Concept:Free-living protozoans exchange respiratory gases directly through their exposed body surface.
Explanation:Protozoans are unicellular organisms that lack specialized respiratory organs such as gills, spiracles, or a buccal cavity.
The thin, moist body surface acts as the respiratory membrane.
Dissolved oxygen (
O2) from the surrounding water enters the body by simple diffusion through this surface.
Carbon dioxide (
CO2) produced inside the cell diffuses outward through the same membrane.
Since the entire body surface is always in contact with the environment, it serves as the most efficient structure for gas exchange in free-living protozoans.
Although a cytochrome system exists for cellular respiration, no special breathing organelle is present.
Answer:A. body surface