page_title: Contractile vacuole
section_title: Water flow into the CV
hierarchical_section_title: Contractile vacuole / Water flow into the CV
caption_attribution_description: A Dictyostellium discoideum cell showing a prominent contractile vacuole on the left side.
context_page_description: A contractile vacuole is a sub-cellular structure involved in osmoregulation. It is found predominantly in protists and in unicellular algae. It was previously known as pulsatile or pulsating vacuole.
context_section_description: 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. 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. This ion flux into the CV causes an increase in CV osmolarity and as a result water enters the CV by osmosis. Water has been shown in at least some species to enter the CV through aquaporins.
  Acidocalcisomes have been implied to work alongside the contractile vacuole in responding to osmotic stress. They were detected in the vicinity of the vacuole in Trypanosoma cruzi and were shown to fuse with the vacuole when the cells were exposed to osmotic stress. Presumably the acidocalcisomes empty their ion contents into the contractile vacuole, thereby increasing the vacuole's osmolarity.
caption: A Dictyostelium discoideum (slime mold) cell exhibiting a prominent contractile vacuole on its left side
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