Osmosis and Diffusion: Definitions and Differences

Though osmosis sounds like a word that you could hear during a magic show, it is one of the most important phenomena that govern the ability for our bodies to remain perfectly hydrated. Not to be confused with diffusion, which is the movement of solutes between various compartments, osmosis is particularly concerned with the movement of water. Today, we will discuss the difference between the two processes and their importance in our daily lives!

Concentration and Diffusion

Whether it is water, particles in a solution, or gasses in the atmosphere, substances spontaneously move from spaces with a larger concentration, into spaces with a lower concentration to ultimately achieve a uniform concentration across the total area.

Let’s start with the basics!

Concentration is defined as the amount of a given particle per unit volume. A common unit of concentration is ppm or parts per million. If you are familiar with chemistry, another common unit is molarity which is defined as moles per liter of fluid. One mole is 6.02 x 10²³ atoms or molecules.

This spreading of a substance to achieve a uniform distribution of that substance across an area is defined as diffusion. An example of this would be the release of a balloon filled with air. When you fill up a balloon, there is an increased concentration of gas in the balloon relative to the outside environment. Therefore, as diffusion would suggest, when you release the balloon, air flows out to create an equal air concentration (or pressure) both inside and outside of the balloon. In the context of gas, concentration (recall: amount of gas particles per unit volume) is often referred to as pressure.  This is a little confusing, but the diagram below may help!

 

Osmosis and the Human Body

The balloon example above is an example of gas diffusion, but diffusion also occurs between two compartments filled with two fluids that have different concentrations. Diffusion can occur in any medium, both gases and solids along with fluids, but osmosis is specific to fluid.

The movement of particles dissolved in water goes hand in hand with the movement of water or solvent between two compartments such as osmosis. Before our discussion of osmosis, we need to define a few important terms:

Solute: a substance dissolved in a solvent, i.e. in ocean water, salt is a solute and water is the solvent

Solvent: the medium that dissolves the solute. In the above example, water is the solvent that dissolves salt.

Semi-permeable membrane: a barrier that only lets a specific substance through. The cells in our bodies are an example of containers surrounded by semi permeable membranes that only allow some substances to pass through. In this discussion, our semi-permeable membrane will only allow water to pass through.

 

Osmosis is the movement of water or another solvent from areas of low solute concentration to areas of high solute concentration that are separated by a semi permeable membrane. This results in an equal concentration of solute in both compartments. The diagram below describes how this would look during a real experiment!

 

Though osmosis happens throughout the body to maintain a proper balance of fluid in all of the organ systems, our kidneys rely on this process to maintain the body’s perfect balance of electrolytes, buffers, calcium, and many other components that allow for healthy bodily function. As blood enters the kidneys, diffusion allows for our blood to be filtered, and solutes to move from the blood into the kidney’s filtering system. Then by osmosis, most of the water is reabsorbed into the blood stream within the kidney, but small amounts are filtered and turned into urine to make sure that we maintain a very specific balance of water in our bodies.

Our kidneys also impact our thirst reflex, and when they sense that the blood is too concentrated, trigger signals that lead us to have the sensation of thirst. It is truly amazing how much we experience the effects of osmosis and diffusion every day!

 

Author: Sydni Britton