In biological systems, molecules and ions move across the plasma membrane out and into the cell. When there is a net movement of molecules from higher to lower concentration, it is referred to as diffusion. It does not require energy unlike in active transport of molecules and ions. Diffusion may be simple or facilitated. A facilitated diffusion requires a protein in the plasma membrane. A transmembrane or a carrier protein assists in the transport of molecules or ions based on the concentration gradient. Facilitated diffusion may occur differently depending on the coupling mechanisms. It may be through symport or antiport. In symport and antiport, the molecule is transported across the plasma membrane by coupling to the transport of another molecule. The difference in the two mechanisms lies on the direction of the transport. In symport, two different molecules move in the same direction whereas in antiport, in different directions. A uniport is the transport of only one molecule, without coupling to the transport of another molecule or ion. In uniport, the transport process makes use of a uniporter (i.e. an integral membrane protein, such as ion channel or carrier protein). The uniporter binds to one molecule at a time and transporting it with its concentration gradient.