Editing Endomembrane system (section)

=== Plasma membrane ===
{{main|Plasma membrane}}

The [[plasma membrane]] is a phospholipid bilayer membrane that separates the cell from its environment and regulates the transport of molecules and signals into and out of the cell. Embedded in the membrane are proteins that perform the functions of the plasma membrane. The plasma membrane is not a fixed or rigid structure, the molecules that compose the membrane are capable of lateral movement. This movement and the multiple components of the membrane are why it is referred to as a fluid mosaic. Smaller molecules such as carbon dioxide, water, and oxygen can pass through the plasma membrane freely by [[diffusion]] or [[osmosis]]. Larger molecules needed by the cell are assisted by proteins through [[active transport]].<ref name=Cooper7>{{cite web|url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?highlight=plasma%20membrane&rid=cooper.section.1967|title=Structure of the Plasma Membrane|access-date=2008-12-09|last=Cooper|first=Geoffrey| name-list-style = vanc |year=2000|work= The Cell: A Molecular Approach|publisher=Sinauer Associates, Inc}}</ref>

The plasma membrane of a cell has multiple functions. These include transporting nutrients into the cell, allowing waste to leave, preventing materials from entering the cell, averting needed materials from leaving the cell, maintaining the pH of the cytosol, and preserving the [[osmotic pressure]] of the cytosol. Transport proteins which allow some materials to pass through but not others are used for these functions. These proteins use ATP hydrolysis to pump materials against their concentration gradients.<ref name="Cooper7"/>

In addition to these universal functions, the plasma membrane has a more specific role in multicellular organisms. Glycoproteins on the membrane assist the cell in recognizing other cells, in order to exchange metabolites and form tissues. Other proteins on the plasma membrane allow attachment to the [[cytoskeleton]] and [[extracellular matrix]]; a function that maintains cell shape and fixes the location of membrane proteins. Enzymes that catalyze reactions are also found on the plasma membrane. Receptor proteins on the membrane have a shape that matches with a chemical messenger, resulting in various cellular responses.<ref name=Lodish3>{{cite web|url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?highlight=Cell%20Membrane&rid=mcb.section.1146#1168|title=Section 5.3. Biomembranes: Structural Organization and Basic Functions|access-date=2008-12-09|last=Lodish|first=Harvey| name-list-style = vanc |year=2000|work= Molecular Cell Biology|publisher=W. H. Freeman and Company|display-authors=etal}}</ref>