Diffusion is the process in which particles move from an area of high concentration to an area of low concentration. It is driven by the tendency of particles to spread out and become evenly distributed in a given space.
The cell membrane is a thin layer that surrounds the cell. It is partially permeable, which means that it allows certain substances to pass through while blocking others.
A cell has a lower concentration of a substance when there are fewer particles of that substance inside the cell than outside. For example, if there are more water molecules outside a cell than inside, the cell has a lower concentration of water.
For diffusion to take place, there must be a difference in concentration (a concentration gradient). In this example, there was initially a higher concentration of solute outside the cell. So, the particles move from the outside to the inside of the cell.
As the particles continue to move, the concentration of the substance will become equal both inside and outside the cell. When this occurs, there is no longer a concentration gradient.
Diffusion will still take place when there is a small concentration gradient; however, it will occur at a slower rate.
Below are some examples of diffusion in biology:
In plants, root hair cells have a lower concentration of water inside the cell than in the soil (where there is a high concentration of water). Water molecules will move by diffusion, from the high concentration outside the cell, to the low concentration inside the cell.