Co-transport: Secondary Active Transport
Recall that several things have happened as a result of the primary active transport process. At this point, there are more sodium ions outside of the cell than inside and more potassium ions inside than out. For every three ions of sodium that move out, two ions of potassium move in. This results in the interior being slightly more negative relative to the exterior.
This difference in charge is important in creating the conditions necessary for the secondary process. The sodium-potassium pump is, therefore, an electrogenic pump (a pump that creates a charge imbalance), creating an electrical imbalance across the membrane and contributing to the membrane potential.
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 action of the primary active transport process, an electrochemical gradient is created. If a channel protein exists and is open, the sodium ions will be pulled through the membrane. This movement is used to transport other substances that can attach themselves to the transport protein through the membrane (see image below).
An electrochemical gradient, created by primary active transport, can move other substances against their concentration gradients. We refer to this process as co-transport or secondary active transport. Image Credit: modification of work by Mariana Ruiz Villareal
Many amino acids, as well as glucose, enter a cell this way. In fact, this secondary process is also used to store high-energy hydrogen ions in the mitochondria of plant and animal cells for the production of 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. Afterwards, that energy is used to convert ADP into ATP.
Video Summarizing Active Transport
Don’t worry if you did not understand much of the transport processes above and in the previous lessons. What’s more important is grasping the basic ideas. This video by FuseSchool summarizes active transport in cells.
Active transport works in the opposite direction to passive transport. Generally, it moves molecules from a low concentration to a high concentration, against the concentration gradient. This is the opposite of diffusion and osmosis. In fact, because it is not the natural direction, the cell requires energy to make this work.
Active transport is carried out by protein carriers. The protein carriers are within the cell membrane. In fact, they have a specific binding site for the exact molecule they are transporting. The substance binds to the protein carrier on the side that it is at low concentration. And using energy from respiration, the protein carrier releases the substance on the other side of the membrane—where it is already at a higher concentration.