How do Contractile Vacuoles work and maintain Homeostasis?

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How do Contractile Vacuoles work? – (The Overview)

The main function of the contractile vacuole is to maintain the osmoregulation and so the balance between the water and the concentration of solutes between the cell and the external medium.

Here, the external medium can be either hypertonic or hypotonic in nature. And, the contractile vacuole helps the cell to take in and out the water through a process called osmoregulation just in order to make the cell volume stable with that of the external medium.

The task of waste removal of water out of the cell is performed by the contractile vacuole. By doing so, it also regulates the water concentration inside the cell.

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

The contractile vacuole works by periodic expansion (diastole) and contraction (systole). It can take about several seconds for one contration and expanison cycle to complete.

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

So, if the protozoan species is in a hypertonic solution then there will be movement of water from inside of the cell to the external medium, and at that time the contractile vacuole will expel the water only to an extent to adjust its cytoplasm to become even more hyperosmotic than the environment. This makes the cell’s volume stable.

And, if the protozoan species is in a hypotonic solution then there will be movement of water from the external medium to inside of the cell, and at that time the contractile vacuole will remove the excess water from the cell just in order to protect the cell from lysing (rupturing) due to the excessive internal pressure. This also makes the cell volume stable.

Here, in the above cases as mentioned, if the external medium is a Hypertonic solution that it means that the concentration of solute is more per litre of water.

And, if the external medium is a Hypotonic solution then it means that the concentration of solute is less per litre of water.

Now, it is also to be noted that the contractile vacuole being a sub-cellular organelle remains connected with the cell membrane with the help of the various minute tubules, and canals along with other accessory vacuoles.

This connection between the cell membrane and the contractile vacuole helps in the movement of water inside the cell through the cell membrane pores and out of the cell through the contractile vacuole pores as the vacuole periodically contracts and expands to maintain the stable cell volume.

In simple words, the contractile vacuole has one or many pores or canals connecting it with the outside world through the cell membrane for expelling the excess of water and wastes out of the cell whenever required.

Also Read This Related Post: Function of Contractile Vacuole in Protozoa & More Info About It

CONFUSION CLEARED

Water from the external medium enters the cell’s cytoplasm through the cell membrane, and not in any way through the contractile vacuole.

The function of the contractile vacuole is just to remove the water out of the cell in order to maintain the cell volume as required.

So, the water that is already there in the cytoplasm is taken into the contractile vacuole directly from the cytoplasm only for expulsion outside of the cell whenever the need arises based on the osmotic pressure.

And so, the Contractile vacuole expands to take in water from the cytoplasm, and the vacuole contracts to remove the water from inside of the vacuole to outside of the cell.

And so, we can also say that water enters the cell through the cell membrane to get into the cell’s cytoplasm. And that water from the cell’s cytoplasm enters into the contractile vacuole and finally that water gets expelled water through that vacuole.

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

Working of Contractile Vacuole in some of the well-known Protozoan species:

In Amoeba

As seen in the amoebae species of Amoeba proteus there is a single large contractile vacuole (average diameter of 13 and 45 µm) that is present at the most posterior end of the amoeba’s body.

This single membrane-bound contractile vacuole that keeps on constantly contracting and expanding itself remains well-surrounded by the various food vacuoles, mitochondria, and sub-cellular crystals while being surrounded by the cytoplasm all throughout.

This vacuole remains directly connected with the endoplasm of the cell membrane by means of the vacuole’s single-layer membrane and many microtubules.

So, this contractile vacuole can directly expel the water out of the cell due to its direct membrane connection with the endoplasm by bursting to release its contents outside of the cell, and also due to the little passage of water through the microtubules.

In Paramecium

As seen in Paramecium caudata, there are two contractile vacuoles, occupying somewhat fixed positions in the endoplasm.

One vacuole lies near each end of the body, close to the dorsal surface.

Each of the contractile vacuoles is surrounded by atleast 6 to 10 numbers of spindle-shaped radial canals extending far into the cytoplasm. These radial canals are also called spongiomes.

Not only the spongiomes, but the contractile vacuole of paramecium also includes some of the tubules of the endoplasmic reticulum, nephridial tubules, and feeder canals surrounding the vacuole on all sides.

Each of the contractile vacuole opens to the outside of the cell through a permanent pore in pellicle part of the dorsal side of the body.

So, it has been well seen that the spongiomes (radial canals) of the contractile vacuole aid in the collection of water from the cytoplasm and passing it to the contractile vacuole.

And also that the direct opening of the contractile vacuole through the pellicle part of the Paramecium cell aids in the direct expelling of water out of the cell very easily based on the osmotic pressure.

While the nephridial tubules and feeder canals help in the excretion of wasted water out of the contractile vacuole in a few of the cases.

In Euglena

As seen in Euglena viridis, inside the cell there is a dense osmoregulatory zone where the main Contractile Vacuole is situated.

This main contractile vacuole remains surrounded by various accessory vacuoles, which probably fuse together to form a large single vacuole. This together forms the Contractile apparatus of Euglena.

The whole contractile apparatus serves to expel the water via. the reservoir, cytopharynx, and cytostome directly with the help of the accessory vacuoles as well.


Is Contractile Vacuole active or passive?

We know that contractile vacuole is always busy in osmoregulation by expelling water out of the cell just in order to maintain the cell’s stability.

However, it is also to be noted that this contractile vacuole also works to collect excretory waste, such as ammonia, from the intracellular fluid to remove it outside of the cell by following the active (facilitated) or passive diffusion process.

So, if the contractile vacuole is doing its job of osmoregulation, then we say that the vacuole is passive. It’s because during osmoregulation water moves from areas of low concentration of solute to areas of high concentration of solute which is actually a very passive process and doesn’t require the cell’s ATP energy.

And, if the contractile vacuole is doing its job of excretion, then it can work either passively or actively.

If the contractile vacuole is working passively due to simple diffusion then it will be passive. It’s because during simple diffusion the excretory materials move in the direction of a concentration gradient and so no energy is required.

And if the contractile vacuole is working actively due to facilitated diffusion or active diffusion then the vacuole will need energy to work. It’s because during facilitated diffusion the excretory material moves against a concentration gradient.

Note that, simple diffusion (also called passive diffusion) and osmosis are both forms of passive transport and require none of the cell’s ATP energy. While facilitated diffusion (also called active diffusion) requires the cell’s ATP energy.

So, if the contractile vacuole performs simple diffusion and osmosis then it is working in a passive way. And if the contractile vacuole performs facilitated diffusion then it is working in an active way.


Why is Contractile Vacuole more active in freshwater?

Freshwater can be considered as a hypotonic solution because in freshwater the solute concentration is comparatively lesser per litre of water as compared to the solute concentration inside the cell.

So, in order for the cell to maintain osmoregulation, there is a net movement of water from the external freshwater environment to the inside of the cell.

And, as the cell gets filled up with a lot of water molecules than actually required, then at that time the contractile vacuole starts its job to remove the excess water from the cell just in order to protect the cell from lysing (rupturing) due to the excessive internal pressure.

So, if the cell is present in freshwater it is very very much mandatory for the contractile vacuole to be more active thus in order to protect the cell from lysing (rupturing) due to excessive accumulation of water inside the cell.

However, if we talk about marine animals then it is to be noted that the seawater is hypertonic and so water will move from the cell’s cytoplasm to the external medium by osmosis.

And so marine animals do not have contractile vacuoles or if present it remains in a very less active state because they need to conserve their cellular water content from getting lost in their hyperosmotic environment.


How do Contractile Vacuoles work as excretory systems in microorganisms? What’s the concept here?

The excretory function of contractile vacuole has not been known much. But it has been known that the contractile vacuole works both passively and actively to perform its excretory function very well.

In many protozoans, like Amoeba, it has been well-seen that the contractile vacuoles work both passively and actively to take in the excess of carbon dioxide and ammonia in the dissolved state along with the cytoplasmic water just in order to expel it outside of the cell.

Here, in Amoeba, the contractile vacuoles can merge with the cell membrane very well and expel wastes into the outer environment.

By the way, don’t even compare Contractile Vacuoles which expels water outside with Food Vacuoles which store food or water. Both of these vacuoles are totally different in working, cause, and functioning.

In many species like Paramecium, the contractile vacuole also does the same job both passively and actively but in a little different way.

Here, in Paramecium, the central region of the contractile vacuole receives water and dissolved excretory particles with the help of the various radial canals (spongiomes). The water of the central region is then expelled out periodically through the permanent opening of the contractile vacuole present in the pellicle.

The same excretory role of contractile vacuole can also be seen in Euglena as well. In Euglena, the contractile vacuole fuses well with the cell membrane and removes the waste along with the water following the systole phase.

Kindly note that, the chief role of the contractile vacuole is actually to perform osmoregulation, not in excretion. Excretion is only a peripheral role of contractile vacuoles.

Some scientists have referred the contractile vacuole as the kidney of the unicellular protozoans.

Many workers have attributed contractile vacuoles to having the power of excretion along with osmoregulation. Others have suggested that they perform a respiratory function as well.


How Contractile Vacuole maintain Homeostasis?

Contractile Vacuoles work to perform osmoregulation very well. In fact, osmoregulation is the Contractile Vacuoles’ main role.

And it is also to be noted that, Osmoregulation balances concentrations of solutes and water across the semi-permeable membranes between the cell and the external environment leading to homeostasis in the form of the cell’s stable volume.

Osmoregulation is the biological process through which the proper balance of electrolytes in the organisms’ body is also maintained despite external factors such as temperature, diet, and weather conditions.

This homeostatic osmoregulation due to the contractile vacuole done by expelling the excess water is mandatory for the biological cell to survive, or else, it can lead the cell into absorbing too much water and possibly lysing (rupturing) through excessive internal pressure.

Here, the control of water content within the cell is an example of homeostasis which is actually the maintenance of a controlled internal environment.

So, it can also be stated that the Contractile Vacuole maintain Homeostasis by controlling the water content within the cell.

Also that the Contractile Vacuole performs its peripheral role of excretion by removing the nitrogenous waste products like ammonia along with water out of the cell.

So, through excretion organisms control the osmotic pressure too, just in order to bring the balance between inorganic ions and water and also to maintain the acid-base balance in the intracellular environment.

The excretory process of contractile vacuole thus promotes homeostasis as well.

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