Osmosis is a special type of diffusion where only water molecules move across a semipermeable membrane—a barrier that lets water pass but blocks larger molecules like salts or sugars. Water moves from the side with more water and fewer solutes (low concentration of dissolved substances) to the side with less water and more solutes (high concentration of dissolved substances).

This continues until the water levels on both sides become balanced, or reach equilibrium. Osmosis does not require energy, so it's a form of passive transport.

Why osmosis is important:

In plants, osmosis helps absorb water from the soil into the roots.

In animals, osmosis keeps cells from shrinking or bursting by regulating water flow.

In medical treatments, like dialysis, osmosis helps remove waste from the blood.

Example:

If a cell is placed in fresh water, water enters the cell by osmosis and it swells.

If it's placed in salty water, water leaves the cell and it shrinks.