The function of the membranes is complex and multifaceted, and can be divided into eight general categories. First, membranes compartmentalize, providing continuous, relatively unbroken sheets. Second, membranes prevent unrestricted exchange of molecules, providing a selectively permeable barrier. Third, membranes contain the machinery for the physical transport of substances from one side of the membrane to the other. Fourth, the membrane is involved in response of the cell to external stimuli, via signal transduction. Fifth, membranes allow cells to recognize one another, to adhere, and to exchange materials in an intracellular interaction. Sixth, membranes provide a means to organize cellular biochemical activities, through an extensive framework or scaffolding within which components can be arranged for effective interaction. Seventh, membranes maintain cell polarity. Eighth, membranes are involved in the process of energy transduction (i.e. conversion of one type of energy to another), such as the transfer of chemical energy from carbohydrates and fats to ATP.