Simple diffusion Molecules/ions are in constant state of random motion due to their thermal energy. >Simple diffusion occurs >whenever there is a concentration difference across the membrane >the membrane is permeable to the diffusing substance26 1. Simple Diffusion ➢Molecules/ions are in constant state of random motion due to their thermal energy. ➢Simple diffusion occurs ➢whenever there is a concentration difference across the membrane ➢the membrane is permeable to the diffusing substance