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.

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.

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.

active transport

the movement of materials across the membranes and epithelial layers of a cell by means of chemical activity that allows the cell to admit otherwise impermeable molecules against a concentration gradient. Expediting active transport are carrier molecules within the cell that bind and enclose themselves to incoming molecules. Active transport is the means by which the cell absorbs glucose and other substances needed to sustain life and health. Certain enzymes play a role in active transport, providing a chemical "pump" that typically uses adenosine triphosphate (ATP) to help move substances through the plasma membrane. Compare osmosis, passive transport.
enlarge picture
Active transport: the sodium-potassium pump

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.

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.

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.
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.

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.

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.

active

not passive.

active principle
the drugs or chemicals in a pharmaceutical preparation that exert an effect pharmacologically; as distinct from the inert fillers, wetting agents and other excipients also often included.
active site
that region of a protein, usually an enzyme, that binds to another molecule such as the substrate of the enzyme.
active transport
the movement of ions or molecules assisted by a carrier protein across the cell membranes and epithelial layers, usually against a concentration gradient, resulting directly from the expenditure of metabolic energy. For example, under normal circumstances more potassium ions are present within the cell and more sodium ions extracellularly. The process of maintaining these normal differences in electrolytic composition between the intracellular fluids is active transport. The process differs from simple diffusion or osmosis in that it requires the expenditure of metabolic energy.

transport

1. movement of materials in biological systems, particularly into and out of cells and across epithelial layers.
2. transport of animals, see transit, transportation.

active transport
see active transport.
transport death
death during transportation, e.g. porcine stress syndrome.
transport host
transport media
see transport medium.
membrane transport proteins
specific proteins associated with the plasma membrane of cells that are responsible for transferring solutes including ions, sugars, amino acids, nucleotides and many metabolites across cell membranes.
transport myopathy
see exertional rhabdomyolysis.
transport stress
stress imposed by lack of access to water and feed, physical exhaustion caused by standing for long periods, heat stress, aggression by other animals.
transport tetany
see transit tetany.
References in periodicals archive ?
Gastrointestinal magnesium absorption occurs via passive paracellular diffusion and active transport via apical membrane magnesium channels (TRPM-6/7) in enterocytes.
GABA is commonly inactivated after release into the synapse by active transport into the astrocyte glial cells that are closely associated with synapses.
Interpretation & conclusion: Passive mechanisms are enhanced by high calcium diet, while low calcium diet favours active transport.
The 21 papers are organized into sections dealing with the historical perspective; biosynthesis, metabolism, active transport, and existence; release; pharmacology; and neurotoxicology.
For example, while walk-to-school and other active transport programs hold much promise for increasing activity in many school settings, their effectiveness in rural areas may be limited (primarily due to poor sidewalk accessibility and large catchment areas).
Drugs with low lipid solubility due to ionization need an active transport system since ionized molecules do not penetrate membranes.
Other investigators have suggested a possible connection between the two because breast tissue, like the thyroid, concentrates iodine through membrane active transport.
Construction of the Tractebel Calypso Pipeline is scheduled to begin in 2005, with active transport of natural gas in 2007.
The cells contain a large number of mitochondria, which are necessary for active transport.
This is not merely a passive process of diffusion; there is an active transport mechanism that delivers the aminoglycoside across the membrane itself.
Currently, our social, economic and physical environment promotes the consumption of recreational and non-basic foods that are profitable, energy-dense and nutrient-depleted, and discourages active transport and other forms of physical activity.