Osmosis
The plasma membrane of a cell is semi-permeable, which means it allows the entry of certain molecules in or out. It lets small molecules pass through, but blocks larger molecules,. The membrane also has ports or gateways which get certain macromolecules through. This is active transport, which uses energy and is selective. It is the outermost covering of the animal cell, made of proteins and lipids. Example: exchange of gases like oxygen and carbon dioxide.