Do molecules crossing a membrane using active transport with or against the concentration gradient?

Against the concentration gradient.

Explanation:

Active transport implies the movement of molecules across a membrane from a lower concentration to a higher concentration, that is to say, against the concentration gradient. The molecules requires energy to go against the concentration gradient, which is usually coming from ATP or an electrochemical gradient.

In passive transport, the molecules move across a membrane down a concentration gradient without the need for energy.

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