7/24/2019 Contractile vacuole

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Contractile vacuole

A contractile vacuole (abbreviation: CV) is a sub-cellular structure (organelle) involved in

osmoregulation.

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.

The contractile vacuole 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. 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 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 as contractilevacuolecomplex (CVC).

Paramecium possess large contractile vacuole (average diameter of 13m), 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 out of the cell

which can be opened and closed.

Water flow into the CV

Water could theoretically cross the CV membrane by osmosis, but only if the inside of the CV is

hyperosmotic (higher solute concentration) to the cytoplasm. The discovery of proton pumps in the

CV membrane and the direct measurement of ion concentrations inside the CV using

microelectrodes led to the following model: the pumping of protons either into or out of the CV

causes different ions to enter the CV. For example, some proton pumps work as cation exchangers,

whereby a proton is pumped out of the CV and a cation is pumped at

the same time into the CV. In other cases, protons pumped into the CV drag

anions with them (carbonate, for example), to balance the pH.