The movement of substances across cell membranes against their concentration gradient, requiring energy input (usually ATP). This process allows cells to maintain concentration differences and accumulate needed materials.
The term combines 'active' from Latin 'activus' meaning energetic or busy, with 'transport' from Latin 'transportare' meaning to carry across. The concept emerged in the 1940s when scientists realized some cellular transport required energy expenditure.
Active transport is like swimming upstream - it takes energy to move things where they don't naturally want to go! This is how your cells can concentrate essential nutrients inside while keeping toxins out, defying the natural tendency for everything to spread out evenly.
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