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Sodium-potassium pump molecule. Computer model showing the structure of a molecule of Sodium-Potassium Adenosine Triphosphatase, or the sodium-potassium pump. This molecule is an enzyme located in the plasma membrane of cells in all animals. It is responsible for maintaining the high concentration of potassium ions and low concentration of sodium ions within the cell by 'pumping' these ions in opposite directions across the plasma membrane, against their concentration gradient. This process is a form of active transport. (KEYSTONE/SCIENCE PHOTO LIBRARY/LAGUNA DESIGN)