* Direct Diffusion: Single-celled organisms are small, meaning their surface area is relatively large compared to their volume. This allows gases like oxygen and carbon dioxide to easily diffuse across their cell membrane, directly into and out of the cell.
* Short Diffusion Distances: Because the cell is so small, the distance gases need to travel within the cell to reach all parts is very short. This makes diffusion efficient and fast.
In contrast, multicellular organisms are much larger. Their cells are located further from the external environment, so diffusion alone wouldn't be efficient enough for gas exchange. They have evolved complex structures like lungs, gills, or specialized respiratory systems to facilitate gas exchange across larger distances.
