Porin is one of the most ABUNDANT POLYPEPTIDES. It forms channels that allow for the diffusion of solutes. It is different from other 'membrane transport proteins', and large enough to allow passive diffusion.
Porins are proteins (beta barrel proteins) that form huge pores in the outer membranes of plastids, mitochondria and some bacteria (gram-negative bacteria, gram-positive bacteria) allowing molecules up to the size of small proteins to pass through.
