Porins are pore-like structures that help in the diffusion of molecules.
Porins act as a channel, they are specific to different molecules
They allow large molecules to pass through them.
They are hydrophilic channels that are lined by proteins.
They are located in the outer membrane of plastids, mitochondria, etc.
They allow the passage of molecules ranging from smaller to larger.
Final Answer
Thus the porins are channels that allow the passage of small to large molecules as per the diffusion gradient.