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Principles of Sports Medicine & Exercise Science

Osmosis

by Richard M. Renneboog

Category: Physiology

Specialties and related fields: Biochemistry, genetics, molecular biology, physiology

Definition: a process by which molecules of a solvent tend to pass through a semipermeable membrane from a less concentrated solution into a more concentrated one, thus equalizing the concentrations on each side of the membrane

KEY TERMS

concentration gradient: the gradual change in the concentration of solutes in a solution across a specific distance

diffusion: the process by which different particles, such as atoms and molecules, gradually become intermingled due to random motion caused by thermal energy

equilibrium: the state that exists when the forward activity is exactly equal to the reverse activity of that process

hypertonic: describes a solution with a greater concentration of solutes than the solution to which it is being compared; in biology, a solution with a greater solute concentration than the cytoplasm of a cell

hypotonic: describes a solution with a lower concentration of solutes than the solution to which it is being compared; in biology, a solution with a lower solute concentration than the cytoplasm of a cell

isotonic: describes a solution with the same concentration of solutes as the solution to which it is being compared; in biology, a solution with the same solute concentration as the cytoplasm

osmotic pressure: the pressure that would have to be applied to a solution to prevent the flow of solvent through a semipermeable membrane

reverse osmosis: the application of pressure to a solution to overcome the osmotic pressure of a semipermeable membrane and force water to pass through it in the direction opposite to normal osmotic flow

semipermeable membrane: a membrane that allows the passage of a material, such as water or another solvent, from one side to the other while preventing the passage of other materials, such as dissolved salts or another solute

solute: any material dissolved in a liquid or fluid medium, usually water

solvent: any fluid, most commonly water, that dissolves other materials

VISUALIZING THE CONCEPT OF OSMOSIS

Osmosis is the process by which molecules of a solvent pass through a semipermeable membrane that separates two solutions with differing concentrations of solute. The solvent moves from the solution with the lower concentration (the hypotonic solution) toward the one with the higher concentration (the hypertonic solution). The process will continue until the concentrations of both solutions are equal, and equilibrium is achieved.

A semipermeable membrane is necessary for osmosis to occur. Such a membrane acts as a porous barrier that will allow the passage of solvent molecules but not dissolved materials, such as various mineral salts. Water molecules, though polar, are electrically neutral and very small. When salts such as sodium chloride are dissolved in water, they dissociate into ions, which are electrically charged and significantly larger than the surrounding water molecules. A porous membrane with pores big enough to allow electrically neutral water molecules to pass through, but not the larger electrically charged dissolved ions, is said to be semipermeable. Other types of dissolved materials, such as various sugars and proteins, are too large to pass through the membrane’s pores and so are subject to the process of osmosis as well. The presence of a semipermeable membrane can produce an osmotic system. In an osmotic system, the hypotonic solution is confined to one side of the membrane, and the hypertonic solution is contained on the other side. Osmosis occurs spontaneously and continues until the two solutions become isotonic, meaning that both solutions have the same concentration of solutes.

When an athlete drinks a sports drink, water diffuses into cells by osmosis, leading to rapid rehydration.

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Photo via iStock/mihailomilovanovic. [Used under license.]

DIFFUSION AND OSMOTIC PRESSURE

Diffusion can be demonstrated simply by adding a few drops of food coloring to a container of water, being careful not to mix them, and then letting the water stand undisturbed. At first, the food coloring will remain where it was placed. Still, over time it will become evenly distributed throughout the water. The water molecules are in constant motion. As they continually bump into the food coloring molecules, they eventually spread them throughout so that the two become mixed. During this process, the distribution of the food coloring in the water follows a concentration gradient, which is the difference in concentration of a solute when the concentration is not constant throughout the solution. Once the food coloring is evenly distributed, the result of this mixing is the same as if the solution had been stirred or agitated, but it requires a much longer time.

Diffusion is the mechanism that drives water molecules through the pores of a semipermeable membrane. Once they are through the membrane, the water molecules interact with the dissolved salts in that solution and remain. The process is reversible, so some water molecules are driven through the membrane in the opposite direction simultaneously. However, the difference in concentration ensures that the net flow of water molecules is toward the hypertonic solution until equilibrium is achieved.

Osmosis can be prevented by applying pressure to the hypertonic solution. The amount of pressure that must be applied to stop osmosis is termed the membrane’s osmotic pressure, and it depends on the temperature and the difference in concentration between the two solutions. Osmotic pressure was first described by Jean-Antoine Nollet (1700-1770), also known as Abbé Nollet, in 1748 and first measured directly by Jacobus Henricus van’t Hoff (1852-1911) in 1877. The osmotic pressure is given the symbol ϖ and, in the case of an ideal solution, is defined by the van’t Hoff equation as:

ϖ = RT(CB – CA)

where T is the temperature in kelvins and C is the concentration in moles per liter (mol/L), or molars (M). R is the gas constant, 8.314 J K–1 mol–1. An ideal solution is a solution in which the molecules of solute and solvent interact with each other in the same way they interact with themselves. If the solution is not ideal, then an osmotic coefficient must be included in the equation.

By applying pressure above the osmotic pressure, the process can be driven in reverse. Reverse osmosis forces water molecules to pass through a semipermeable membrane from the hypertonic solution into the hypotonic solution. Through this process, salt-free potable water can be produced from salty seawater or other nonpotable sources.

A DEMONSTRATION OF OSMOSIS

The process of osmosis and osmotic pressure can be readily demonstrated and observed. The essential feature of the demonstration is that two solutions are separated by a semipermeable membrane and cannot mix. This can be done by using the membrane as a partition to separate one half of the inside of a beaker from the other half. The solution on one side of the membrane is a salt solution in water, while on the other is just plain water. As osmosis takes place, the level of the saltwater solution will increase as the level of the unadulterated water decreases. The rate at which the levels change depends on the area of the membrane that is exposed to both solutions.

As water molecules pass through the membrane, the concentration of dissolved salt in the hypertonic solution decreases, and the concentration in the hypotonic solution increases. The same effect will be observed if both solutions contain dissolved salt but in different amounts. Still, it will cease when the concentrations of the two solutions become equal. When the two solutions become isotonic at the equilibrium point, water molecules pass through the membrane in both directions at the same rate.

OSMOSIS IN BIOLOGICAL SYSTEMS

Cell membranes function as semipermeable membranes in living systems, allowing water, oxygen, carbon dioxide, sugars, enzymes, ions, hormones, metabolites, and various other cellular components to pass through as necessary. Living systems use a complex mechanism of osmoregulation that actively brings water into the cells to replace water lost through osmosis to maintain the proper amount of water in cells and prevent dehydration. Anything that interferes with this mechanism, such as the consumption of alcohol, use of drugs, smoking, or lack of sufficient water in the diet, adversely affects the system’s viability. Hangovers, for example, are partly the result of dehydration of cell fluids as alcohol is metabolized and often persists until the osmotic balance of the cells is restored.

Further Reading

1 

Cooke, David, editor. Microbiology: Concepts and Applications. Callisto Reference, 2018.

2 

Costanzo, Linda S. Physiology: Cases and Problems. 4th ed., Lippincott, 2012.

3 

Kucera, Jane. Reverse Osmosis: Design, Processes, and Applications for Engineers. Wiley, 2010.

4 

Lafferty, Peter, and Julian Rowe, editors. The Hutchinson Dictionary of Science. 2nd ed., Helicon, 1998.

5 

Lodish, Harvey, et al. Molecular Cell Biology. 9th ed., W. H. Freeman, 2021.

6 

Urry, Lisa A., et al. Campbell Biology. 12th ed., Cummings, 2020.

Citation Types

Type
Format
MLA 9th
Renneboog, Richard M. "Osmosis." Principles of Sports Medicine & Exercise Science, edited by Michael A. Buratovich, Salem Press, 2022. Salem Online, online.salempress.com/articleDetails.do?articleName=POSpKin_0036.
APA 7th
Renneboog, R. M. (2022). Osmosis. In M. A. Buratovich (Ed.), Principles of Sports Medicine & Exercise Science. Salem Press. online.salempress.com.
CMOS 17th
Renneboog, Richard M. "Osmosis." Edited by Michael A. Buratovich. Principles of Sports Medicine & Exercise Science. Hackensack: Salem Press, 2022. Accessed September 16, 2025. online.salempress.com.