The major difference between active and passive transport is that passive transport does not require the application of external energy, while active transport does. Both processes move molecules across the cell membrane, but passive transport always takes advantage of existing concentration gradients to allow a natural diffusion of molecules.
Unlike passive transport mechanisms, active transport can work against a concentration gradient to distribute necessary molecules in a way that helps to maintain the chemical balance of the cell. Passive transportation may be as simple as diffusion through the bilayer of the cell membrane, or it can take the form of pressure-facilitated osmosis. Sometimes, as is the case with the cell's calcium pump, special transport proteins can work to facilitate absorption and secretion without the direct application of energy from adenosine triphosphate, or ATP, molecules.
Active transport requires energy to do work. The process is used to transport molecules against a concentration or pressure gradient and consumes ATP. Active transport is responsible for the exchange of glucose in the cells of the small intestine and maintaining the balance of ions in brain cells, which allows them to efficiently conduct electrical impulses. The sodium pump is another example of primary active transport.