Why does a mammal require a transport system where as a one celled organism suchas Amoeba does not?

The reason is our size.Because mammals are so large(increased distance from the nutrients and the cells requiring them), have a high metabolic rate and a high level of activity, we have high oxygen and nutrient requirements.We also produce a relatively large amount of waste that has to be removed- all this is achieved by our complex transport system.While the single celled creature is able to recieve/absorb oxygen and nutrients as well as get rid of waste products directly to the environment ocer the organisms cell membrane.