It is essential for every living cell to get food and oxygen. The survival of the organisms depends upon the availability of energy which a cell obtains by the oxidation of food. Food and digested materials are transported from the place of synthesis or absorption to different places where they are used. It is equally necessary for the organisms to get rid of metabolic wastes. These requirements are satisfied by a transport system which moves substances from where they are plentiful to where they are required. This is done by blood vascular system.
Once a compound is transported to a place where it is needed, it has to be brought inside the cells for its utilization. All cells are bounded by a thin, flexible and dynamic cell membrane or plasma membrane. If this compound happens to enter the cell, i.e. transported from the exterior of the cell to the interior by a simple process of diffusion, this kind of transportation across the membrane barrier is called passive transport. On the other hand, some compounds cannot make their entry into the cell on their own because of their being large-sized. If such compounds are useful, the cell may guide their entry or transportation across the plasma membrane even if it is against the law of simple diffusion. In this type of transport, there is an expenditure of cellular energy. This is called active transport.