passive diffusion A few substances, such as glycerol, can cross the plasma membrane by passive diffusion. Passive diffusion is the process in which molecules move from a region of higher concentration to one of lower concentration as a result of random thermal agitationA few substances, such as glycerol, can cross the plasma membrane by passive diffusion. Passive diffusion is the process in which molecules move from a region of higher concentration to one of lower concentration as a result of random thermal agitation. passive diffusion