According to cell biology, molecular transport involves the movement of molecules into and out of cells across a cell membrane. Two different types of molecular transport exist: active transport and passive transport.
Active transport is transporting molecules from areas of low concentration to areas of high concentration. It requires energy to happen and occurs with the help of special proteins or vesicles in the membrane, which are fluid- or air-filled cavities. The process of active transport involving membrane vesicles is called endocytosis; it occurs when vesicles assist molecules moving into a cell. Exocytosis is the opposite of endocytosis and happen when molecules leave a cell.
Passive transport moves molecules from areas of high concentration to areas of low concentration. Unlike active transport, passive transport does not require energy to accomplish. It occurs via simple diffusion, osmosis or facilitated diffusion. Simple diffusion is the tendency of molecules to spread across an open space, as perfume does in a room. Osmosis is the movement of water across a permeable membrane. Facilitated diffusion is when a protein rapidly assists the movement but is otherwise similar to simple diffusion.