The two basic types of active transport that take place when molecules and other larger materials cross cell membranes are active transport using pumps and active transport using vesicles. Pumps are proteins that push or pull molecules through the cell membrane by making use of cellular energy which is stored in the form of adenosine triphosphate, or ATP. Vesicles are organelles that physically engulf large materials outside the cell to bring them inside, or they surround interior materials and then expel them.
The active transport process that takes place when vesicles expel materials from the cell is called exocytosis. The inverse process, by which vesicles bring materials into the cell, is called endocytosis.
The protein pumps that either pull or push molecules through the cell membrane require the energy obtained from ATP, which is the cell's energy currency, because the pumps are working against the normal dynamics of diffusion. Molecules in a solution normally pass through a permeable membrane when there is an unequal concentration of those molecules between the two sides. The flow is from the side with the higher concentration of solute to the side with the lesser concentration. This process, which is called diffusion, normally continues until the solute concentrations on either side of the permeable membrane are equal, or isotonic. The protein pumps, however, are working against that process, and energy is required to overcome it.
When it is in the best interests of the cell for solute molecules to cross the cell wall in the direction of the lesser solute concentration, the process is called osmosis. Because the cell is not required to expend any of its ATP energy currency during the diffusion-driven process of osmosis, this form of cross-membrane molecular transport is referred to as passive transport.