The concentration of sodium ions in the extracellular fluid is usually substantially higher than in the intracellular fluid, due to the action of the sodium potassium pump. Therefore, there is a strong concentration gradient of sodium ions across the plasma membrane.

Because of this concentration gradient, the sodium ions in the extracellular fluid can be described as having 'potential energy'.

If there is a route for the sodium ions to diffuse back across the plasma membrane, this potential energy is converted into free kinetic energy.

Antiporter
During secondary active transport, some carrier proteins called antiporters use this mechanism to move sodium ions, and at the same time transport other substances in the opposite direction.

Calcium levels are usually higher outside the cell. A sodium/calcium antiporter uses the kinetic energy of sodium ions moving into the cell to transport calcium ions in the opposite direction, against their concentration gradient.

Similarly, levels of hydrogen ions can be higher outside the cell. A sodium/hydrogen ion antiporter uses the kinetic energy of the sodium ions moving into the cell to transport hydrogen ions out of the cell, against their concentration gradient.



Symporter
Other carrier proteins, called symporters, use a similar mechanism, but transport substances in the same direction as the sodium.

Levels of glucose may be higher inside the cell. A sodium/glucose symporter uses the kinetic energy of sodium ions moving into the cell to transport glucose molecules into the cell at the same time, against their concentration gradient.

Levels of amino acids may also be higher inside the cell. A sodium/amino acid symporter uses the kinetic energy of sodium ions moving into the cell to transport amino acids into the cell at the same time, against their concentration gradient.





All these mechanisms are types of secondary active transport because they rely on the electrochemical gradient of sodium ions established by primary active transport.

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