various organisms. These organelles were found in the vacuole located inside
. When cells were exposed to osmotic stress, acidocalcisomes were observed to fuse with the vacuole. Presumably, these organelles pour their contents of ions into the contractile vacuole, thereby increasing the osmolarity of the vacuole.
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Contractile vacuoles are present in the cells of lower organisms to play an important role in excretion and osmoregulation. Impurities such as ammonia and other salts can be released through the vacuoles in order to maintain osmotic balance. Thus, the role of the contractile vacuole complex (CVC) is to act as an osmoregulatory organelle in nature.
In free-living amoebae and protozoa, CVC controls intracellular water balance by accumulating and expelling excess water from the cell. For example, in pond water, this allows cells to survive under hypotonic stress. Without a functional CV complex, cells cannot expel water. They, therefore, become more swollen and lyse
The contractile vacuole is an organelle found in protists such as paramecium, amoeba, euglena, Dictyostelium, and Trypanosoma. These organisms have very well understood contractile vacuoles and the function of the contractile vacuole in these unicellular organisms is to remove excess water from the cell. The number of contractile vacuoles per cell varies according to the type. For example, Amoeba has one CV while organisms like
, they have many CVs. The number of CVs in each species is usually constant and is often used for species selection in the system.
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Paramecium (paramecia) is a single-celled organism that lives in aquatic environments. This organism has a large bacuole with an average of 13 um. There are two types of vacuoles seen in a
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