Plants can be the source if the insect feeds on a plant that has taken up metal from its surroundings. For aquatic organisms the primary source of metals is an environmental source and the secondary a food source. Environmental sources include sediment release, and metal accumulation from an external source such as dumping of material into or near the water source. However not all the metal that an organism is exposed to is taken up by the animal. There are a number of ways in which a metal is believed to be taken up by a cell.
Simple diffusion is due to an electrochemical gradient that exists across the membrane of a cell.
The movement of ions across the membrane occurs through pores of channels in the membrane. Cadmium appears to mimic calcium and can pass through calcium channels. Facilitated diffusion involves the use of membrane carriers that bind to and transport metal ions across the membrane and release the metal on the other side. The mechanism appears to be the route by which zinc and copper are commonly taken up. Endocytosis appears the route of uptake for iron and lead.