Folded membranes improve the efficiency of a variety of processes that occur across a cell's membrane by increasing the surface area available for the proteins that manage those processes. Proteins facilitate respiration, the exchange of nutrients and waste. They also block harmful molecules from entering the cell.
Cell membranes are selectively permeable, only allowing free passage of particular useful molecules such as water. Diffusion, the movement of molecules through a barrier, occurs because of the tendency towards equalizing the environment on both sides of an obstacle. The rate of diffusion is the speed at which a substance can pass through a membrane.
Proteins can transport molecules that do not naturally diffuse across the membrane. Ion channel proteins embedded in the membrane allow specific substances to pass through a protein channel when they are opened by the cell. Transfer proteins bind to the blocked molecules and change shape to carry the molecules across the membrane. They then release the molecules and return to their original location on the outside of the membrane. This process of facilitated diffusion is used to transport glucose, sodium ions and chloride ions into the cell for use in metabolism.
In more advanced cells, the folded membrane can pinch off to create sacs called vacuoles through which larger particles can transfer in and out of the cell. Some vacuoles have evolved to create holding areas for harmful substances such as digestive acids.