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Active transport is the movement of molecules across a cell membrane 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. ==Details== 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.〔(Active Transport Process ). Buzzle.com (2010-05-14). Retrieved on 2011-12-05.〕 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. In an ''antiporter,'' one substrate is transported in one direction across the membrane while another is cotransported in the opposite direction. In a symporter, two substrates are transported in the same direction across the membrane. Antiport and symport processes are associated with secondary active transport, meaning that one of the two substances is transported in the direction of its concentration gradient, utilizing the energy derived from the transport of such substance (mostly Na, K or H ions) down its concentration gradient. If substrate molecules are moving from areas of lower concentration to areas of higher concentration〔(Active Transport ). Biologycorner.com. Retrieved on 2011-12-05.〕 (i.e., in the opposite direction as, or ''against'' the concentration gradient), specific transmembrane carrier proteins are required. These proteins have receptors that bind to specific molecules (e.g., glucose) and transport them across the cell membrane. Because energy is required in this process, it is known as 'active' transport. Examples of active transport include the transportation of sodium out of the cell and potassium into the cell by the sodium-potassium pump. Active transport often takes place in the internal lining of the small intestine. Plants need to absorb mineral salts from the soil or other sources, but these salts exist in very dilute solution. Active transport enables these cells to take up salts from this dilute solution against the direction of the concentration gradient. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Active transport」の詳細全文を読む スポンサード リンク
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