Osmosis is a biophysical phenomenon in which water (or another solvent) moves from a less concentrated solution to a more concentrated solution through a partially permeable membrane (in other words, it lets some particles pass, while blocking others).
The solvent will maintain this migration until equilibrium in concentration is reached.
So whenever there’s a net migration of the water molecules from a solution that has a low solute concentration towards one that has a higher solute concentration, we call this phenomenon osmosis. This movement is also sometimes referred to as “down the concentration gradient”.
Osmotic pressure is the force required to prevent water movement across the semipermeable membrane.
The term osmosis, which is Greek for ‘thrust’ or ‘impulse’, was first coined by J.A. Nollet, who in 1747 described an experiment in which he used an animal bladder to separate two chambers containing water and wine. He noticed that the volume in the chamber containing wine increased and, if the chamber was closed, pressure rose.
How osmosis works
A classic experiment for osmosis involves splitting a beaker of water into two halves, with a semipermeable membrane in between and salt added to one of the sides. You’ll soon notice water migrating from the side of the beaker with no salt at all to the side with the saline solution. This movement of water will continue until the concentration of salt is the same on both sides.
It’s the same reason why you should never put a snail near salt, which would cause the poor creature to die as its water is extracted.
Key to osmosis is the presence of a semipermeable membrane that makes it more likely for water molecules in a low concentration solution to collide with the membrane and pass through, whereas water molecules in a concentrated solution will have far fewer molecules of water colliding with the membrane and passing through. This mismatch means that there’s a greater statistical probability of more water molecules passing through the membrane from a less concentrated solution. Once the statistical probability of water molecules passing through the membrane is equal, osmosis stops.
Osmosis in nature
Osmosis is one of the essential processes of life. Each cell of our body, plants, and animals around us owe their survival to osmosis.
Take plants, for instance. When we water them, we pour it on the stem end and soil. If the plant’s cells are surrounded by a solution that contains a higher concentration of water molecules than the solution inside the cells, water will enter the leaves, fruits, and flowers by osmosis. During this process, the plant cell will become firm.
However, if a plant is surrounded by a solution that contains a lower concentration of water, then the water molecules of the solution inside the plant’s cells will be expelled by osmosis, turning the plant flaccid.
When we water plants, we usually water the stem end and soil in which they are growing. Hence, the roots of the plants absorb water and from the roots, water travels to different parts of the plants; be it leaves, fruits or flowers. Every root acts as a semipermeable barrier, which allows water molecules to transfer from high concentration (soil) to low concentration (roots). Roots have hair, which increases surface area and hence the water intake by the plants.
Perhaps a more relatable example is within our own bodies. When we drink water, cells absorb it by osmosis just like plant roots. The cell wall acts as a semipermeable membrane, creating osmotic pressure between the inside and outside of the cell. Blood is a more dilute solution than the cell’s cytoplasm, so water will cross through the cell wall. The same applies for nutrients and minerals, which are also transferred by osmosis.
Humans have recognized the potential of osmosis since antiquity, employing it to preserve foods. The ancients observed that adding salt or sugar removes water from tissues. At the time, the process was called imbibition due to the fact that solutes like salt and sugar attracted the water from the material they touched.
What’s the difference between osmosis and diffusion
Diffusion and osmosis are both passive transport processes, meaning they require no energy input to move substances. Both processes are essential to the proper functioning of biological processes such as the transport of water or nutrients between cells.
The main difference between the two is that diffusion can occur in any mixture, even when two solutions aren’t separated by a semipermeable membrane, whereas osmosis exclusively occurs across a semipermeable membrane.
Diffusion makes air composition uniform by redistributing chemical species, such as oxygen in the air, until equilibrium is reached: in other words, until the concentration gradient — the difference in concentration between two areas — has been eliminated. If the concentration of a species is not initially uniform, over time, diffusion will cause a mass transfer in favor of a more uniform concentration.
Bottom line: osmosis — the natural movement of water into a solution through a semipermeable membrane — is central to all of biology. It is a passive transport process like diffusion, but the two are distinct.
Tibi is a science journalist and co-founder of ZME Science. He writes mainly about emerging tech, physics, climate, and space. In his spare time, Tibi likes to make weird music on his computer and groom felines.