Explain why a large multicellular organism cannot rely solely on diffusion across its outer body surface to meet its oxygen demands, whereas a single-celled organism such as Amoeba can.

Large multicellular organisms have a smaller surface area to volume ratio than small organisms, so the amount of oxygen that can diffuse in per unit volume is limited.
The volume of respiring tissue is too great relative to the surface area available for diffusion, meaning the metabolic demand cannot be met.
Diffusion distance from the outer surface to inner cells is too great, so oxygen cannot reach all cells quickly enough by diffusion alone.
In contrast, a single‑cell organism such as an amoeba has a high surface area to volume ratio and all parts of the cell are close to the external surface, so diffusion distance is short and enough oxygen can diffuse in to meet its demand.