The Endomembrane System: The Plasma Membrane:
The plasma membrane surrounds the cell and separates it from the external environment. The membrane is
selectively permeable, meaning that it allows some substances to cross but not others. In this way, the plasma membrane controls what substances can get into and out of the cell.
The plasma membrane is mostly a phospholipid bilayer, of course, with various
inclusions. The “inclusions” are molecules other than phospholipids that are embedded in the plasma membrane. These include cholesterol molecules, which help to strengthen and stabilize the membrane, and protein molecules, which help determine the functional properties of the membrane.
The basic structure of a cell’s plasma membrane. Cholesterol helps
to strengthen the membrane, and protein inclusions are important
in determining the functionality of the plasma membrane.