Secondary active transport is a method cells use to move a substance against its concentration gradient, from an area of lower to higher concentration. Unlike primary active transport, this mechanism does not directly use cellular energy, such as ATP. Instead, it harnesses the potential energy stored in an electrochemical gradient established by a separate process. This form of transport couples the movement of one substance down its gradient to the movement of another substance up its own gradient.
Generating the Electrochemical Gradient
The electrochemical gradient is a combination of a chemical force, due to the unequal distribution of ions, and an electrical force, from the difference in charge across the membrane. A primary example of how this gradient is created is the action of the sodium-potassium (Na+/K+) pump. This pump is a protein that operates using primary active transport, meaning it consumes ATP directly for energy.
The Na+/K+ pump works continuously to move three sodium ions (Na+) out of the cell for every two potassium ions (K+) it moves into the cell. This action results in a higher concentration of sodium ions outside the cell. This steep concentration gradient for sodium represents a powerful source of stored energy.
The constant work of the Na+/K+ pump is analogous to charging a battery. By creating and maintaining this imbalance of sodium ions, the cell establishes a state of readiness. The tendency for these sodium ions to flow back into the cell down their gradient is the driving force harnessed by secondary active transport systems.
Types of Secondary Active Transport
These proteins bind to the ion moving down its gradient (e.g., sodium) and, at the same time, to the substance that needs to be moved against its gradient. The movement of the first ion releases the stored energy, which the protein then uses to change its shape and transport the second substance. This process occurs through two main mechanisms.
One type of secondary active transport is known as symport, or cotransport. In this arrangement, the transporter protein moves both the driving ion and the transported molecule in the same direction across the membrane. The downhill movement of the ion, such as sodium flowing into the cell, provides the necessary force to pull the other molecule along with it.
The other mechanism is called antiport, or countertransport. In this case, the transporter protein moves the two substances in opposite directions. As the driving ion moves into the cell down its established gradient, the protein uses that energy to actively push a different molecule out of the cell against its own gradient. The process can also work in reverse, with an ion moving out to power the import of another substance.
Physiological Significance and Examples
The principles of secondary active transport are fundamental to many processes in the body, from nutrient absorption to muscle function. These systems allow cells to accumulate necessary molecules and expel waste products by linking these tasks to the sodium gradient maintained by the Na+/K+ pump.
A specific example of a symporter is the sodium-glucose linked transporter, or SGLT1. This protein is located in the epithelial cells lining the small intestine and is responsible for absorbing glucose from digested food. The SGLT1 protein simultaneously binds to both sodium ions and glucose molecules. The strong electrochemical drive for sodium to enter the cell provides the energy to pull glucose inside as well, concentrating it within the intestinal cells before it passes into the bloodstream.
An example of an antiporter is the sodium-calcium exchanger (NCX), which is particularly important in cardiac muscle cells. For the heart muscle to relax after each contraction, calcium ions (Ca2+), which trigger the contraction, must be promptly removed from the cell’s interior. The NCX protein accomplishes this by allowing three sodium ions to rush into the cell down their steep gradient. This influx provides the energy to transport one calcium ion out of the cell against its concentration gradient, which allows the heart muscle to relax in preparation for the next heartbeat.