The cell membrane (also known as the plasma membrane or cytoplasmic membrane) is a biological membrane that separates the interior of all cells from the outside environment. Cell membranes are involved in a variety of cellular processes such as cell adhesion, ion conductivity and cell signaling and serve as the attachment surface for several extracellular structures, including the cell wall, glycocalyx, and intracellular cytoskeleton.
- Fungi, bacteria and plants also have a cell wall in addition, which provides a mechanical support to the cell and precludes the passage of larger molecules.
- The cell membrane is selectively permeable and able to regulate what enters and exits the cell, thus facilitating the transport of materials needed for survival.
- The movement of substances across the membrane can be either “passive”, occurring without the input of cellular energy, or “active”, requiring the cell to expend energy in transporting it.
- The cell membrane consists primarily of a thin layer of amphipathic phospholipids that spontaneously arrange so that the hydrophobic “tail” regions are isolated from the surrounding water while the hydrophilic “head” regions interact with the intracellular (cytosolic) and extracellular faces of the resulting bilayer.
The Cell’s Transport Mechanisms
- Passive osmosis and diffusion: Some substances (small molecules, ions) such as carbon dioxide (CO2) and oxygen (O2), can move across the plasma membrane by diffusion, which is a passive transport process.
- Transmembrane protein channels and transporters: Nutrients, such as sugars or amino acids, must enter the cell, and certain products of metabolism must leave the cell.
- Endocytosis: Endocytosis is the process in which cells absorb molecules by engulfing them.
- Exocytosis: Just as material can be brought into the cell by invagination and formation of a vesicle, the membrane of a vesicle can be fused with the plasma membrane, extruding its contents to the surrounding medium.