Osmosis is the random but directional movement of free water molecules from places where there are many of them to places where there are fewer. Free water molecules are free the move around, as they are not busy separating salt ions or other molecules. Table salt dissolves in water because water molecules surround and separate the salt ions, preventing them from recombining into a solid crystal. The movement of free water molecules into and out of a cell can dramatically change its shape.
The Tonicity of a Solution
Tonicity can be a confusing topic for biology students. It refers to the saltiness of a liquid that is outside of a cell. The confusion comes from the fact that the cell also has salt inside of it. There are three conditions of tonicity: hypertonic, isotonic and hypotonic. It helps to keep in mind that each condition describes the saltiness of the liquid outside of the cell in relation to the liquid inside the cell. A tonic is a medicine that someone drinks to make themselves feel better. So, an easy way of remembering that hypertonic, isotonic and hypotonic refer to the tonic and not the cell is to picture yourself holding a cup of tonic medicine -- it is outside of your body, just like tonics are outside of a cell.
An isotonic liquid has the same amount of salt in it as a cell that is submerged in the liquid. Since there is the same amount of salt outside the cell as inside the cell, the same amount of free water molecules flow into the cell as they do out of the cell. The result is that the cell does not change shape. The isotonic condition is said to have no net movement of water in or out of the cell, but this does not mean that water molecules stop moving in and out -- they are constantly moving in and out. A mnemonic to remember that isotonic solutions don’t change the shape of a cell is that the word isotonic has an “s,” which stands for “same” -- as in, same shape.
A hypertonic liquid has more salt in it than is in a cell. Since free water molecules love surrounding and separating salt crystals, water from inside the cell will flow out. There will be a net movement of water out of the cell, because the free water molecules that leave the cell will become “busy” water molecules that surround salts on the outside of the cell. Their "busyness" prevents them from going back into the cell. Hypertonic solutions cause a cell to change shape by shriveling. A mnemonic to remember that hypertonic solutions shrink cells is that the word hypertonic contains an “e,” which is like an “o” that has shriveled.
A hypotonic liquid has less salt in it than is inside of a cell. In this condition, free water molecules will move from the hypotonic liquid into the cell. Once inside, the free water molecules will meet salt molecules and become busy surrounding them. This busyness keeps the water molecules from leaving the cell. Thus, there is a net movement, or influx, of water molecules into the cell. This causes the cell to swell or perhaps burst. A mnemonic to remember that hypotonic solutions cause cells to swell is that “hypo” is spelled with an “o,” which resembles the round shape of a swollen cell.