Why is the process of simple diffusion sufficient for taking in food, exchange of gases and removal of waste in unicellular organism but not sufficient in multicellular organism

(Q.74)  Why is the process of simple diffusion sufficient for taking in food, exchange of gases and removal of waste in unicellular organism but not sufficient in multicellular organism?

(Ans) The process of simple diffusion is sufficient to carry different activities of life in unicellular organisms as in them, the entire surface of the organism is in contact of the environment.
In case of multicellular organisms all the cells are not in direct contact of the environment so simple diffusion will not work.