Toxicants may move across biological membranes by all the following mechanisms except aqueous diffusion. This is because for the diffusion of the toxicant molecules, the aqueous pores are too small. Hence, the option a is the correct answer. Solution Toxicants may move across biological membranes by all the following mechanisms except aqueous diffusion. This is because for the diffusion of the toxicant molecules, the aqueous pores are too small. Hence, the option a is the correct answer..