The plasma membrane acts as a barrier in cells of all types to control the movement of substances into and out of the cell. a) Explain how the plasma membrane acts as a semi-permeable barrier to various types of molecules based on its molecular structure. b) Describe, in detail, three ways in which proteins can cross the plasma membrane. NOTE: A complete answer will require some information from earlier in the course, especially the information on plasma membranes in the Prokaryotes topic.