The principle of osmosis is extremely important in understanding many biological processes. Osmosis is most simply defined as the movement of fluid from a highly concentrated area through a semipermeable membrane to a less concentrated area. Thus, osmosis results in  equal amounts of fluid on either side of the membrane. This state of dynamic equilibrium is called an isotonic state. A basic overview of key osmosis-related vocabulary is essential to understanding how osmosis works. Osmosis itself is a specific example of the principle of diffusion: the spontaneous movement of particles from a very concentrated area to a less concentrated area. You can visualize the process of diffusion if you picture what happens when a few drops of food coloring are dropped into a glass of water, or when the scent of supper flows from one room into the rest of the house. Concentration refers to how much of one substance is mixed with a given amount of another substance. To describe osmosis, we can start with the fluid solution, which is comprised of two substances: the solvent and the solute. The solute is the material which dissolves into a solution; the solvent is the dissolver. For example, in a saltwater solution, the solute is salt, because salt is dissolved. Accordingly, the solvent is water, since water is what makes salt dissolve. When a solvent and a solute are combined, the resulting fluid is a solution. If a solution contains low levels of a solute, it is called hypotonic. Solutions with high levels of solute are termed hypertonic. The solution passes through a barrier called the membrane. Most often, osmosis refers to water molecules moving across a cell membrane. Cell membranes are semipermeable–that is, they allow exclusive substances to pass through, but not others. Sometimes this kind of membrane is referred to as selectively permeable.
Osmosis Demystified
Osmosis occurs when a mass migration of solvent molecules in a solution move through the membrane from an area highly concentrated with solute to one with lower solute levels. This process occurs independently, without any input of energy from outside sources. During osmosis, fluid passes in and out of the semipermeable membrane, though there is usually a net directional flow, depending on which side of the membrane has the higher solute concentration. The isotonic state of equilibrium is obtained when the solution is neither hypotonic nor hypertonic, but the solute is equally diffused throughout the solution. Osmosis works because less-concentrated solution contains a larger amount of free energy. Therefore, solvent molecules diffuse into a place with less free energy in order to equalize it.
Examples of Osmosis
An easy way to visualize the effects of osmosis is to picture a bowl of water and a cup of rice. If you were to drop the cup of rice into the water, after several minutes, the grains of rice would begin to puff up with the water they had absorbed. Meanwhile, the volume of water in the bowl would drop. One commonly cited example of osmosis relates to the way in which plants get their nutrients. Plants use their roots to absorb water and nutrients directly from soil. The roots contain a hypertonic solution. Therefore, since roots are selectively permeable membranes, they will admit both water and some nutritious solutes, like minerals the plant needs. The roots draw in this material from the ground through osmosis. Osmosis plays a large role in maintaining the health of plant and animal cells, but it can also be destructive in unusual environments. For example, when gardeners need to dispose of slugs, they often sprinkle table salt on top of the creatures. Through osmosis, water passes out of the slug, and it shrinks until it dies. The same principle can be applied to leeches. Osmosis can also harm aquatic creatures if they are placed into an unnatural environment. The potentially harmful effects of osmosis are dramatically illustrated when a saltwater fish is placed in freshwater. Saltwater fish are ill-equipped to deal with the drastic change in salinity. Osmosis moves freshwater into the fish, which has no mechanism to rid itself of extra water. If left in freshwater, it will die.
Variations of Osmosis
Reverse Osmosis Reverse osmosis is a process used to filter large molecules and particles out of solutions. When a solution is on one side of the semi-permeable membrane, pressure is applied. External pressure reverses the normal flow of solvent. As a result, the solute stays on the pressurized side, while the newly-pure solvent passes through the membrane. Reverse osmosis is used for purifying saltwater by removing the salt particles, thus turning it into drinkable fresh water. Additionally, most at-home water filtration systems use the principal of reverse osmosis for purification of drinking water. Forward Osmosis Like reverse osmosis, forward osmosis also utilizes a semipermeable membrane to separate pure water from solutes. However, the means of separation differ. Reverse osmosis utilizes hydraulic pressure. In the case of forward osmosis, an additional highly concentrated solute is used to induce a flow of water through the membrane barrier in order to separate water from the solutes it contains. Forward osmosis (sometimes referred to as engineered osmosis or manipulated osmosis) has many applications. One example are emergency hydration bags, which can be used to make potentially unhealthy surface waters, such as from streams or ponds, drinkable. The bag contains a solute which can be digested, such as glucose or fructose. The solute draws out harmful toxins that the membrane will not allow to pass through. The solution that remains is then drinkable.
Additional Resources
Resources for teachers: The Osmosis Game Science Net: Osmosis and Diffusion Diffusion and Osmosis Web Quest Egg Osmosis Lab (Video)