• hold the cell together, keeping its parts in place.
  
• are partially porous, helping to keep water and its dissolved chemicals inside the cell so they can do their work. Proteins in the membrane create adjustable holes (pores) so the cell can make adjustments in chemical composition - as they do when generating electrical currents.

Membrane Proteins Do Two Things

• Send signals. These proteins provide a way for communication between substances on the outside with target molecules on the inside.

• Transport.  Like trucks, these proteins carry other molecules. They move molecules from one side of the membrane to the other side, even against a high concentration of similar molecules. This is like breaking the "laws of diffusion." Think of it like driving a truck up a steep hill.