Cells exist in a watery environment
Many substances may be "dissolved" in the water.
Some of those substances are impermeable to the cell--meaning they cannot cross the plasma membrane
These impermeable solutes are able to "get in between" the water molecules and push them a little away from each other, thereby decreasing the concentration (not the volume) of the water.
Diffusion is the tendencies of molecules to move from a higher concentration to a lower concentration.
Since these impermeable molecules cannot enter the cell and water freely diffuses across the membrane the concentration of water changes outside of the cell, but the concentration inside remains the same (because those solutes cannot enter the cell).
There is now a difference in concentration of water outside of the cell versus inside of the cell.
Osmosis is the diffusion of water across a membrane.
Because there is now a lower concentration of water outside the cell as compared to inside of the cell, water diffuses from the inside of the cell into the extracellular space and due to the loss of water the cell may shrivel or crenate.
A solution that has an increased concentration of non-permeable solutes as compared to the cell is called a hypertonic solution.
A solution that has a decreased concentration of non-permeable solutes as compared to the cell is called a hypotonic solution.
This solution has an opposite effect on the water molecules, allowing them to become more concentrated as compared to inside of the cell.
Now based on the created concentration gradient, water will diffuse from the extracellular space into the cell, causing the cell to swell due to the increased water content.
In an isotonic solution the concentration of non-permeable solutes inside and outside of the cell are the same. Therefore water freely diffuses in a balanced manner with no demonstrable change in the cell shape or size.
Tonicity is the ability of a solution to change the shape or tone of a cell by altering its internal water content.
