Imagine trying to fill a balloon with air using only your breath. When the balloon is small and empty, it’s easy. But as it inflates, the air pressure inside becomes higher than outside. To add more air, you must exert force—pushing against that pressure. Your cells face a similar challenge every second. They often need to move substances into or out of a space where those substances are already concentrated. To solve this, cells rely on a vital mechanism known as active transport . What is Active Transport? In biology, active transport is the movement of molecules or ions across a cell membrane against their concentration gradient—from an area of low concentration to an area of high concentration.