The protozoo Paramecium aurelia with highlighted contracted vacuoles
The contractile vacuum (abbreviated as V.C., also known as a pulsatile vacuum or button) is a vacuole involved in osmodulation. It is predominantly found in protozoa and unicellular algae.
The contractile vacuum pumpes excess water out of the cell. In fresh water environments, the concentration of solutes within the cell is higher than that of the external (i.e., the environment is hypotonic). Under these conditions water flows from the environment into the osmosis cell. The contractile vacuum serves as a protection mechanism that prevents the cell from absorbing too much water and eventually explodes.
As the name suggests, the contractile vacuum exposes the water out of the contracting cell. Growth (water collection) and contraction (water expulsion) of V.C. are periodical. A cycle takes a few seconds, depending on the species and the osmolarity of the environment. The phase in which the water flows in the V.C. is called diastole. The contraction of V.C. and the expulsion of water outside the cell is called systole.
Water always flows from the outside of the cell into the cytoplasm, and only then from the cytoplasm in the V.C .. Species that possess a V.C. they always use it, even in high concentrations of solutes (hypertonic), since the cell tends to make its cytoplasm become even more hyperactive than the environment. The amount of water ejected from the cell and the shrinkage rate are linked to the humor of the environment. In hyperosmotic environments less water is expelled and the contraction cycle is longer. Bibliografiamodifica wikitesto
wiki
