Osmosis
Osmosis is important in biological (living) systems, as biological membranes are semipermeable. In general, these membranes are impermeable to large  molecules such as ions, proteins, and polysaccharides. They are  permeable to non-polar or hydrophobic molecules like lipids and small molecules like oxygen, carbon dioxide, nitrogen, and nitric oxide. Permeability depends on solubility, charge, or chemistry, as well as solute size. Water molecules travel through the plasma membrane, vacuole or protoplast by diffusing across the phospholipid bilayer.