Contractile vacuole

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 best-understood contractile vacuoles belong to the protists Paramecium, Amoeba, Dictyostelium and Trypanosoma, and to a lesser extent the green alga Chlamydomonas.

The contractile vacuole has several structures attached to it in most cells, such as membrane folds, tubules, water tracts and small vesicles.

Paramecium and Amoeba possess large contractile vacuoles (average diameter of 13 and 45 μm, respectively), which are relatively comfortable to isolate, manipulate and assay.

[2] 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.

The way in which water enters the CV had been a mystery for many years, but several discoveries since the 1990s have improved understanding of this issue.

In other cases, protons pumped into the CV drag anions with them (carbonate, for example), to balance the pH.

[5] Acidocalcisomes have been implied to work alongside the contractile vacuole in responding to osmotic stress.