Interior of a cell must be protected from certain toxic compounds, and metabolites must be taken in and waste products removed. Same as brownian motion of molecules in fluids (i. e. , molecular diffusion: results in an equilibrium between concentrations on exterior and interior spaces. P (permeability coefficient) will increase with increasing solubility of the substance in the lipid phase. Non-mediated diffusion tends to increase as the hydrophobic character of the substance increases. For ions and other hydrophilic substances, non-mediated diffusion through lipid membranes is extremely low. The rate of non-mediated transport across lipid membranes is faster for nonpolar substances. Rapid transport of water is achieved by membrane-spanning channels called aquaporins. Three general types of facilitated transport occur by: transport through pores or channels formed by transmembrane proteins, transport by carrier molecules, and transport by permeases (can all include the rate of diffusion across a membrane) Facilitated transport can be passive or active.