The main types of cellular transport are passive transport and active transport. Passive transport occurs spontaneously through diffusion, which is the movement of chemicals across the cell membrane from regions of higher concentrations to lower ones. Active transport requires energy and involves special proteins called pumps and transporter proteins.
The cell membrane allows the passing of molecules of water and some small, uncharged molecules, such as oxygen and carbon dioxide. However, the membrane is impermeable to the positive metal ions of sodium, potassium and calcium, as well as negative ions, such as chlorine. The cell membrane contains specialized proteins that consume energy to actively transport these chemicals inside or outside the cell.