All the material that the cell gets from the environment or sends to the environment has to go through the cell or plasma membrane. The membrane is basically a phospholipid bi-layer with various proteins embedded into it. What's key to realize is that the cell membrane is semi permeable, that is some materials can get through the membrane while others can't.
Transport mechanisms fall into two catagories passive and active. Passive transport mechanisms do not require the cell to do work for the substance to enter or leave the cell. Instead the energy involved comes from the kinetic energy of the molecules in solution. Active transport mechanisms involve the cell to use cellular energy usually in the form of ATP to power special protein pumps to bring material into the cell.
The passive transport mechanisms and the protein pump mechanisms involve movement of substances as single molecules across the membrane. The "bulk" flow mechanisms endocytosis and exocytosis enable the cell to take in very large packages of molecules...say a food item from the environment. Many books treat these bulk flow mechanisms as seperate from active transport for that reason.
Cell Transport Mechanisms
