What is the function of Contractile Vacuole in Protozoa? What actually is the contractile vacuole? Read More About It!

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Contractile Vacuole in Paramecium, Amoeba, and Euglena
Contractile Vacuole in Paramecium, Amoeba, and Euglena (Source: CNX OpenStax)

What actually is the contractile vacuole?

Contractile Vacuole is a sub-cellular structure or organelle that is seen inside the protozoan cell. It is a single, clear, transparent, rounded, and pulsating cell organelle.

Previously it was called a pulsatile or pulsating vacuole. It is filled with watery fluid and remains enclosed by a unit membrane while being floating in the cytoplasm.

As the name suggests, it is a vacuole that contracts and expels water out of the cell. It shows periodic expansion and contraction.

The expansion of the contractile vacuole leads to water gathering inside the organelle, and the contraction of the vacuole leads to water expulsion out of the contractile vacuole.

The contractile vacuole is the kidney of the unicellular protozoans. It shows the rhythmic movement of systole and diastole.

One systole and diastole cycle takes several seconds, depending on the species and the environment’s osmolarity.

The stage in which water flows into the Contractile Vacuole is called diastole. And, the contraction of the contractile vacuole and the expulsion of water out of the cell is called systole.

In some protozoans like Amoeba proteus, there is only one contractile vacuole in each individual. In other protozoans like Dictyostelium discoideum, Paramecium aurelia, and Chlamydomonas reinhardtii there are two contractile vacuoles, and in giant Amoeba, such as Chaos carolinensis, there are many.

Depending on the species type, the vacuole can remain at a fixed position while being attached to the endoplasm, or it can circulate inside the cell by keeping close interactions with the endoplasm.

Electron microscopy of the contractile vacuoles in Paramecium has revealed some of the tubules of the endoplasmic reticulum, nephridial tubules, feeder canals, accessory vacuoles, and of the main contractile vacuole. All of these helps in the proper functioning of the contractile vacuole.

The regular evacuation of water suggests a comparison with the kidneys of more complicated animals, and in fact, many workers have attributed to contractile vacuoles the power of excretion or osmoregulation. Others have suggested that they perform a respiratory function.

The formation and growth (or “diastole”) of contractile vacuoles may be accompanied by important secretory processes, although there is no direct evidence as to what is dissolved in the vacuolar fluid.

What is the function of contractile vacuole? (Explained in detail)

The main function of Contractile Vacuole is to maintain the task of osmoregulation and waste removal of water out of the cell. It helps in regulating the water concentration inside the cell.

It does so by the proper maintenance of constant osmotic pressure in the fluids of an organism by the control of water and salt concentrations in a well-balanced way so that the cell doesn’t get ruptured due to cytolysis.

In simple words, the significant function of the contractile vacuole is to pump water out of the cell through a process called osmoregulation, which is actually the regulation of osmotic pressure.

The wastes like the ammonia can be excreted through the vacuoles. The water and the other salts are also removed through these vacuoles only in order to maintain the osmotic balance.

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. All thanks to the Contractile Vacuole.

In freshwater environments, the concentration of solutes is hypotonic, lesser outside than inside the cell. Under these conditions, osmosis 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 rupturing through excessive internal pressure.

Contractile vacuole simple acts like the water pump. They can slowly accumulate water during diastole, and can periodically expel the liquid rapidly into the medium during systole.

According to Patel and Docampo, 2010 as studied in D. discoideum, contractile vacuoles are considered acidic calcium stores and they have been proposed to be involved in Ca2 + secretion and signaling.

Also, according to Sriskanthadevan, 2009 the Dictyostelium CV is reported to contain a Ca2 +-sensitive cell-adhesion molecule DdDAD-1. This molecule is synthesized on free ribosomes and transported to the CV for insertion into the plasma membrane.

What does the contractile vacuole look like?

If you look a protozoan like Ameoaba, Paramecium, etc. under a microscope then the Contractile Vacuole simply looks like a transparent, spacious, rounded, membrane-bounded, water-filled cell organelle inside the cell.

If you keep looking at the contractile vacuole you will observe the rhythmic phases of collecting water (expansion) and expelling water (contraction) after every several seconds, depending on the species and the environmental osmolarity.

In Paramecium you will observe a highly complex and advanced contractile vacuole in comparison to other protozoans.

You will also find various tubules, and canals along with accessory vacuoles all on the contractile vacuole that together maintains the osmoregulation.

In other protozoan species like the Tokophrya, as in Suctoria and Ciliata in general, the contractile vacuole has a permanent canal connecting it with the outside. The canal appears to have a very elaborate structure and is composed of a pore, a channel, and a narrow tubule located in a papilla that is well-protruding into the cavity of the contractile vacuole.

Depending on the species there may be only one, or two, or many contractile vacuoles in the cell. The number of vacuoles present inside the cell is directly influenced by the freshwater environment.

The contractile vacuole has a pore or canal connecting it with the outside world for expelling the excess of water and wastes out of the cell.

Contractile Vacuole cannot be seen in multicellular organisms except in some blood cells of the frog, guinea pig, and man as recently discovered. However, it still exists in the unicellular stage of several multicellular fungi and in several types of cells in sponges, including amoebocytes, pinacocytes, and choanocytes.

The Contractile Vacuole remains either fixed or moving in the cytoplasm while being in a close association with the endoplasm. In a broad sense, the CV morphologies and behaviors differ among various organisms.

What would happen if the contractile vacuole stops working?

A contractile vacuole (CV) is a membrane-bound osmoregulatory organelle of freshwater and soil amoebae and other protozoans that segregates excess cytosolic water that was acquired osmotically and expels it to the cell exterior so that the cytosolic osmolarity is kept constant under a given osmotic condition.

For freshwater and aquatic protozoans, it is a must that they keep their cell from having excessive water in the cytoplasm, or else the cell will burst or rupture due cytolysis.

In freshwater environments, the concentration of solutes inside the cell is higher than outside the cell. Under these conditions, water flows from the environment into the cell by osmosis.

So, that’s why when the protozoans are present in water, the surrounding water from the environment always flows into the cell’s cytoplasm through the membrane pores. Then the water is moved from the cytoplasm into the contractile vacuole for expulsion if there is the excess entry of water.

The contractile vacuole helps the cell to store those excessive water at very hypertonic (high concentration of solutes) environments so that the cell can adjust its cytoplasm to become even more hyperosmotic than the environment.

Whereas, in the case when the cell is in hyperosmotic environments, less water will be expelled and the contraction cycle of CV will be longer.

Thus, it is well-cleared that the contractile vacuole is the organelle that protects the cell against cellular expansion and explosion due to too much water in the cytoplasm. It protects so by expelling the excess water from the cell by contracting.

And so, if the contractile vacuole stops working the cell would explode and rupture leading to the death of the protozoan.

In which Protozoans contractile vacuole is seen?

The contractile vacuole is present in many protozoans. Some of the most well-known protozoans that have CV are Amoeba, Paramecium, Euglena, Chlamydomonas, Chaos, etc.

In Amoeba, a single contractile vacuole is present at the posterior end of the endoplasm. This rounded, and filled with a watery fluid being enclosed by a membrane. Surrounding the CV you will find various food vacuoles and mitochondria.

In Paramecium, there are 2 CV in the cell that are fixed positioned in the endoplasm. One vacuole lies near each end of the body, close to the dorsal surface. In each CV you will find radial canals extending into the cytoplasm along with various accessory vacuoles, canals, and tubules.

In Euglena, the CV is situated in a dense osmoregulatory zone having being surrounded by various accessory vacuoles, which probably fuse together to form a large vacuole. The CV discharge the water via. the reservoir, cytopharynx, and cytostome.

In Chlamydomonas, two contractile vacuoles are situated near the front-end of the cell i.e. near the bases of flagella. The smallest known contractile vacuoles belong to Chlamydomonas, with a diameter of 1.5 µm.

In Chaos which falls under the Genus of giant Amoeba, there are multiple Contractile vacuoles i.e 2 or more than 2. In Chaos carolinensis, there is more than 2 CVs to maintain the large osmotic pressure of the giant cell.

What is the difference between Contractile Vacuole and Food Vacuole?

The key difference between the food vacuoles and the contractile vacuoles is in its function. The food vacuoles are involved in digestion whereas contractile vacuoles are involved in osmoregulation.

Both contain watery fluid. The contractile vacuoles will contain the excess cytoplasmic water along with various wastes to be expelled out.

Whereas, the food vacuoles will contain water along with the various digestive enzymes. During the digestion process, the reaction inside the food vacuole is first acidic and then alkaline.

Food vacuoles are small sacs that are distributed in the cell cytoplasm of protists, plants, fungi, and in some animals.

Whereas, Contractile vacuoles are present mostly in aquatic protozoa and are involved in osmoregulation in the cell.

Without the presence of food vacuoles, the organism can’t digest food and so can’t acquire energy for its day to day activities.

And, without Contractile vacuoles, there will be no osmoregulation of the cell and so the cells may burst and rupture when in water.

Endocytosis is the method that forms food vacuoles. Whereas, contraction and relaxation of the vacuole to push water out of the cell is the method which forms contractile vacuoles.

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