What mechanism of absorption involves moving substances against their concentration gradient with the help of another molecule?

The mechanism of absorption that involves moving substances against their concentration gradient with the help of another molecule is co-transport. Co-transport utilizes energy stored in the form of a concentration gradient of another substance, typically ions like sodium or protons, to drive the import or export of a second substance across the cell membrane.

In this process, when one substance moves down its concentration gradient, it provides the necessary energy for another substance to be transported against its gradient. For example, in the epithelial cells of the intestines, sodium ions are actively transported out of the cell into the bloodstream. The gradient created allows glucose, which is moving against its gradient, to be co-transported into the cell alongside sodium as it moves downhill.

This differs from passive diffusion, which relies solely on the concentration gradient without any additional energy input, and from facilitated diffusion, which also moves substances down their concentration gradient but does not involve coupling to another molecule. Endocytosis involves the engulfing of substances into cells but does not specifically refer to the movement against concentration gradients in the way co-transport does.