Final answer:
The process by which molecules move against their concentration gradient by using energy, such as ATP, is known as active transport.
Step-by-step explanation:
Molecules moving against their concentration gradient require energy, usually in the form of adenosine triphosphate (ATP). This process is known as active transport.
Unlike passive transport methods such as osmosis or facilitated diffusion, which require no energy input to move substances down their concentration gradient, active transport uses cellular energy to move substances from an area of lower concentration to an area of higher concentration.
It's akin to moving something really heavy that requires a lot of energy; similarly, active transport requires ATP to move molecules "uphill" or against a concentration gradient, employing membrane proteins that function like pumps.