Paramecium are unicellular eukaryotes that belong to the kingdom Protista. These are usually found in aquatic habitats.
Contractile vacuoles in paramecium are the organelles involved in pumping out excess water from the cytoplasm of the cell. They are majorly responsible for maintaining the osmotic homeostasis in fresh water organisms that lack cell wall.
Let us discuss a few more aspects of the contractile vacuoles complex presence in paramecium.
Where are contractile vacuoles found in paramecium?
Contractile vacuoles are often found in the cytoplasm of paramecium that inhabit fresh water habitats. Their hypotonic external environment would cause the cell to undergo endosmosis. As a result, the cell would swell up and burst. The contractile vacuole prevents this from happening.
How are contractile vacuoles formed in paramecium?
Several vesicles combine together to form the contractile vacuoles and they can detach from it as well.
Contractile vacuoles are formed by the fusion of several smaller vesicles. These new vesicles appear randomly in the close vicinity to the central vacuole. They eventually end up fusing with the central vacuole following which the collective contents of the vacuoles are expelled out.
Contractile vacuole functions in paramecium
Major function of a contractile vacuole in a cell is osmoregulation. Let us discuss the steps in which this osmoregulatory organs performs its function.
1. The fluid filling phase
In fluid filling phase, the radial structures of the central vacuole segregate the excess cytosolic water. This excess water which enters the cell through endosmosis gets collected in the vacuole. This causes the vacuole to swell.
2. The rounding phase
In the rounding phase, the vacuole swells up to become round in shape. Studies suggest that rounding of the cell causes tension in the membrane of both the vacuole, radial structures and the cell.
Due to the tension created, the radial structures get detached from the central vacuole. It also causes the pore to open up when the vacuole fuses with the plasma membrane at the pore region.
3. The fluid discharging phase
In fluid discharging phase, through the open pore, all of vacuoles contents are released. Loss of excess water reduces the vacuole size and along with it, the tension in the membrane. A protein called disgorging is responsible for the fusion and the discharge.
Lack of membrane tension closes the pore and allows the radial structures reattach to the shrunk vacuole so that the cycle continues.
Contractile vacuole structure in paramecium
Paramecium possesses one or a few contractile vacuole complexes. They are composed of a central large vacuole possessing several radial arm like structures with tubular network.
The structure of a contractile vacuole in paramecium is given below:
1. Central vacuole:
- Central vacuole is a contractile vacuole to which the radial arms like structure attach and detach from.
- The membrane of the central vacuole lacks V-ATPase holoenzymes and act as the reservoir. These can fuse with the plasma membrane.
2. Radial structures:
- The central vacuole possesses around 5-10 radial arms or vesicles or tubules like structures.
- The membrane of such structures possesses V-ATPase holoenzymes which are primarily responsible for maintaining a proton gradient but mediating proton-translocation.
Contractile vacuoles in paramecium play an irreplaceable role without which it would be nearly impossible for the organism to survive in a hypotonic environment.