Osmosis is the special type of diffusion that applies specifically to water. It involves the movement of water molecules from a region of higher water concentration (hypotonic solution) to a region of lower water concentration (hypertonic solution) through a semipermeable membrane. During osmosis, water selectively passes through the membrane, while solute particles cannot freely move across.

The semipermeable membrane in cells acts as a barrier, allowing water molecules to pass through while preventing the passage of dissolved solutes like salts, sugars, and proteins. This process of osmosis is crucial for maintaining water balance and regulating the concentration of substances inside and outside the cells. It ensures proper hydration, turgidity of plant cells, and plays a vital role in various biological processes, such as nutrient absorption and excretion.