Answer:
cell membrane
Explanation:
Single cell organisms like the bacteria depend mostly on diffusion for gaseous exchange. They take in food that does not require digestion. The cell membrane is selectively permeable and allows absorption of nutrients like glucose that have molecules of small size.
In multicellular organisms like humans, oxygen is absorbed by the endothelium of the capillaries into the blood. Multicellular organisms contain organs that have inner membrane enhancing the absorption.
