Hypotonic Solution – Definition, Examples, Effects and Uses

A hypotonic solution is a solution that has a lower concentration of osmotically effective solutes compared to another solution across a selectively permeable membrane. In the case of cells, the fluid present inside the cell (cytoplasm) is taken as the reference. The surrounding solution is considered hypotonic when it has a lower concentration of solutes that cannot readily cross the cell membrane. Water then moves into the cell by osmosis.

The term “hypotonic” does not simply mean a solution having a low concentration of dissolved substances. It is a relative term. A solution may be hypotonic to one cell but not to another, depending on their internal solute concentrations and the permeability of the cell membrane.

Osmosis in a Hypotonic Solution

• A cell placed in a hypotonic solution has a higher concentration of nonpenetrating solutes inside the cell compared to its surrounding solution. The cell is surrounded by a selectively permeable membrane, which allows water to pass through it but restricts the movement of certain solutes.
• Water from the surrounding solution is taken up by the cell through osmosis. In this process, water moves in both directions across the cell membrane. However, the amount of water entering the cell is more than the water moving out. This results in a net inward movement of water without the use of cellular energy.
• As more water enters, the cell increases in volume and begins to swell.
• The ability of a surrounding solution to change the volume of a cell by osmosis is referred to as tonicity. It mainly depends on the concentration of nonpenetrating solutes present on either side of the cell membrane.
• Osmolarity is the total concentration of dissolved solute particles, including both penetrating and nonpenetrating solutes. Some solutes, such as urea, can pass through the membranes of many cells. Once these solutes become equally distributed on both sides of the membrane, they do not maintain a lasting osmotic gradient. Such solutes contribute to total osmolarity, but their effect on tonicity is different from that of nonpenetrating solutes.

Effects of Hypotonic Solutions on Cells

When a cell is placed in a hypotonic solution, water enters into the cell by the process of osmosis. The surrounding solution has a lower concentration of non-penetrating solutes compared to the cell. As a result, there is a net movement of water into the cell.

• Swelling of animal cells- Animal cells lack a rigid cell wall. When placed in a hypotonic solution, they take up water and begin to swell. Excessive entry of water may cause the plasma membrane to rupture. This process is called cytolysis.
• In red blood cells (RBCs), water enters through the plasma membrane, causing the cells to swell and lose their normal biconcave shape. In a strongly hypotonic solution, the cells may burst, releasing haemoglobin into the surrounding solution. This is known as hemolysis.
• Effect on plant cells- Water is absorbed by the plant cell and accumulates mainly in the central vacuole. The vacuole increases in size, pressing the cytoplasm and plasma membrane against the cell wall. The cell becomes turgid.
• The entry of water produces turgor pressure, which acts against the rigid cell wall. It prevents the cell from swelling beyond the limit of the wall. Turgor pressure also helps in maintaining the rigidity of non-woody plant tissues.
• Some freshwater protozoans, such as Amoeba and Paramecium, possess contractile vacuoles. These vacuoles collect and expel the excess water entering the cell from its hypotonic surroundings, preventing excessive swelling.

Examples of Hypotonic Solutions

Some of the common examples of hypotonic solutions are:

1. Distilled Water- Distilled water is hypotonic to most living cells because it contains almost no dissolved solutes. When human red blood cells (RBCs) are placed in distilled water, water enters the cells by osmosis. The cells swell and may eventually burst (hemolysis).
2. Tap Water- Tap water is generally hypotonic to the cells of many plants. For example, when carrot pieces are soaked in tap water, water enters their cells and increases the turgor pressure. The cells become turgid. The actual effect depends on the concentration of dissolved substances present in the water and the cells.
3. Dilute Salt Solution- A 0.45% sodium chloride (NaCl) solution is an example of a hypotonic solution relative to normal human blood plasma. It is also called “half-normal saline”. Compared to 0.9% NaCl solution, it contains a lower concentration of salt. Human red blood cells placed in this solution take up water and swell. More dilute salt solutions can produce greater osmotic swelling and may cause hemolysis.
4. Freshwater- The water present in freshwater ponds and lakes is usually hypotonic to the cytoplasm of freshwater protozoans such as Amoeba and Paramecium. Water continuously enters their cells by osmosis. These organisms possess contractile vacuoles, which collect and expel the excess water from the cells.
5. Water Used for Soaking Vegetables- When wilted celery or carrot pieces are placed in fresh water, the surrounding water may act as a hypotonic solution relative to their cells. Water is absorbed by osmosis, increasing the volume of the vacuoles and restoring turgor pressure in the cells.

Note: A solution is termed “hypotonic” only in comparison with another solution or the internal environment of a particular cell. It is based on the relative concentration of solutes that cannot readily cross the cell membrane. The same solution may be hypotonic to one type of cell but isotonic or hypertonic to another, depending on the concentration of these solutes and the permeability of the cell membrane.

Uses of Hypotonic Solutions

Some of the important uses of hypotonic solutions are:

• Medical Treatment- Hypotonic saline solutions, such as 0.45% sodium chloride (NaCl), are used in selected cases of hypernatremia, where the sodium concentration in the blood is abnormally high. These solutions provide water along with electrolytes and help in correcting the water deficit. The administration is done under medical supervision, with regular monitoring of serum sodium concentration to prevent complications.
• Osmotic Fragility Test- In this test, a series of hypotonic salt solutions of different concentrations are used to determine the osmotic fragility of red blood cells (RBCs). The RBCs are exposed to these solutions, and the extent of hemolysis is measured. It is used in laboratory investigations of certain blood disorders, particularly hereditary spherocytosis.
• Chromosome Preparation- Hypotonic potassium chloride (KCl) solution is used during the preparation of metaphase chromosomes for karyotyping. The cells are treated with the solution, which causes them to swell due to the entry of water. It provides more space for the chromosomes and facilitates their spreading on microscopic slides.
• Plant Tissue Rehydration- Watering wilted plants helps in restoring their turgidity when the surrounding water is hypotonic relative to the cell sap. Water is absorbed by the roots and enters the plant cells. The vacuoles increase in size and turgor pressure is restored.
• Osmosis Experiments- Hypotonic solutions are commonly used in biological laboratories to study osmosis and its effects on living cells. For example, distilled water is used in experiments with onion epidermal cells to observe the entry of water and development of turgor pressure. These cells can also be exposed to concentrated salt solutions to study plasmolysis.

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