The term transport is used to refer to systems that facilitate the movement of solutes from one side of a membrane to the other. A simplistic, but useful, view of a cell is that it functions like a manufacturing facility. Raw materials must be continuously transported into the facility while waste and finished products must be continuously transported out. Nutrients must enter the cell from outside, while waste products and substances manufactured in the cell must be transported out of the cell. Furthermore, there are processes such as electron transport and oxidative phosphorylation, for which membrane transport of materials is an essential part of the process. Therefore, specific transport systems are present that not only facilitate the flow of materials from one side of the membrane to the other but also regulate the flow. Transport systems can be subclassified into two broad categories.