Active transport is the movement of
molecules across a
cell membrane from a region of their lower concentration to a region of their higher concentration in the direction against some
gradient or other obstructing factor (often a
concentration gradient). Unlike
passive transport, which uses the kinetic energy and natural
entropy of molecules moving down a gradient, active transport uses cellular energy to move them against a gradient, polar repulsion, or other resistance. Active transport is usually associated with accumulating high concentrations of molecules that the cell needs, such as
ions,
glucose and
amino acids. If the process uses chemical energy, such as from
adenosine triphosphate (ATP), it is termed primary active transport. Secondary active transport involves the use of an
electrochemical gradient. Examples of active transport include the uptake of glucose in the intestines in humans and the uptake of mineral ions into
root hair cells of plants.