Hypertonic Solution: Definition And Role In Cell Biology

A hypertonic solution refers to a solution that has a greater concentration of solute than another solution. In the context of biology, when two aqueous solutions are separated by a cell membrane, if the concentration of solute is greater outside the cell than inside the membrane, the solution is called hypertonic.

Thus, hypertonicity refers to a relative difference between the concentration of solute inside a cell and the concentration of the solute outside the cell. When a cell is placed in a hypertonic solution, osmotic pressure will force water out of the cell to balance the concentration of solute across the membrane.

Since water tends to flow out of the cell, cells placed in a hypertonic solution will shrink. The process by which water moves out a cell in a hypertonic solution is called plasmolysis. Cells that lose too much water can be damaged, and organisms immersed in strongly hypertonic solutions can become dehydrated.  The other kinds of solutions relevant to osmosis are called hypotonic and isotonic. Hypotonic solutions have a lower concentration of solute than inside the cell so water rushes in, while isotonic solutions have an equal concentration of solute inside and outside the cell, so there is no net diffusion of water.

Diffusion, Osmosis, And Tonicity

In order to fully understand hypertonicity, we need to step back and look at the basic behavior of fluids in certain environments.

For any sample of fluid, the molecules in that fluid are subject to random motion. Over time, this random motion will compound, and the molecules in the fluid will change from being close together to being evenly spread apart. Diffusion is the name for this tendency for molecules in a fluid to move from regions of high concentration to regions of low concentration. Diffusion is a result of the random motion of molecules and does not require any net input of energy to occur. For example, if I put a drop of ink in a cup of water, over time, the ink will naturally spread out to be evenly mixed with the water.

Animals and plant cells have a selectively permeable membrane around them that lets some chemicals pass (like oxygen and water) and keeps other things out (like proteins and DNA). The term osmosis refers to the diffusion of water across a selectively permeable membrane. Osmosis is extremely important for living organisms as it regulates the amount of water inside and outside of cells. The presence of a semipermeable membrane affects what kinds of molecules it can diffuse across.

The tonicity of a solution refers to the direction and magnitude of the osmosis of water. In the presence of a semipermeable membrane, water has a tendency to diffuse from regions of low solute concentration to regions of high solute concentration. So, if a cell is placed in salt water, water will move out of the cell into the salt water. Conversely, if a cell is placed in fresh water, water will flow into the cell, as the cell has a higher concentration of solute. This movement is due to the diffusion of water from regions where itself is highly concentrated (in low solute concentrations), to regions where it is less concentrated (in high solute concentrations). Additionally, if the solution is equally concentrated on either side of the cell membrane, water flows in at the same rate it flows out, so there is no net diffusion of water.

Given that there are three possible directions for the net water flow during osmosis, out, in, and neutral, we have three possible kinds of tonicity, labeled: hypertonic, hypotonic, and isotonic.

A hypertonic solution has a greater solute concentration than inside a cell. When a cell is placed in a hypertonic solution water will move from inside the cell where there is a low solute concentration (and so a high water concentration) to areas with a high solute concentration (and so a low water concentration). Since water is leaving the cell, the cell shrinks.

A hypotonic solution has a lesser solute concentration than that in the cell. When a cell is placed in a hypotonic solution, water will move from outside the cell where there is a low solute concentration (and so a high water concentration) to inside the cell where there is a high solute concentration (and thus a low water concentration). Since water is entering the cell, it expands.

An isotonic solution has an equal solute concentration to that of the cell. When a cell is placed in an isotonic solution since the solute concentration is equal, water will flow into the cell at the same rate it will leave the cell. The result is that there is no net movement of water molecules. Since the amount of water stays the same, the cell stays the same size.

Keep in mind that the standard definitions of these terms are defined from the point of view of the cell. The designation of a solution as hyper-, hypo-, or isotonic depends on the relative difference of concentration of a solution outside the cell. In one way, you could consider hyper-and hypotonicity as two sides of the same coin: A solution that is hypertonic from the view of the cell is hypotonic from the point of the view of the outside solution.

Examples Of Hypertonic Solutions

Salt Water

Seawater and water from other brackish sources is hypertonic. Salt water has a much greater concentration of dissolved minerals and ions than the water in the human body. As such, when cells are submerged in salt water, water moves out of the cell into the salt water, which shrinks the cell. This is the main reason why a person cannot hydrate themselves by drinking ocean water. Sure, they are drinking water, but the high concentration of salt in that water draws water out of cells instead of hydrating them.

Corn Syrup

Corn syrup is a thick mixture made out of starch which contains sugars and other saccharides. Due to this high concentration of sugars, corn syrup is hypertonic with respect to an average animal cell. In fact, one can use corn syrup to easily see the effect of hypertonic solutions of animal cells. First, grab some eggs and soak them in vinegar for about 24 hours. This will dissolve the shell but leave the outer membrane. Next, fill a bowl with corn syrup and drop the egg in the mixture. Over time, you will see the egg shrink as water leaves the egg and goes into the corn syrup. Eggs are useful for this experiment because eggs are, for all and intents and purposes, a single cell.

Salt

One of the main reasons salt is useful as a food preservative is that salt is very hypertonic. This may sound strange as crystallized salt is not a liquid, however, any sample of salt has some amount of water in it (even if it is small) and so it counts as a solution. A solution of salt removes water from nearby areas, which can kill microbes or limit their ability to reproduce. This is also the same reason why putting salt on a snail or slug can hurt them; the hypertonic salt draws water out of their thin skin cells and dehydrates them.

IV Fluids

Many commonly administered IV fluids, like saline and glucose, are hypertonic solutions. Saline, for example, is often given by doctors to dehydrated patients. This may sound strange at first; if saline is hypertonic, and hypertonic solutions draw water out of cells, then how does saline help fix dehydration? The trick is that the saline is just slightly hypertonic compared to the average blood cell, but slightly hypotonic with respect to other cells that contain water. The result is that, while the slight hypertonicity of saline relative to blood cells draws some water out of the blood cells, the relative hypotonicity of saline compared to other cells ensures that water does not leave the bloodstream. This is why a normal saline drip has a very low concentration of salt (about 0.9%); any higher and this balance would be offset, and the solution would dehydrate the patient.

Fish And Hypertonic Solutions

Saltwater tends to be hypertonic to the fish that live in it. Since they require a lot of surface area of their bodies to contact water (or their gills) they lose a lot of water through osmosis of the gill linings. Many fish have evolved to get around this problem though; they drink large amounts of salt water and excrete any excess salt from their bodies.

Plasmolysis In Plants

Hypertonic solutions can be dangerous for plants as they remove water from the cell that is necessary for the plant to stand upright. In hydrated plant cells, water contained in the vacuoles exerts turgor pressure on the cell wall, causing the plant to stand upright. When a plant is immersed in a hypertonic solution, water leaves the plant cell and it shrivels up. The contracting of the cell pulls apart the cell’s membrane from the cell wall, which can result in a complete cell collapse if too much water is lost.

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