Osmosis is an example of diffusion in which molecules tend to distribute themselves evenly in space. Unlike diffusion, which can take place in all mediums (gas, liquid, and solid), osmosis only occurs in liquids and (very often in) gases.
What Exactly is Osmosis?
Osmosis is a process in which solvents, such as water, pass from a solution of lower concentration to a solution of higher concentration through a semipermeable membrane. It’s a passive process, which means it happens without any expenditure of energy.
But what’s a semipermeable membrane? Well, it’s a kind of a barrier that allows some molecules or substances to pass through, but not others. For example, a plastic wrap allows water vapor and air to travel across it, but not food or water. Similarly, membranes of cells allow water and specific solutes (tiny molecules dissolved in a solvent) to pass through while blocking other solutes.
This process was first documented by French physicist Jean-Antoine Nollet in 1748. More than a century later, a German chemist invented highly selective precipitation membranes, advancing the art and technique for measuring osmotic flow.
How Does It Work?
Osmosis always tries to equalize the concentration on both sides of the membrane. Since solute cannot travel through the membrane, it’s the solvent (water) that has to move. The solution gets more stable as it gets closer to equilibrium. Thus, osmosis favors the laws of thermodynamics.
Types of Osmosis
Typically, there are two types of osmosis:
- Endosmosis: When a cell is placed in a solution that has a higher concentration of water than the cell, the solvent (like water) moves into the cell. This makes the cell become turgid or undergo deplasmolysis.
- Exosmosis: When a cell is placed in a solution that has a higher concentration of solute than the cell, the solvent moves out of the cell. This makes the cell become flaccid or undergo plasmolysis.
To better explain this phenomenon, we have listed a few very good examples of osmosis that we encounter in everyday life.
13. Raisin In Water
If you put a raisin into pure water, it swells up over time. This happens because raisin contains a higher concentration of sugar and other solutes than water, so water moves into the raisin cells by osmosis.
Since the solvent (pure water) enters into the raisin cell, this is an example of endosmosis. The same thing happens with a wilted carrot stick or limp.
12. Potato In Sugar Solution
If you put a potato into a sugar solution, it shrinks over time. This is because there is a much higher concentration of water in potato cells than the sugar solution, so water moves out of the potato through its membrane into the sugar solution.
Since solvent moves out of the potato cells and enters the sugar solution in an attempt to reach equilibrium, it’s an example of exosmosis.
11. Plants Absorb Water From The Soil
While plants absorb water through their whole surface (leaves, stems, and roots), the majority of the water is absorbed by root hairs. These root hairs act as a semipermeable barrier, allowing water molecules (solvent) to move from high concentration (soil) to low concentration (roots).
As a result, root hair cells become more turgid, and their osmotic pressure (tendency to take in solvents) drops.
The water molecules then move into tubes called xylem vessels to be transported to the leaves. Inside the xylem cells, water molecules exert a strong force on each other due to hydrogen bonding. When water evaporates through the stomata (tiny pores on leaves), more water is drawn up via root xylem cells to replace what’s been lost.
10. Salt on Slugs
Salt and slugs do not pair well. Have you ever wondered why does salt kills slugs and snails? The moist skin of a slug act as a semipermeable membrane. The high concentration of salt on the slug’s skin draws water out of its cells through osmosis.
The water comes out because this equalizes salt concentration between the outside and inside of the slug’s skin. Like most other living organisms, slugs need water to service. And when they lose too much water, they shrivel up and die.
9. Red Blood Cells Placed Into Freshwater
Red blood cells give the blood its characteristic color and carry oxygen from the lungs to the tissues. Mammalian red blood cells are small, round, and biconcave — they appear dumbbell-shaped in profile and have a semipermeable cell membrane.
When placed in freshwater, the water travels into the cells via osmosis, causing the cells to swell. This happens because the concentration of ions and other solute particles is higher inside the red blood cell than outside it.
The amount of water that can travel into the cells is controlled by the cell membrane’s pressure acting on the contents of the cell. In most cases, the cell consumes more water than its membrane can handle, which causes the cell to burst. This phenomenon is called hemolysis.
However, when red blood cells are placed in a solution with a higher solute concentration, water moves out of the cell. As a result, cells become smaller and crenated in shape.
8. Fish Absorb Water Through Their Skin and Gills
Type: Endosmosis or Exosmosis depending on the type of fish
If you place a saltwater or freshwater fish in the water with different salt concentrations, it will die due to the entry or exit of water in its cells.
The blood and bodily fluids of freshwater fish are much saltier than the water they swim in. Thus, water moves in through their gills. Similarly, fish that live in the ocean tend to lose water through their gills.
Just like human bodies, fish bodies need a specific concentration of salt to function best. They cannot withstand too much water flowing in or out through their gills. The freshwater fish would explode and the saltwater fish would wither.
However, this doesn’t happen because their gills contain specialized cells that selectively pump salt in or out of their blood. Freshwater fish’s cells regularly move salt in and saltwater fish’s cells regularly pump salt out. And since ocean water is so salty, fish pump out the excess salt through their gills as well as their kidneys.
7. Gargling With Saltwater Provide Relief From Sore Throat
Type: Exosmosis (excess fluid rushes out of throat tissues)
Saltwater doesn’t actually cure a sore throat, but it helps in reducing pain and discomfort. This is because saltwater contains a higher concentration of solute (salt) than what is present in the tissues of our throat.
More specifically, the saltwater’s osmotic pressure is greater than the pressure in the fluid of the surrounding cells. So, when we gargle, the excess fluid rushes out of our throat tissues, reducing swelling and easing out the pain.
6. Sugar On Strawberries
The outer membrane of the strawberry acts as a semipermeable layer between its interior and exterior. The interior already contains water and natural sugar. When you sprinkle sugar on the cut strawberries, the higher amount of sugar outside the fruit’s cells (combined with sugar’s ability to attract water) causes the water to move outside to the strawberry’s surface.
This process can be used to make delicious foods such as macerated strawberries, jellies, and jams. It can also be utilized to prolong the fruits’ shelf life.
5. Food Preservation
Type: Exosmosis (bacteria cells lose water)
The reason why we can enjoy jams and pickles for a long time without any fear of their spoilage is osmosis. They both are concentrated food products, loaded with high amounts of sugar (in case of jams), salts, oils, vinegar, and other spices (in case of pickles).
Not only do they act as taste enhancers but also as excellent preservatives by killing bacteria and preventing the growth of other harmful microorganisms.
The high concentration of sugar and salt is hypertonic to bacteria cells. Bacteria cells lose water due to higher concentrations outside and become less conductive to support the growth of microorganisms.
4. Digested Food Absorbed In Small and Large Intestines
Image credit: Cancer.gov
When you drink water or eat food, it moves from the mouth, down the esophagus (a 10-inch stretchy pipe), and into the stomach. Inside your stomach, the food breaks down into many small parts that are mixed with stomach liquids. The mixture forms a thick semifluid mass called chyme. When chyme moves into the small intestine, osmosis takes place.
Intestinal epithelial cells (that form the lining of intestines) have a lower concentration than the chyme. Thus, in order to reach equilibrium, solvent (water) travels into these cells through semipermeable membranes, taking some nutrients along with it.
There are capillaries near the epithelial cells. Both nutrients and water move through the capillaries’ cells and into the bloodstream.
3. Pathogenic Bacteria Interfere With Intestinal Cells
Cholera bacteria imaged by a scanning electron microscope
A few pathogenic bacteria, such as Vibrio cholerae, are capable of interfering with human intestinal cells’ ion transport channels. They produce enterotoxins that change the permeability of the epithelial cells of the intestinal wall by generating pores.
As the result of osmosis, the water and other fluid compounds flow out of the body leading to severe dehydration and diarrhea. These bacteria can hold on tight to the intestinal cells, while common bacteria that live in our stomach get washed away. Thus, cholera bacteria get enough region to grow and replicate. Clever tiny microbe, isn’t it?
2. Contact Lens-Induced Dry Eye (CLIDE)
Did you know why we put contact lenses in a saline solution? Why not pure water? This is because the saline solution for contact lenses contains the same concentration of saltwater as your eye.
When you keep your lenses inside the solution, they stay moist, soft, and comfortable. Otherwise, they tend to absorb moisture from the eye via osmosis as they lose water during wear.
1. Water Purification
Type: Reverse Osmosis
One of the most popular and cost-effective water filtration techniques is Reverse Osmosis (RO). As the name suggests, it is the process of osmosis in reverse — the solvent passes through a semipermeable membrane in the direction opposite to that of natural osmosis when subjected to a hydrostatic pressure greater than osmotic pressure.
In simple terms, reverse osmosis is a separation technique that uses pressure (greater than osmotic pressure) to force the solvent through a semipermeable membrane, which retains the solute (contaminants) on one side and allows the pure solvent (drinkable water) to pass to the other side.
This process is widely used to remove common pollutants from water, including lead pesticides, nitrates, fluoride, sulfates, arsenic, bacteria, and much more.