Simple diffusion processes
permit the absorption of non-ionic hydrophilic substances, e.g. urea, and anionic
substances with hydration radii of less than 2.9 Å, e.g. nitrates. Many lipophilic
compounds such as barbiturates, food additives and industrial solvents are absorbed by
passive diffusion. The term ‘facilitated diffusion process’ is used to describe some
absorption processes which act down a concentration gradient involving a carrier protein,
but do not have a requirement for metabolic energy. Cations are often absorbed by a
carrier-mediated transport system which is energy dependent.