Porins : These are proteins which form huge pores in outer membranes of plastids, mitochondria as well as certain bacteria allowing molecules of small proteins to pass through.
Role of diffusion : Some are small and specific; e.g., voltage gated potassium channel in plasma membrane of neurons. It permits K+ to diffuse into cytosol as part of a sequence of electrical events or a nerve impulse. Channels open and close according to voltage across a membrane