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The difference between facilitated diffusion and active transport is that facilitated distribution occurs down a concentration gradient and active transport occurs against a concentration gradient. I understand that, as a result of these differences, active transport uses ATP so that it can bind to the carrier protein, however I don't know what is meant by going against a concentration gradient or down a concentration gradient.

In short, I would like to know the difference between going against and down a concentration gradient. A definition of what a concentration gradient is might clarify things a little further as well, since the definition I have doesn't help much.

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"Concentration" is "how much stuff is there someplace?" "Concentration gradient" is "how much is concentration changing from point A to point B?"

Imagine a terrain where concentration is represented by height.

"Down" a concentration gradient means from high concentration to low concentration - this is the direction things would move via simple diffusion.

"Up" or "against" a concentration gradient means from low concentration to high concentration. Moving in this direction requires some sort of energy input, such as active transport using ATP, or an electrical gradient, or something else.

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