from The American Heritage® Dictionary of the English Language, 5th Edition.
- noun 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.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
- noun transport of a substance (as a protein or drug) across a cell membrane against the concentration gradient; requires an expenditure of energy
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