Final answer:
Active transport is the energy-requiring method of moving particles across a cell membrane against their concentration gradient, often using ATP as the source of energy.
Step-by-step explanation:
The method of movement of particles across a cell membrane which requires energy is known as active transport. Unlike passive transport, which does not necessitate energy and allows molecules to move down a concentration gradient, active transport is needed when the movement of substances is against their concentration gradient, or when it involves the movement of large molecules or particles. The cellular energy used in active transport typically comes in the form of ATP (adenosine triphosphate), which is produced by the cell through various metabolic processes.
Examples of active transport include the sodium-potassium pump, which moves sodium and potassium ions against their respective concentration gradients, and vesicle transport, used for the movement of large particles and macromolecules. In essence, active transport allows cells to maintain a specific concentration of ions and molecules that would not occur naturally due to concentration differences.