If the concentration of molecule X is greater inside a cell than outside, and the cell acquires X from its surroundings, it must cross the membrane by means of active transport.

Here's why:

* Passive transport relies on the concentration gradient, meaning molecules move from an area of high concentration to an area of low concentration. Since X is more concentrated inside the cell, passive transport would move it *out* of the cell, not into it.

* Active transport requires energy to move molecules against their concentration gradient. This is necessary to bring X into the cell, where it is already more concentrated.

Examples of active transport mechanisms:

* Protein pumps: These proteins use energy (often from ATP) to move molecules across the membrane against their concentration gradient.

* Endocytosis: The cell membrane engulfs the molecule and brings it into the cell.

Let me know if you have any other questions!