Plasmid or vector transformation is the process by which exogenous DNA is transferred into the host cell. Transformation usually implies uptake of DNA into bacterial, yeast or plant cells, while transfection is a term usually reserved for mammalian cells. Typically the method for transformation of a DNA construct into a host cell is chemical transformation, electroporation or particle bombardment. In chemical transformation, cell are made competent (able to take up exogenous DNA) by treatment with divalent cations such as calcium chloride, which make the bacterial cell wall more permeable to DNA. Heat shock is used to temporarily form pores in the cell membrane, allowing transfer of the exogenous DNA into the cell. In electroporation, a short electrical pulse is used to make the bacterial cell temporarily permeable. Particle bombardment, is typically used for the transformation of plant cells. Gold or tungsten particles are coated with the DNA construct and physically forced into the cell by gene gun.