A solution contains a solute, usually a solid, and a solvent, usually a liquid. In the case of a dye diffusing in water, dye is a solute and water is the solvent. Membrane chemical and physical properties allow only a few types of molecules to cross by diffusion. Gases readily diffuse through lipid bilayer. Water moves in and out of cells with ease, probably through channels.
Osmosis is the diffusion of water across a differentially permeable membrane.
The membrane does not permit passage of the solute. Although sugars and salts pass through membranes, differences in permeability between water and these solutes is so great that cells in sugar and salt solutions must deal with osmotic movement of water. Isotonic is where the relative solute concentration of two solutions are equal. Hypotonic is where a relative solute concentration of one solution is less than another solution. Hypertonic is where relative solute concentration of one solution is greater than another solution.