Two examples of active transport include the root hair cells in plants taking in mineral ions and humans taking in glucose through their intestines. In general terms, active transport refers to a substance moving from areas in which it has low concentration to an area with high concentration, and the substance is generally one that a cell needs for sustenance, like amino acids, ions or glucose.
When active transport consumes chemical energy, as from ATP (adenosine triphosphate), it is called primary active transport, but when it uses an electrochemical gradient instead, it is known as secondary active transport. In either event, cells must have energy in order for active transport to take place.
Before active transport can begin, specialized proteins have to recognize the needed substance and allow it to enter the cell. In the instance of secondary transport, the proteins use energy to force the substance across the membrane. In both primary and secondary transport, this is not a movement that would normally have taken place because the concentration of the substance on the other side of the membrane is higher or the membrane is not permeable. In order to move from low to high concentration, particles of a substance must have specific proteins in the membrane waiting to carry them across.