A protein channel across the outer membrane of a Gram-negative bacterium that allows the diffusion of molecules into the periplasm, which is located between the outer and inner membranes.

A trimeric transmembrane protein that forms channels in the bacterial outer membrane. Similar porins are found in the mitochondrial outer membrane.

Porins are proteins which 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 which are specific to different types of molecules. They are prevalent in the outer membrane of the mitochondria and Gram-negative bacteria.