Active transport is a biological process that moves molecules across a cell membrane against their concentration gradient. This means that substances are transported from areas of lower concentration to areas of higher concentration, which requires energy. The energy for active transport typically comes from ATP (adenosine triphosphate), a molecule that stores and provides energy for various cellular functions. There are two main types of active transport: primary and secondary. Primary active transport directly uses ATP to pump ions or molecules, while secondary active transport relies on the energy created by the movement of other substances. Both types are essential for maintaining cellular functions and homeostasis.