Contractile Vacuoles
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A contractile vacuole (CV) is a sub-cellular structure (
organelle In cell biology, an organelle is a specialized subunit, usually within a cell (biology), cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as Organ (anatomy), organs are to th ...
) involved in
osmoregulation Osmoregulation is the active regulation of the osmotic pressure of an organism's body fluids, detected by osmoreceptors, to maintain the homeostasis of the organism's water content; that is, it maintains the fluid balance and the concentration ...
. It is found predominantly in
protists A protist ( ) or protoctist is any Eukaryote, eukaryotic organism that is not an animal, Embryophyte, land plant, or fungus. Protists do not form a Clade, natural group, or clade, but are a Paraphyly, paraphyletic grouping of all descendants o ...
, including
unicellular A unicellular organism, also known as a single-celled organism, is an organism that consists of a single cell, unlike a multicellular organism that consists of multiple cells. Organisms fall into two general categories: prokaryotic organisms and ...
algae Algae ( , ; : alga ) is an informal term for any organisms of a large and diverse group of photosynthesis, photosynthetic organisms that are not plants, and includes species from multiple distinct clades. Such organisms range from unicellular ...
. It was previously known as pulsatile or pulsating vacuole.


Overview

The contractile vacuole is a specialized type of
vacuole A vacuole () is a membrane-bound organelle which is present in Plant cell, plant and Fungus, fungal Cell (biology), cells and some protist, animal, and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water ...
that regulates the quantity of
water Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...
inside a
cell Cell most often refers to: * Cell (biology), the functional basic unit of life * Cellphone, a phone connected to a cellular network * Clandestine cell, a penetration-resistant form of a secret or outlawed organization * Electrochemical cell, a de ...
. In
freshwater Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. The term excludes seawater and brackish water, but it does include non-salty mi ...
environments, the
concentration In chemistry, concentration is the abundance of a constituent divided by the total volume of a mixture. Several types of mathematical description can be distinguished: '' mass concentration'', '' molar concentration'', '' number concentration'', ...
of
solutes In chemistry, a solution is defined by IUPAC as "A liquid or solid phase containing more than one substance, when for convenience one (or more) substance, which is called the solvent, is treated differently from the other substances, which are ...
is
hypotonic In chemical biology, tonicity is a measure of the effective osmotic pressure gradient; the water potential of two solutions separated by a partially-permeable cell membrane. Tonicity depends on the relative concentration of selective memb ...
, lower outside than inside the cell. Under these conditions,
osmosis Osmosis (, ) is the spontaneous net movement or diffusion of solvent molecules through a selectively permeable membrane, selectively-permeable membrane from a region of high water potential (region of lower solute concentration) to a region of ...
causes water to accumulate in the cell from the external environment. The contractile vacuole acts as part of a protective mechanism that prevents the cell from absorbing too much water and possibly
lysing Lysis ( ; from Greek 'loosening') is the breaking down of the membrane of a cell, often by viral, enzymic, or osmotic (that is, "lytic" ) mechanisms that compromise its integrity. A fluid containing the contents of lysed cells is called a ...
(rupturing) through excessive internal pressure. The contractile vacuole, as its name suggests, 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, depending on the species and the
osmolarity Osmotic concentration, formerly known as osmolarity, is the measure of solute concentration, defined as the number of osmoles (Osm) of solute per litre (L) of solution (osmol/L or Osm/L). The osmolarity of a solution is usually expressed as Osm/ ...
of the environment. 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 The cytoplasm describes all the material within a eukaryotic or prokaryotic cell, enclosed by the cell membrane, including the organelles and excluding the nucleus in eukaryotic cells. The material inside the nucleus of a eukaryotic cell a ...
, 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 best-understood contractile vacuoles belong to the protists ''
Paramecium ''Paramecium'' ( , , plural "paramecia" only when used as a Common name, vernacular name) is a genus of eukaryotic, unicellular ciliates, widespread in freshwater, brackish, and Ocean, marine environments. Paramecia are often abundant in stagna ...
'', ''
Amoeba An amoeba (; less commonly spelled ameba or amœba; : amoebas (less commonly, amebas) or amoebae (amebae) ), often called an amoeboid, is a type of Cell (biology), cell or unicellular organism with the ability to alter its shape, primarily by ...
'', ''
Dictyostelium ''Dictyostelium'' is a genus of single- and multi-celled eukaryotic, phagotrophic bacterivores. Though they are Protista and in no way fungal, they traditionally are known as "slime molds". They are present in most terrestrial ecosystems a ...
'' and ''
Trypanosoma ''Trypanosoma'' is a genus of kinetoplastids (class Trypanosomatidae), a monophyletic group of unicellular parasitic flagellate protozoa. Trypanosoma is part of the phylum Euglenozoa. The name is derived from the Ancient Greek ''trypano-'' (b ...
'', and to a lesser extent the
green alga The green algae (: green alga) are a group of chlorophyll-containing autotrophic eukaryotes consisting of the phylum Prasinodermophyta and its unnamed sister group that contains the Chlorophyta and Charophyta/ Streptophyta. The land plants ( ...
''
Chlamydomonas ''Chlamydomonas'' ( ) is a genus of green algae consisting of about 150 species of unicellular organism, unicellular flagellates, found in stagnant water and on damp soil, in freshwater, seawater, and even in snow as "snow algae". ''Chlamydom ...
''. Not all species that possess a contractile vacuole are freshwater
organism An organism is any life, living thing that functions as an individual. Such a definition raises more problems than it solves, not least because the concept of an individual is also difficult. Many criteria, few of them widely accepted, have be ...
s; some marine,
soil Soil, also commonly referred to as earth, is a mixture of organic matter, minerals, gases, water, and organisms that together support the life of plants and soil organisms. Some scientific definitions distinguish dirt from ''soil'' by re ...
microorganism A microorganism, or microbe, is an organism of microscopic scale, microscopic size, which may exist in its unicellular organism, single-celled form or as a Colony (biology)#Microbial colonies, colony of cells. The possible existence of unseen ...
s and
parasite Parasitism is a Symbiosis, close relationship between species, where one organism, the parasite, lives (at least some of the time) on or inside another organism, the Host (biology), host, causing it some harm, and is Adaptation, adapted str ...
s also have a contractile vacuole. The contractile vacuole is predominant in species that do not have a
cell wall A cell wall is a structural layer that surrounds some Cell type, cell types, found immediately outside the cell membrane. It can be tough, flexible, and sometimes rigid. Primarily, it provides the cell with structural support, shape, protection, ...
, but there are exceptions (notably ''Chlamydomonas'') which do possess a cell wall. Through
evolution Evolution is the change in the heritable Phenotypic trait, characteristics of biological populations over successive generations. It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, re ...
, the contractile vacuole has typically been lost in
multicellular A multicellular organism is an organism that consists of more than one cell (biology), cell, unlike unicellular organisms. All species of animals, Embryophyte, land plants and most fungi are multicellular, as are many algae, whereas a few organism ...
organisms, but it still exists in the unicellular stage of several multicellular
fungi A fungus (: fungi , , , or ; or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified as one ...
, as well as in several types of cells in
sponge Sponges or sea sponges are primarily marine invertebrates of the animal phylum Porifera (; meaning 'pore bearer'), a basal clade and a sister taxon of the diploblasts. They are sessile filter feeders that are bound to the seabed, and a ...
s (
amoebocyte An amebocyte or amoebocyte () is a motile cell (moving like an amoeba) in the bodies of invertebrates including cnidaria, echinoderms, mollusca, molluscs, tunicates, sponges, and some chelicerata, chelicerates. Moving by pseudopodia, amebocytes c ...
s,
pinacocyte Pinacocytes are flat cells found on the outside of sponges, as well as the internal canals of a sponge. Pinacocytes are not specific to the sponge however. It was discovered that pinacocytes do not have as many sponge specific genes. These genes su ...
s, and
choanocyte Choanocytes (also known as "collar cells") are cells that line the interior of asconoid, syconoid and leuconoid body types of sponges that contain a central flagellum, or ''cilium,'' surrounded by a collar of microvilli which are connected by ...
s). The number of contractile vacuoles per cell varies, depending on the
species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...
. ''Amoeba'' have one, ''
Dictyostelium discoideum ''Dictyostelium discoideum'' is a species of soil-dwelling Amoeboid, amoeba belonging to the phylum Amoebozoa, infraphylum Mycetozoa. Commonly referred to as slime mold, ''D. discoideum'' is a eukaryote that transitions from a collection of unic ...
'', ''
Paramecium aurelia ''Paramecium aurelia'' are unicellular organisms belonging to the genus ''Paramecium'' of the phylum Ciliophora. They are covered in cilia which help in movement and feeding.''Paramecium'' can reproduce sexually, asexually, or by the process ...
'' and ''
Chlamydomonas reinhardtii ''Chlamydomonas reinhardtii'' is a single-cell green alga about 10 micrometres in diameter that swims with two flagella. It has a cell wall made of hydroxyproline-rich glycoproteins, a large cup-shaped chloroplast, a large pyrenoid, and a ...
'' have two, and giant amoeba, such as ''
Chaos carolinensis ''Chaos'' is a genus of single-celled amoeboid organisms in the family Amoebidae. The largest and most-known species, the so-called "giant amoeba" (''Chaos carolinensis''), can reach lengths up to 5 mm, although most specimens fall betwee ...
'', have many. The number of contractile vacuoles in each species is mostly constant and is therefore used for species characterization in
systematics Systematics is the study of the diversification of living forms, both past and present, and the relationships among living things through time. Relationships are visualized as evolutionary trees (synonyms: phylogenetic trees, phylogenies). Phy ...
. The contractile vacuole has several structures attached to it in most cells, such as membrane folds,
tubule In biology, a tubule is a general term referring to small tube or similar type of structure. Specifically, tubule can refer to: * a small tube or fistular structure * a minute tube lined with glandular epithelium * any hollow cylindrical body stru ...
s, water tracts and small
vesicles Vesicle may refer to: ; In cellular biology or chemistry * Vesicle (biology and chemistry), a supramolecular assembly of lipid molecules, like a cell membrane * Synaptic vesicle In a neuron, synaptic vesicles (or neurotransmitter vesicles) s ...
. These structures have been termed the spongiome; the contractile vacuole together with the spongiome is sometimes called the "contractile vacuole complex" (CVC). The spongiome serves several functions in water transport into the contractile vacuole and in localization and docking of the contractile vacuole within the cell. ''Paramecium'' and ''Amoeba'' possess large contractile vacuoles (average diameter of 13 and 45 μm, respectively), which are relatively comfortable to isolate, manipulate and assay. The smallest known contractile vacuoles belong to ''Chlamydomonas'', with a diameter of 1.5 μm. 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 Pore may refer to: Biology Animal biology and microbiology * Sweat pore, an anatomical structure of the skin of humans (and other mammals) used for secretion of sweat * Hair follicle, an anatomical structure of the skin of humans (and other ...
in the cytoplasm which can be opened and closed. Other protists, such as ''
Amoeba An amoeba (; less commonly spelled ameba or amœba; : amoebas (less commonly, amebas) or amoebae (amebae) ), often called an amoeboid, is a type of Cell (biology), cell or unicellular organism with the ability to alter its shape, primarily by ...
'', have CVs that move to the surface of the cell when full and undergo
exocytosis Exocytosis is a term for the active transport process that transports large molecules from cell to the extracellular area. Hormones, proteins and neurotransmitters are examples of large molecules that can be transported out of the cell. Exocytosis ...
. In ''Amoeba'' contractile vacuoles collect excretory waste, such as
ammonia Ammonia is an inorganic chemical compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pu ...
, from the
intracellular fluid The human body and even its individual body fluids may be conceptually divided into various fluid compartments, which, although not literally fascial compartment, anatomic compartments, do represent a real division in terms of how portions of the ...
by both
diffusion Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...
and
active transport In cellular biology, active transport is the movement of molecules or ions across a cell membrane from a region of lower concentration to a region of higher concentration—against the concentration gradient. Active transport requires cellula ...
.


Water flow into the CV

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 pump A proton pump is an integral membrane protein pump that builds up a proton gradient across a biological membrane. Proton pumps catalyze the following reaction: : n one side of a biological membrane/sub> + energy n the other side of the m ...
s in the CV membrane and the direct measurement of ion concentrations inside the CV using
microelectrode A microelectrode is an electrode used in electrophysiology either for recording neural signals or for the electrical stimulation of nervous tissue (they were first developed by Ida Hyde in 1921). Pulled glass pipettes with tip diameters of ...
s led to the following model: the pumping of protons either into or out of the CV causes different
ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
s to enter the CV. For example, some proton pumps work as
cation exchanger An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
s, 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
anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...
s with them (
carbonate A carbonate is a salt of carbonic acid, (), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word "carbonate" may also refer to a carbonate ester, an organic compound containing the carbonate group ...
, 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
aquaporin Aquaporins, also called water channels, are channel proteins from a larger family of major intrinsic proteins that form pores in the membrane of biological cells, mainly facilitating transport of water between cells. The cell membranes of ...
s.
Acidocalcisome Acidocalcisomes are rounded electron-dense acidic organelles, rich in calcium and polyphosphate and between 100 nm and 200 nm in diameter. Acidocalcisomes were originally discovered in Trypanosoma, Trypanosomes (the causing agents of Afri ...
s have been implied to work alongside the contractile vacuole in responding to
osmotic stress Osmotic shock or osmotic stress is physiologic dysfunction caused by a sudden change in the solute concentration around a cell, which causes a rapid change in the movement of water across its cell membrane. Under hypertonic conditions - conditi ...
. They were detected in the vicinity of the vacuole in ''
Trypanosoma cruzi ''Trypanosoma cruzi'' is a species of parasitic euglenoids. Among the protozoa, the trypanosomes characteristically bore tissue in another organism and feed on blood (primarily) and also lymph. This behaviour causes disease or the likelihood ...
'' 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.


Unresolved issues

The CV does not exist in higher organisms, but some of its unique characteristics are used by them in their osmoregulatory mechanisms. Research on the CV can therefore help us understand how osmoregulation works in all species. Many issues regarding the CV remain, as of 2010, unsolved: * Contraction. It is not completely known what causes the CV membrane to contract, and whether it is an active process which costs
energy Energy () is the physical quantity, quantitative physical property, property that is transferred to a physical body, body or to a physical system, recognizable in the performance of Work (thermodynamics), work and in the form of heat and l ...
or a passive collapse of the CV membrane. Evidence for involvement of
actin Actin is a family of globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in muscle fibrils. It is found in essentially all eukaryotic cells, where it may be present at a concentration of ...
and
myosin Myosins () are a Protein family, family of motor proteins (though most often protein complexes) best known for their roles in muscle contraction and in a wide range of other motility processes in eukaryotes. They are adenosine triphosphate, ATP- ...
, prominent contractile
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...
s which are found in many cells, are ambiguous. * Membrane composition. Although it is known that several proteins decorate the CV membrane (V−H+−ATPases, aquaporins), a complete list is missing. The composition of the membrane itself and its similarities to and differences from other cellular membranes are also not clear. * Contents of the CV. Several studies have shown the ion concentrations inside some of the largest CVs but not in the smallest ones (such as in the important model organism ''Chlamydomonas rheinhardii''). The reasons and mechanisms for ion exchange between the CV and cytoplasm are not entirely clear.


References

{{DEFAULTSORT:Contractile Vacuole Organelles