Cellular diffusion is the process that causes molecules to move in and out of a cell. Molecules move from an area of high concentration to an area of low concentration. When there is a higher concentration of molecules outside of a cell, then more molecules enter the cell than leave. When there is a higher concentration of molecules inside of a cell, then more molecules leave the cell than enter.
When molecules are evenly distributed, then an equal number of molecules are entering and leaving the cell. This is called equilibrium.
It is possible to observe osmosis, or the diffusion of water across a cell membrane, by performing a simple experiment using two slices of potatoes, two glasses of water and table salt. Fill one glass with water, and place a slice of potato in the water. Fill another glass with water, and add a slice of potato and 2 tablespoons of table salt. Let the potato slices soak. Then observe the slices.
The potato slice in the salt water look smaller and feel mushier. This is because more water molecules left the potato's cells than entered them. The water molecules moved to where there was less concentration of water molecules (the salt water solution surrounding the potato).
The potato slice in water only looks bigger and feels firmer. This is because there is more salt and other dissolved chemicals (solutes) within the potato. The water molecules moved from the outside area where there is a higher concentration of water molecules to the potato's cells, where there is more solute and less concentration of water molecules.