Secondary Active Transport (Co-transport)


This illustration shows a membrane bilayer with two integral membrane proteins embedded in it. The first, a sodium-potassium pump, uses energy from A T P hydrolysis to pump three sodium ions out of the cell for every two potassium ions it pumps into the cell. The result is a high concentration of sodium outside the cell and a high concentration of potassium inside the cell. There is also a high concentration of amino acids outside the cell, and a low concentration inside. A sodium-amino acid co-transporter simultaneously transports sodium and the amino acid into the cell.
An electrochemical gradient, which primary active transport creates, can move other substances against their concentration gradients, a process scientists call co-transport or secondary active transport. (credit: modification of work by Mariana Ruiz Villareal)

OpenStax Biology 2e

Secondary active transport brings sodium ions, and possibly other compounds, into the cell. As sodium ion concentrations build outside of the plasma membrane because of the primary active transport process, this creates an electrochemical gradient. If a channel protein exists and is open, the sodium ions will pull through the membrane. This movement transports other substances that can attach themselves to the transport protein through the membrane. Many amino acids, as well as glucose, enter a cell this way. This secondary process also stores high-energy hydrogen ions in the mitochondria of plant and animal cells in order to produce ATP. The potential energy that accumulates in the stored hydrogen ions translates into kinetic energy as the ions surge through the channel protein ATP synthase, and that energy then converts ADP into ATP.

– What are a subcategory of membrane transport proteins that couple the favorable movement of one molecule with its concentration gradient and unfavorable movement of another molecule against its concentration gradient?

Secondary active transporters couple the free energy of the electrochemical potential of one solute to the transmembrane movement of another. As a basic mechanistic explanation for their transport function the model of alternating access was put forward more than 40 years ago, and has been supported by numerous kinetic, biochemical and biophysical studies. According to this model, the transporter exposes its substrate binding site(s) to one side of the membrane or the other during transport catalysis, requiring a substantial conformational change of the carrier protein.


Clark, M., Douglas, M., Choi, J. Biology 2e. Houston, Texas: OpenStax. Access for free at:


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