The cell membrane is comprised of phospholipids and proteins. The phospholipids are oriented so that their hydrophilic, polar heads face outwards and their hydrophobic, non-polar tails face inwards toward the middle of the cell. Proteins dot the cell membrane to allow solutes to be transported in and out of the cell membrane.
The cell membrane is a biological membrane that separates the interior of the cell from the outside environment. It is selectively permeable and allows ions and small polar molecules through via passive diffusion. Large molecules such as proteins must be transported into the cell via transmembrane or carrier proteins that are embedded in the membrane. Cell membranes also allow water to pass through them via osmosis. Cell membranes are flexible and allow the cell to be fluid and to maintain their volume as they change shapes. They are also involved in a variety of cellular functions. These include cell adhesion, ion conductivity and cell signaling. Cell membranes can also be very diverse. In eukaryotes, they are single-layered and follow the fluid mosaic model. In prokaryotes, the cell membrane is surrounded by an outer membrane. The cell membrane and outer membrane are separated by a periplasmic space. However, some prokaryotes have no cell membrane at all.