<span>The Excretory System Which includes the: </span><span>Kidneys </span><span>Ureters </span><span>Urethra </span><span>Bladder </span><span>Skin And Lungs</span>
Cell membranes serve as barriers and gatekeepers. They are semi-permeable, which means that some molecules can diffuse across the lipid bilayer but others cannot.
