active transport

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active transport

 [ak´tiv trans´port]
the movement of ions or molecules across cell membranes and epithelial layers, usually against a concentration gradient, as a direct result of the expenditure of metabolic energy. For example, under normal circumstances more potassium ions are present within the cell and more sodium ions are present extracellularly. The process of maintaining these normal differences in electrolytic composition between the intracellular and extracellular fluids is active transport. The process differs from passive transport, simple diffusion, and osmosis in that it requires the expenditure of metabolic energy.
Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition. © 2003 by Saunders, an imprint of Elsevier, Inc. All rights reserved.

ac·tive trans·port

the passage of ions or molecules across a cell membrane, not by passive diffusion but by an energy-consuming process at the expense of catabolic processes proceeding within the cell; in active transport, movement takes place against an electrochemical gradient.
Farlex Partner Medical Dictionary © Farlex 2012

active transport

n.
The movement of a chemical substance through a gradient of concentration or electrical potential in the direction opposite to normal diffusion, requiring the expenditure of energy: active transport across a cell membrane.
The American Heritage® Medical Dictionary Copyright © 2007, 2004 by Houghton Mifflin Company. Published by Houghton Mifflin Company. All rights reserved.

Active Transport

The transportation across membranes by a membrane-bound protein complex of ions, nutrients or other molecules against a concentration gradient, which requires hydrolysis of high-energy phosphate bonds—e.g., ATP.
Segen's Medical Dictionary. © 2012 Farlex, Inc. All rights reserved.

ac·tive trans·port

(ak'tiv trans'pōrt)
The passage of ions or molecules across a cell membrane, not by passive diffusion but by an energy-consuming process against an electrochemical gradient.
Medical Dictionary for the Health Professions and Nursing © Farlex 2012

active transport

The movement of dissolved substances across a membrane in the direction opposite to that of normal diffusion. Active transport operates against gradients of chemical concentration, electrical charge or electrochemical state. It requires the expenditure of energy.
Collins Dictionary of Medicine © Robert M. Youngson 2004, 2005
Active transportclick for a larger image
Fig. 12 Active transport . Active transport across a membrane.

active transport

movement of a substance from a region of low concentration to another of higher concentration, i.e. against the CONCENTRATION GRADIENT. Such transport typically occurs in cell membranes, which are thought to contain carriers which move molecules from one side of the membrane to the other. Since these processes involve movement up a free-energy gradient, they require the expenditure of energy from the breakdown of ATP and are therefore sensitive to factors affecting metabolism (temperature, oxygen, pH, etc.). Compare DIFFUSION. See ACTIVE ABSORPTION.
Collins Dictionary of Biology, 3rd ed. © W. G. Hale, V. A. Saunders, J. P. Margham 2005

transport, active 

A process by which particles (e.g. ions, molecules) are transported across cell membranes, against, in almost all instances, the concentration gradient. It requires energy, which is provided by the metabolism of carbohydrates, proteins or lipids and cellular energy, which is obtained from splitting adenosine triphosphate (ATP).(Example: the sodium/ potassium pump that keeps sodium ions out of a cell and potassium ions in). When this process results in a compound being released, it is termed 'secretion'. This process is one of the mechanisms by which aqueous humour is produced in the ciliary body. See action potential; ultrafiltration.
Millodot: Dictionary of Optometry and Visual Science, 7th edition. © 2009 Butterworth-Heinemann

ac·tive trans·port

(ak'tiv trans'pōrt)
Passage of ions or molecules across a cell membrane by an energy-consuming process at the expense of catabolic processes proceeding within the cell.
Medical Dictionary for the Dental Professions © Farlex 2012