Final answer:
Active transport is the energy-dependent movement of substances against their concentration gradient, from an area of lower concentration to one of higher concentration, requiring ATP.
Step-by-step explanation:
The best definition of active transport is the movement of molecules from an area of their lower concentration to an area of their higher concentration. This process is facilitated by membrane proteins and requires energy, usually in the form of adenosine triphosphate (ATP). Unlike passive transport where substances move along their concentration gradient without the use of energy, during active transport, substances move against their gradient. This is essential for maintaining concentrations of various substances inside a cell, such as ions. An example of active transport is the sodium-potassium pump which actively pumps sodium out of cells and potassium into cells.
It is different from passive transport, which does not require energy and moves molecules from an area of higher concentration to an area of lower concentration. An example of active transport is the sodium-potassium pump, which moves sodium out of cells and potassium into cells, against their concentration gradients.