Hypotonic vs Isotonic: Detailed Insights

This article would highlight the details of Hypotonic vs Isotonic solutions in describing the ability of a solution to move water in or out of a particular cell. 

The ability associated with an extracellular solution in making the water either move in or move out of a particular cell through the process of osmosis is termed as tonicity. Hypotonic, Hypertonic and isotonic are the three different terms used in describing the process of tonicity. 

Hypotonic Vs Isotonic Solutions

Important terms for understanding hypotonic and isotonic solutions 

Solution

A solution is termed as the homogenous mixture of either two components or more than two components. It has two constituents, a solute and a solvent. The solute is dissolved in the solvent to form the solution. 

Concentration

The concentration of a solution is determined by evaluating the amount of solute present in the solution per unit volume. 

Osmosis

The process of movement of water molecules through a semipermeable membrane from a lower solute concentration area to a higher solute concentration area. 

Osmotic pressure

The pressure which is required in preventing the inward flow of the solutes across the semipermeable membrane is termed as the osmotic pressure. 

Isotonic solutions 

The solution with equal amount of osmotic pressure is defined to be isotonic which is due to the equal concentration of solutes among the solutions. The isotonic solutions have been identified to have an equal amount of solutes per unit volume within the solutions with equal amounts of water. On separating the isotonic solutions through semi-permeable membranes, the overall movement of the solutes is identified to be zero as there is no existence of any kind of concentration gradient.

Free movement of the water molecules in and out of the cells can be witnessed but the rate of the movement remains same in both the directions. Thus, when a cell is placed within an isotonic solution, there can be no change in the normal state of the cell witnessed. The shape of the cells remains the same with neither swelling nor shrinking. In terms of the osmotic pressure, the isotonic solutions have equal pressure and hence no movement of solutes or the water can be witnessed. 

Few examples of the isotonic solutions in terms of the animal cells are saline (0.98%) or 5% Dextrose in water.

Hypotonic solutions 

The solutions which are identified to have a lower osmotic pressure are termed as the hypotonic solutions. The lower osmotic pressure is the resultant of the lower concentration of the solutes within the solutions. When a cell is placed into a hypotonic solution, the amount of water present within the cell is lesser than the water concentration in the solution. This is due to the fact that the concentration of the solutes is lower than the volume of the water. This causes water to move into the cells and causes it to swell up. This can lead to the increase of the overall internal pressure and causes it to burst.

In plants, the hypotonic solutions have the ability to cause “turgidity” within the plant cells. As the water enters the cells of the plants, they swell up. This causes the cell membrane to push more towards the cell wall of the plants. The cell wall which is present only in the plant cells has the ability to avoid further bursting of the cells. This entire process is termed as turgidity and the swelled up cell is termed as “turgid cell”. 

Hyponatremia (Water Overdose)

One rare occurring consequence caused due to hypotonicity is “water overdose” where people tend to consume more than required water that the cells of our body can endure or the kidneys can process. This leads to the creation of an imbalance in the body. The excess drinking of water causes the salt solution to the exterior of the blood to become extremely diluted due to which the water enters into the blood cells and leads to them swelling. The symptoms of such a condition can be identified to be extreme exhaustion and heatstroke. 

Know more on: What Happens to a cell in a Hypotonic Solution: Detailed Insights

Tonicity in Living systems 

In terms of living cells, when placed in an isotonic solution, there is no net movement in water witnessed and hence no change in the size of cells, whereas in the hypotonic solution, water would enter the cells to cause them to swell up. As an example, for the red blood cells, the isotonic conditions are identified to be ideal. Our body being able to regulate the homeostatic environment helps in keeping such necessary conditions ideal. If the red blood cells are placed in the hypotonic solutions, it will bloat up and may cause it to explode.

Another example regarding the plant cells, it has been identified that the hypotonic (extracellular) solutions are ideal. The plasma membrane within the cells of the plants has the ability to expand only till a certain maximum limit, beyond which it would not expand and would not allow the cell to further burst or lyse. The cytoplasm within the cells are identified to be hypertonic in nature which would allow the water to enter within the cell until the internal pressure, which is termed as the turgor pressure works on preventing further influx within the cell. 

Similarities between Hypotonic and isotonic Solutions

In terms of the similarity between the hypotonic and the isotonic solutions, it can be identified that both the solutions are not helpful in preserving any kind of food and cannot be used as food preservatives.

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