 Dear students, in this topic we shall discuss the mechanisms of absorption with particular emphasis on simple diffusion and carrier mediated diffusion. Several transfer processes are used during absorption. The utilization of a particular mechanism is based on the type of molecule being absorbed. These mechanisms include simple diffusion, carrier mediated facilitated diffusion, core transport and counter transport mechanisms, active transport and endocytosis. In this topic we shall discuss simple diffusion and carrier mediated facilitated diffusion in detail. Simple diffusion takes place across the plasma membrane of epithelial cells. The diffusion occurs through either lipid bilayer or through the water filled pores. The lipid soluble substances for example monoglycerides, fatty acids and cholesterol diffuse through the lipid bilayer while water soluble non-electrolytes and alcohol diffuse through the water filled pores. Their net diffusion rate is proportional to the chemical concentration gradient. Certain electrolytes are also absorbed through diffusion. As they are charged particles, their net diffusion rate is proportional to the electrochemical gradient. These students, some larger or charged water soluble substances for example fructose and few amino acids cannot diffuse through water filled pores. So they are absorbed by carrier mediated transport which is a facilitated diffusion that occurs with the help of channel proteins.