Final answer:
Active transport is the movement of biological molecules against a concentration gradient and requires ATP for energy.
Step-by-step explanation:
The movement of biological molecules against a concentration gradient that requires the expenditure of ATP is known as active transport. During this process, substances are moved through a cell membrane from an area of lower concentration to higher concentration. Active transport is necessary when a cell needs to transport molecules against the direction that they would naturally move by diffusion. This process is 'active' because it uses cellular energy, typically in the form of ATP (adenosine triphosphate), to move substances 'uphill' against their concentration gradient.
Active transport is the movement of biological molecules against a concentration gradient, which requires the expenditure of ATP. Unlike diffusion and osmosis, which occur without the need for energy, active transport requires ATP to transport molecules or ions across a cell membrane from a lower concentration to a higher concentration. Examples of active transport include the sodium-potassium pump, which transports sodium out of cells and potassium into cells, and other membrane pumps that move substances against their concentration gradients.