What Is Osmosis?
The term “osmosis” may bring back nightmares from high school biology. Many people might remember that it has something to do with water… and something to do with cells. This omnipresent process is occurring all around us, constantly. Scientists and industries have learned to manipulate this phenomenon in all manners of interesting applications. Today, let’s break the biology textbook back open and take a moment to better understand: what is osmosis?
Osmosis has many practical utilities in both industry and nature. Before we understand how osmosis functions in real life, let’s first take a look at the definition of osmosis and some easy-to-understand examples.
First, let’s get familiar with some vocabulary that will be necessary to understand the technical definition of osmosis.
- Solvation: the process of dissolving something in a liquid. When making hot chocolate, the powder dissolves into the water via the process of solvation.
- Solvent: the liquid that does the dissolving. In the above example, the water is acting as the solvent.
- Solute: the substance that is being dissolved. In the above example, the hot chocolate powder is acting as the solute.
- Semipermeable membrane: a filter of sorts that selectively lets some substances through, but does not allow others to pass. For example, a window screen is a type of semipermeable membrane. Air, water, and dust can all move through the screen, but a cat, coffee mug, or computer cannot pass through.
Okay, now we’re ready! As defined by Merriam-Webster, osmosis is the “movement of a solvent (such as water) through a semipermeable membrane… into a solution of higher solute concentration that tends to equalize the concentrations of solute on the two sides of the membrane.”
Take a look at the diagram below to help understand.
The pink liquid in the beaker exists on both sides of the semi-permeable membrane (the dotted red line). Notice that the concentration of the solute (the small spheres) is higher on the right-hand side than the left-hand side. There are about 7 spheres on the left-hand compared to about 30 on the right-hand side. The solvent will move from left (lower solute concentration) to right (higher concentration) in order to equalize the concentrations. Note that the solute particles do not move – only the liquid solvent.
Armed with a working definition, let’s examine some easy-to-understand examples of osmosis.
Raisin in Water
This is an experiment that is easy to try at home. Take a few raisins and place them into a cup of tap water. Come back 30 minutes later and what will you see? The raisins will have swollen to a larger size. Why? Osmosis. The water contains fewer solutes (like sodium, potassium, and magnesium) than the raisin. In response, the water moves from low solute concentration (the tap water) to high solute concentration (the raisin).
An oft-cited remedy for a sore throat is to gargle salt water. This can be very beneficial, and it is an example of osmosis. When you have a sore throat, the cells in the back of your throat are often swollen, filled with water. When you mix salt into tapwater, it creates a somewhat concentrated saline solution.
This solution has a higher concentration of solutes (salt) than the cells in the back of your throat. Thus the water moves from low solute concentration (your throat) to high solute concentration (the saltwater). This causes temporary relief from the swelling.
Applications of Osmosis
Osmosis is an important process both in nature and society. Let’s take a closer look at some of the applications and instances of osmosis around us.
Osmosis is an age-old technique used to preserve meats. Typically, the process is referred to as “brining.” Meat is cut into smaller pieces and then encased in a very highly concentrated saltwater solution.
Over the course of several days or weeks, the water inside the meat will move from low solute concentration (in the meat) to high solute concentration (in the saltwater solution). The loss of water makes the meat an inhospitable place for bacteria to grow, thus helping to preserve the meat. This was a common tactic for sea voyages before the invention of refrigeration.
Osmosis is what allows plants to survive. Specifically, the process of osmosis draws water from the roots of the plant to the leaves of the plant. This is accomplished by the usage of a salt gradient. The lowest part of the plant, the roots, has the least amount of salt. The highest part of the plant, the leaves, has the highest concentration of salt. This causes water to constantly be drawn upwards towards the leaves.
Plants also use osmosis for other purposes, as well. Specialized cells, called “guard cells”, grow and shrink in response to how much water the plant has. When the plant is engorged with water, the cells swell with water and create tiny openings in the leaf. This allows water to escape. When the plant is low on water, the cells shrink, causing that same opening to shut, thus conserving water.