What will happen to a cell when it is placed in a hypotonic solution

Updated April 24, 2017

By Rochelle Leggett

The function of a cell is directly influenced by its environment, including the substances that are dissolved into its environment. Placing cells in different types of solutions helps both students and scientists understand cell function. A hypotonic solution has a drastic effect on animal cells that demonstrates important and distinctive properties of an animal cell and cell membranes.

A solution is a mixture of two or more substances and is composed of two parts, the solutes and the solvent. The solutes are the substances that are dissolved, and the solvent is the substance that the solutes are dissolved into. Solutions have an even distribution of solvents throughout the mixture. Solutions are compared to one another by describing them as hypertonic, isotonic or hypotonic. If a solution is hypertonic, it has more solutes relative to another solution. An isotonic solution has the same amount of solutes. A hypotonic solution has fewer solutes.

Osmosis refers to the movement of water through a selectively permeable membrane. A selectively permeable membrane is a membrane that only allows the passage of water molecules -- not solutes or ions -- through the membrane. In osmosis, water always moves from a solution with a low number of solutes to one with a high number of solutes. If a solution with a low number of solutes (hypotonic) is placed next to one with a higher number of solutes (hypertonic) and is separated by a selectively permeable membrane, the water would move from the hypotonic solution to the hypertonic solution due to osmosis.

Every cell has a membrane that covers the outside of the cell; it's called a plasma membrane. This membrane has numerous functions, including keeping the contents of the cell separate from the outside world, protecting the cell and moving substances in and out of the cell. These substances can be nutrients, wastes and water. Animal cells are different from those of other organisms in that they lack a cell wall, which is a rigid structure that both protects the cell and gives it shape.

Animal cells have a membrane that is differentially permeable. Similar to a selectively permeable membrane, a differentially permeable membrane only allows certain substances -- including water, but not exclusively water -- to pass through the membrane. An animal cell that is placed in a hypotonic solution will rapidly gain water, because osmosis would cause the water to move to an area with more solutes. In this case, that is the inside of the cell.

A cell in a hypotonic solution may gain enough water to lyse, or rupture, the cell membrane, which destroys the cell. Plant cells have some defense against this phenomenon because their cell walls prevent the cell from rupturing. Organisms that live in freshwater environments, which are usually hypotonic, often have mechanisms that help prevent cells from rupturing. This principle is often demonstrated with red blood cells, which have no mechanisms to defend against lysing.

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In biology, osmosis refers to the movement of water. It is a passive process, meaning it requires no energy and happens automatically. The water moves from an area of greater concentration to an area of lesser concentration until equilibrium is reached. For cells, this movement is from one compartment to another through a semipermeable membrane. The solution is the area outside the cellular compartment, also called the extracellular environment.

Solute is the number of dissolved solids in a solution regardless of what they are. It can consist of proteins, carbohydrates, ions, hormones, etc. When discussing osmosis, comparison is made between the intracellular and extracellular (solution) solute concentrations. The concentration is also called the osmolarity of the solution. The difference between the number of dissolved solids creates an osmotic pressure gradient that forces water to move to achieve equilibrium between the inside and outside environments.

In a hypotonic solution, the solute concentration is lower than inside the cell. The prefix hypo means under or below in Latin. Under these conditions, the osmotic pressure gradient forces water into the cell. Depending on the amount of water that enters, the cell may look enlarged or bloated. If the water continues to move into the cell, it can stretch the cell membrane to the point the cell bursts (lyses) and dies.

Cells don’t have the ability to regulate their water content (remember that osmosis is a passive process), so they rely on the body to provide an environment where intracellular and extracellular solute concentrations are equal or isotonic. The body regulates the composition of extracellular fluid using the kidneys, adrenal glands, and the hypothalamus in the brain which triggers thirst and drives organisms to drink water. Slight fluctuations in the solute concentration of the extracellular fluid throughout the day cause small amounts of water to be exchanged between the intracellular and extracellular compartments to maintain homeostasis.
In contrast to hypotonic and isotonic solutions, a hypertonic solution has a higher solute concentration than inside the cell. When this happens, the osmotic gradient causes water to rush out of the cell and it becomes wrinkled or shriveled. If this happens to red blood cells, it is called crenation

Plant cells respond the same way as animal cells in a hypotonic solution, but the affects may not be as severe. Plants have rigid cell walls made of cellulose covering the plasma membrane. This makes it difficult for the cell to lyse, but the increased pressure causes the sides of the cell to bulge out.

References

• OpenStax College. (2018). Anatomy & Physiology. Houston, TX. OpenStax CNX. Retrieved from //cnx.org/contents/
• Tonicity. (n.d.). In Wikipedia. Retrieved April 25, 2018 from //en.wikipedia.org/wiki/Tonicity