Why do humans need a circulatory system to transport oxygen while single celled organisms like amoeba do not?
An amoeba cell eats other unicellular organisms like paramecium, bacteria, and plant cells. Some are even parasitic! Amoeba cells eat by moving its cytoplasm (also called pseudopods) around its prey. The prey is literally inside the amoeba now. The Amoeba forms a vacuole around it and breaks it down for nourishment and absorbs it through its cytoplasm.