2.49 understand why simple, unicellular organisms can rely on diffusion for movement of substances in and out of cells

Unicellular organisms can rely on diffusion more because they have a larger SA/V ration and its easier for substances to reach all areas of the organism by diffusion 

Complex organisms cannot rely on diffusion alone, as they evolved they had to develop special systems to increase the surface area available for the exchange of materials with their surroundings. These include:

-        Gut wall (absorption of food)

-        Air sacs (diffusion of gases)

-        Root system (absorption of water)