Describe the process of active transport and provide an example.

Active transport is the movement of molecules across a cell membrane against their concentration gradient (from a region of lower concentration to a region of higher concentration). This process requires energy, typically in the form of ATP, and involves specific transport proteins. An example is the sodium-potassium pump, which actively transports three sodium ions out of the cell and two potassium ions into the cell, maintaining electrochemical gradients essential for nerve impulses and other cellular functions.