Porins are beta barrel proteins that cross a cellular membrane and act as a pore through which molecules can diffuse. Unlike other membrane transport proteins, porins are large enough to allow passive diffusion, i.e., they act as channels that are specific to different types of molecules. They are present in the outer membrane of Gram-negative bacteria and some Gram-positive bacteria of the group Mycolata (mycolic acid-containing actinomycetes), the mitochondria, and the chloroplast.