Secondary active transport

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.

In 1848, the German physiologist Emil Heinrich du Bois-Reymond suggested the possibility of active transport of substances across membranes.

Rosenberg (1948) formulated the concept of active transport based on energetic considerations, but later it would be redefined.

Specialized transmembrane proteins recognize the substance and allow it to move across the membrane when it otherwise would not, either because the phospholipid bilayer of the membrane is impermeable to the substance moved or because the substance is moved against the direction of its concentration gradient. There are two forms of active transport, primary active transport and secondary active transport. In primary active transport, the proteins involved are pumps that normally use the chemical energy in the form of ATP. Secondary active transport, however, makes use of potential energy, which are usually derived through exploitation of an electrochemical gradient. This involves pore-forming proteins that form channels across the cell membrane. The difference between passive transport and active transport is active transport requires energy and moves substances against their respective concentration gradient, whereas passive transport requires no energy and moves substances in the direction of their respective concentration gradient.

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