Osmosis

Osmosis is the process by which molecules of a solvent pass through a semipermeable membrane that is separating 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 in nature, are electrically neutral and very small. When salts such as sodium chloride are dissolved in water, they dissociate into ions, which are both electrically charged and significantly larger in size than the surrounding water molecules. A porous membrane that has 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 also too large to pass through the pores of the membrane 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. The process of osmosis will occur spontaneously and continue until the two solutions become isotonic, meaning that both solutions have the same concentration of solutes.

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 together, and then letting the water stand undisturbed. At first, the food coloring will remain where it was placed, but over time it will become evenly distributed throughout the water. The water molecules are in constant motion, and as they continually bump into the molecules of food coloring, they eventually spread them throughout so that the two become mixed together. 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 at the same time. 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 osmotic pressure of the membrane, and it is dependent on both 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 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 temperature in kelvins and C is the concentration in moles per liter (mol/L), or molars (M). R is the gas constant and can be written as

where L is liters, atm is atmosphere (a unit of pressure), mol is moles, and K is kelvins. 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, φ, must be included in the equation.

By applying pressure in excess of 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. It is through this process that salt-free potable water can be produced from salty seawater or other non-potable 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 so are not able to 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 solution of salt in water, while on the other side 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.

If both solutions contain dissolved salt but in different amounts, the same effect will be observed, but it will cease when the concentrations of the two solutions become equal. As water molecules pass through the membrane, the concentration of dissolved salt in the hypertonic solution decreases and the concentration of dissolved salt in the hypotonic solution increases. At the equilibrium point, when the two solutions become isotonic, 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. In order to maintain the proper amount of water in cells and prevent dehydration, living systems use a complex mechanism of osmoregulation that actively brings water into the cells to replace water that is lost through osmosis. 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 viability of the system. Hangovers, for example, are partly the result of dehydration of cell fluids as alcohol is metabolized and often persist until the osmotic balance of the cells is restored.

PRINCIPAL 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 of a process 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 of a cell.
• 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 in order 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 that is dissolved in a liquid or fluid medium, usually water.
• solvent: any fluid, most commonly water, that dissolves other materials.

Bibliography

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

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

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

Lodish, Harvey, et al. Molecular Cell Biology. 7th ed. Freeman, 2013.

Lopez, Michael J., and Carrie A. Hall. "Physiology, Osmosis." StatPearls, 13 Mar. 2023. US National Library of Medicine, www.ncbi.nlm.nih.gov/books/NBK557609/. Accessed 30 Sept. 2024.

"Osmosis." BYJU's, byjus.com/biology/osmosis/. Accessed 30 Sept. 2024.

Pelczar Jr., Michael J., E. C. S. Chan, and Noel R. Krieg. Microbiology: Concepts and Applications. McGraw, 1993.

Reece, Jane B., et al. Campbell Biology. 9th ed. Cummings, 2011.