The resources that animal cells requrie, like nutrients and oxygen, enter the cytoplasm by crossing the plasma membrane. Metabolic byproducts, like __, exit the cell by crossing the same membrane. In unicell organisns, exchange occurs directlly with the external environment. For most multi cell orgaisms, however, direct exchange between every cell and the environment is not possible. Instead, tehse organisms rely on specialized systems that carry out exchange witht eh environment and that transport materials between sites of exchange and the rest of the body.