Final answer:
Organisms use oxygen to carry out their metabolic functions. Larger organisms have evolved specialized respiratory tissues to transport oxygen throughout their bodies.
Step-by-step explanation:
All aerobic organisms require oxygen to carry out their metabolic functions. As animal size increases, diffusion distances increase and the ratio of surface area to volume drops. In unicellular organisms, diffusion across the cell membrane is sufficient for supplying oxygen to the cell. Larger organisms had to evolve specialized respiratory tissues, such as gills, lungs, and respiratory passages accompanied by complex circulatory systems, to transport oxygen throughout their entire body.