Khan Academy
Khan Academy: Osmosis and Tonicity
Provides an overview of osmosis and tonicity and looks at how osmosis works, as well as the role it plays in the water balance of cells. Describes hypertonic, isotonic, and hypotonic solutions and their effect on cells.
Biology Pages
Kimball's Biology Pages: Transport Across Cell Membranes
This site has information on how facilitated diffusion of ions takes place through proteins, or assemblies of proteins, embedded in the plasma membrane. Includes a helpful diagram.