Concept 5 Review

Active Transport

Most living cells maintain internal environments that are different from their extracellular environment, as well as concentration differences between the cytosol and internal compartments. In human tissues, for example, all cells have a higher concentration of Na+ outside the cell than inside, and a higher concentration of K+ inside the cell than outside. These concentration gradients of Na+ and K+ represent a form of energy storage, similar to a battery. An example of a concentration difference between the cytosol and an internal compartment is found in the lysosome, where the concentration of hydrogen ions (H+) can be 100 to 1000 times greater than the concentration outside, in the cytosol.

Like pushing an object uphill, moving a molecule against a concentration gradient requires energy. Cells have evolved active transport proteins that can use energy to establish and maintain concentration gradients.