WHAT IS A CELL MEMBRANE? The cell membrane, also known as the cytoplasmic membrane or plasma membrane, is a semipermeable membrane surrounding the cytoplasm of a cell (not to be confused with the cell wall, only present in plant cells). Cell membranes are made up of proteins and phospholipids and are about 8 nanometers thick. Being semipermeable means that it allows certain substances to pass through it but not others, especially allowing the passage of a solvent but not of certain solutes.

LIPID BYLAYER OF A CELL MEMBRANE Scientists use the fluid mosaic model (image to the left) to show the organization of proteins and phospholipids in a cell membrane. The model shows that phospholipid molecules are shaped with a head and tail region, the head is hydrophilic (meaning it likes water) and the tail is hydrophobic (meaning it does not like water). This feature makes the molecules amphipathic. Since the tails want to avoid water they tend to stick together and let the heads face the aqueous areas inside and outside the cell. This organization is spontaneous, meaning it is a natural process and does not require energy. This ‘lipid bilayer’ is impermeable, as it does not allow molecules to freely pass across it. Only water and gasses can pass through the bilayer, large molecules and small polar molecules cannot, and thus the cell membrane. Many proteins float in the lipid bilayer, some are permanently attached whilst others are only attached temporarily. Some proteins are only attached to the inner or outer layer of the membrane while the transmembrane proteins pass through the entire cell membrane. The transmembrane proteins that cross the bilayer are very important in the active transportation of ions and small molecules.

FUNCTION OF THE CELL MEMBRANE The cell membrane is the boundary between a living cell and its outside environment. It is also responsible for regulating what molecules pass into and out of the cell. All cells take in nutrients and expel waste, which all must pass through the cell membrane. Transport may occur by diffusion and osmosis across the membrane. It can also occur when a vesicle attaches to the cell membrane from inside and then opens to form a pocket, expelling its contents to the outside, this is called exocytosis. Another way of transportation is endocytosis or phagocytosis (same thing) in which the cell membrane envelops something on the outside and surrounds it, taking it inside the cell. Some molecules move across a membrane from a region of low concentration to a region of high concentration, and this requires a source of energy to “pump” the molecules uphill in concentration, this is called active transport.