When the cell excretes sodium from within, against the natural tendency of matter, it is performing active transport.
In active transport the molecules move across a membrane from the region of lower concentration to a region of higher concentration using the cellular energy for the motion against the concentration gradient.
The cell excreting sodium from within is a part of the sodium potassium pump. In sodium-potassium pump the sodium ions are transported from inside the cell to outside actively although the concentration of sodium is higher outside the cell. The energy which is used to transport sodium from inside to outside and potassium from outside to inside both against the concentration gradient is obtained through the breakdown of the ATP molecule.
