Tonicity: Definition, Types, Examples, Meaning & Synonyms

What is Tonicity?

Tonicity refers to the ability of a solution to cause a cell to either gain or lose water. Tonicity concerns life processes and cellular homeostasis. One look at tonicity relieves us of how the cells function concerning the environment in light of medicine, agriculture, and industries.

Tonicity refers to the gradient of osmotic pressure across a semipermeable membrane between two solutions. In other words, it is the directional and degree movement of water across the cell membrane. Hence affecting the volume and pressure of the cell.

Historical Background

The concept of tonicity grew from the early studies on osmosis and cell behaviour in various solutions. The notable contributions of Jean-Antoine Nollet and Thomas Graham, amongst others, have laid the foundation.

Basic Concepts of Tonicity

The basic concepts of tonicity includes:

Osmosis

Osmosis is the flow of water across a semipermeable membrane from a low solute concentration to high solute concentration. This process is crucial for the stability of the cell.

Solute and Solvent

A solute is a substance dissolved in a solvent. In biological systems, the solvent is mostly composed of water with various solutes such as salts, sugars, and proteins.

Semipermeable Membrane

This membrane allows some molecules through while blocking others. It critically plays a very significant role in controlling the movement of substances into and out of the cell.

Types of Solutions (Based on Tonicity)

The types of solutions based on tonicity:

Hypertonic Solutions

A hypertonic solution is one with a higher concentration of solutes in water than the cytoplasm of the cell. Water leaves the cell, and it becomes shrunk. It is called crenation in animal cells and plasmolysis in plants.

Hypotonic Solutions

A hypotonic solution is a solution that contains a lower concentration of solute than the cytoplasm of the cell. Water moves into the cell and the cell swells, it may even burst as in the case of lysis in animal cells. In plant cells, this causes turgidity and is what helps the plant 'hold itself up' and maintain structure.

Isotonic Solutions

An isotonic solution is a solution having the same concentration of solutes as the cytoplasm of the cell. Movements of water in and out of the cell are going to be the same, and hence the cell neither loses nor gains any water. It stays in its original size and remains proportionate to the surrounding solution.

Mechanisms of Tonicity

The cells react to changes in their tonic environment. In the cell membrane, there are special proteins called aquaporins which help in the fast transportation of water. Osmoregulation is the term used to describe how an animal maintains the concentration of water and salts in the body. Osmosis and osmoregulation work together to maintain cellular homeostasis.

Tonicity and Cellular Homeostasis

Tonicity is also very critical to cellular functioning. For example, human kidneys filter the blood to control its osmolarity, such that cells are always in an isotonic solution. Blood cells themselves also need to maintain their tonicity to serve their purpose.

Applications of Tonicity

The applications of tonicity are:

Medical

Understanding tonicity is crucial both in the use of IV fluids and in the treatment of dehydration. It is also used in hemodialysis machines, which remove waste products from the blood by applying principles of tonicity.

Agricultural

Tonicity interferes with water absorption into plants and the general health status of the plant. Proper irrigation and adequate management of the soil are therefore imperative to achieve optimum growth as they provide the proper levels for tonicity.

Industrial

The principles of tonicity extend into food preservation and biotechnology. By careful management of the tonicity of solutions, unwanted growths of microorganisms can be prevented and stability enhanced.

Tonicity NEET MCQs (With Answers & Explanations)

The key concepts to be covered under this topic for different exams are:

• Types of Solutions

• Mechanisms of Tonicity

Practice Questions for NEET

Q1. The concentration (%) of NaCl isotonic to human blood is

1. 0.085-0.09%

2. 1.7-1.8%

3. 3.4-3.6%

4. 0.85-0.9%

Correct answer: 4) 0.85-0.9%

Explanation:

As 0.9% NaCl solution is said to be isotonic: when blood cells reside in such a medium, the intracellular and extracellular fluids are in osmotic equilibrium across the cell membrane, and there is no net influx or efflux of water.

The percentage of concentration of NaCl, isotonic to human blood is 0.85 - 0.9%.

Hence, the correct answer is option (4) 0.85-0.9%

Q2. What would you expect when the cell is placed in a hypertonic solution?

1. An increase in the volume of the cell

2. A decrease in the volume of the cell

3. No net changes in the volume of the cell

4. None of these

Correct answer: 2) A decrease in the volume of the cell

Explanation:

When a cell is placed in a hypertonic solution, water from the cell sap will move into the hypertonic solution because of the higher concentration of the solution. The cell will decrease in its volume. This is called exosmosis. The cell will decrease in its volume. This is called exosmosis. Exosmosis occurs as water moves out of the cell to balance the concentration gradient, causing the cell to shrink. In plant cells, this leads to plasmolysis, where the plasma membrane pulls away from the cell wall. Exosmosis is a vital process in maintaining osmotic balance but can lead to cell damage if it occurs excessively.

Hence, the correct answer is option 2) A decrease in the volume of the cell.

Q3. When an animal cell is placed in a hypotonic solution

1. It will become turgid

2. It will have maximum turgor pressure

3. It will have maximum wall pressure

4. None of these

Correct answer: 4) None of these

Explanation:

Animal cells do not have a cell wall. Turgor pressure and wall pressure are absent in animal cells. These will be quickly lysed when placed in a hypotonic solution due to the influx of water, causing the cell to swell and burst. In the absence of a rigid cell wall, animal cells rely on mechanisms like osmoregulation to maintain their internal environment. This highlights the importance of structures like contractile vacuoles in unicellular animals or the kidneys in higher animals to prevent lysis.

Hence, the correct answer is option 4) None of these.

Also Read:

• MCQs on Tonicity

• Facilitated diffusion

• Water Potential and its Component

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