<P> Active transport is the movement of molecules across a membrane from a region of their lower concentration to a region of their higher concentration--in the direction against the concentration gradient or other obstructing factor (often a concentration gradient). </P> <P> 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 . </P> <P> In 1848, the German physiologist Emil Heinrich du Bois - Reymond suggested the possibility of active transport of substances across membranes . </P>

When do cells need to use active transport
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