The primary function of molecular membranes is the transport of ions and molecules into and out of cells. The movement of molecules from areas of low concentration to those of high concentration against chemical gradients is best described as

Active Transport

Explanation:

A cell uses energy for the movement of substances against a concentration gradient or an electrochemical gradient that maintains a balanced concentration of ions and molecules in living cells. This transport of substances is known as Active transport mechanisms which are the basic function of the cell membranes. The energy used is in the form of ATP.  Active transport occurs when there is a difference in the concentration of particles on both sides.