Final answer:
The mitochondrial outer membrane contains a) porins, integral proteins that form channels allowing molecules and ions to pass through, thus not being a significant barrier to permeability.
Step-by-step explanation:
The outer membrane of the mitochondrion is not a significant permeability barrier because it contains transmembrane proteins called porins. These proteins allow for the movement of molecules and ions through the polar phospholipid bilayer. Porins are a type of integral protein that create channels or pores in the membrane, facilitating the diffusion of certain small molecules and ions across the otherwise restrictive hydrophobic membrane.
Unlike receptors, kinases, and ligases, porins are not involved in signal transduction, enzymatic activity, or the linking of two molecules, respectively. Instead, porins primarily function to permit the passive movement of substances into and out of the mitochondrial matrix, thereby allowing the mitochondrion to maintain its essential functions.