Contractile Vacuole: Definition, Functions and Structure

A contractile vacuole (CV) is a membrane-bound osmoregulatory organelle or sub-cellular structure of organism. It is spherical in shape found in freshwater protozoa and lower metazoans.

• These vacuoles collect excess water and wastes from sponges and hydras and periodically empty them in the surrounding medium.
• Additionally, it may also excrete nitrogenous wastes.
• The process of filling and emptying may last from seconds to minute which depends on the species.
• It changes position with the animal’s movement in amoebas, in most ciliates it follows a definite path through a cell.
• While in Euglena, it remains stationary.

Key Words: Contractile vacuole, Water, Ameoba, Expel, Cell,  Contraction, Excess Water.

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What is Contractile Vacuole?

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A contractile vacuole is a regulatory organelle or sub-cellular structure of fresh water and soil amoebae and protozoa involved primarily in osmoregulation and waste removal along with storage of cellular products. Normally, it is found in protists and in unicellular algae. 

• These are also available in some marine and soil microorganisms. 
• These are mainly used to absorb excess water and wastes from microorganism’s cell and excrete the same into the surrounding environment
• Through the process of contraction it protects a cell from absorbing too much water.

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Function of Contractile Vacuole

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A Contractile vacuole collects and expels the water in a periodical manner. The diastole is concerned about the flow of water into the contractile vacuole while contraction and expulsion of water take place in the systole phase.

• In the diastole phase water flows from outside the cell into cytoplasm and from the cytoplasm into the contractile vacuole. 
• In the systolic phase the amount of water expelled from the cell and the rate of contraction are dependent on the osmolarity of the environment.
• For instance, in the hyperosmotic environment, less water is expelled and thus the contraction phase is longer.
• While in a hypertonic environment more water is expelled and thus the contraction phase is shorter.
• In simple words, a contractile vacuole prevents excessive water influx into cells of aquatic unicellular organisms by contraction.

Also read: Difference between Algae and Fungi

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What is Osmoregulation?

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Osmoregulation is the process of maintaining a balance between fluid and electrolyte within a cell or organism relative to the surrounding by regulating water potential.

Constant optimal osmotic pressure in the cell is maintained through the process of osmoregulation which in turn helps maintain the right amount of solute concentration and water in cells.

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Structure of Contractile Vacuole in Various Organisms

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The number of contractile vacuole per cell varies from species to species.

• For instance, Amoeba contains a single contractile vacuole.
• Paramecium aurelia, Chlamydomonas reinhardtii, and Dictyostelium discoideum have two contractile vacuoles.
• Chaos carolinensis, a giant amoeba has many of it.
• Contractile vacuole has several structures attached to it viz. membrane folds, water tracts, tubules, and small vesicles.
• These structures are together termed as spongiome.
• The contractile vacuole together with spongiome is called the "contractile vacuole complex" (CVC).
• Paramecium and Amoeba have large contractile vacuoles of 13 and 45 µm diameter.
• Chlamydomonas has the smallest contractile vacuole of 1.5 µm diameter.
• In Paramecium contractile vacuoles are surrounded by several canals which absorb water from the cytoplasm.
• This water in turn is pumped into contractile vacuole when the canals get filled.
• When the vacuole becomes full of water, it expels the water into the cytoplasm through a pore. 

In Chlamydomonas reinhardtii, the structure of the contractile vacuole changes through three phases.

• In the early stage, there are numerous small vesicles about 70–120 nm in diameter.
• In the middle stage, these vesicles merge to form the contractile vacuole properly.
• In the later stage, the size of the contractile vacuole increases in diameter due to the merger of these smaller vesicles.
• It makes contact with the plasma membrane.
• From this stage onwards the contractile vacuole decreases in size.

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Things to Remember

• Contractile vacuole protects cells from absorbing excess water by excreting excess water.
• The contractile vacuole also removes waste water containing ammonia or other nitrogenous waste matter.
• Contractile vacuole functions in a cyclic manner by expanding while collecting water and contracting when releasing water.
• Period of water expulsion of the contractile vacuole is called systole while a period of water filling the contractile vacuole is called diastole.
• Amoeba contains a single contractile vacuole while Paramecium aurelia, Chlamydomonas reinhardtii, and Dictyostelium discoideum have two contractile vacuoles, and Chaos carolinensis, a giant amoeba has more than two. 
• Paramecium and Amoeba comprise large contractile vacuoles of 13 and 45 µm diameter and Chlamydomonas has contractile vacuole of 1.5 µm diameter. 

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