The plasma membrane, also often referred to as the cell membrane, is the lipid bilayer barrier that marks the border of the cells of all living things. Not all cells have cell walls, but all have cell membranes. Cell membranes are flexible, in that they do allow substances to enter and exit the cell. This passage in and out of the cell is called endocytosis and exocytosis, respectively. This selective permeability serves to keep the cell functioning in all its activities, as intake and secretion of substances is vital to the cell’s survival in many ways, but it also serves to protect the cell’s contents from its surroundings. Another one of the most important functions of a plasma membrane (and in certain cases, the cell wall) is cell to cell collaboration. This often occurs in the form of cell adhesion, which is when one cell attaches to another. Cell adhesion forms the basis for the functioning of a living tissue – an organization of similar cells that work together to perform a more macro-level function. These functions are all made possible by the cell membrane’s advanced structure.
While glycolipids and sterols are also present, the most important ingredient of a plasma membrane (and of the membranes of many organelles within the cell) is the phospholipid molecule. This molecule has a hydrophilic (water-loving) phosphate “head” and a hydrophobic (water-fearing) fatty acid “tail.” The extracellular matrix (ECM) – the substances surrounding the cell – is mostly water, as is the cytosol – the intracellular fluid that takes up the majority of the cell’s space (cells are considered to be made up of mostly water). The hydrophilic phosphate head is attracted to the water while the hydrophobic tails are repelled by the water. This combination, coupled with a glycerol molecule that connects two phospholipid molecules at the fatty acid tail ends, is why cell membranes are called phospholipid bilayers.
The phospholipid bilayer is prone to the diffusion of hydrophobic molecules, but is almost impermeable otherwise. For its selective permeability, the cell membrane has protein structures such as pores, channels, and gates. These channel proteins open their gates to let welcome substances through their pores, but keep them closed otherwise. The plasma membrane’s channel proteins usually recognize a very limited group of substances; so typically, specific types of substances enter or exit the cell through specific channels only.
Cell membranes have additional structures composed of proteins, such as integrins and cadherins, that are also responsible for controlling what enters or exits the cell, though these proteins have other main functions, namely cell adhesion. These structures are located on the outside of the cell membrane and include caveola (lipid rafts for communication within the cell), postsynaptic density, podosome (like cell feet), invadopodium (ECM cleaners), focal adhesion (cell-cell radio towers), and some types of cell junctions (intercellular bridge). The carbohydrates located on the outside of the cell membrane also participate in cell adhesion. The phospholipid bilayer is also capable of budding off pieces of itself called vesicles – membrane balls that serve as transport vehicles, usually within the cell – and absorbing vesicles from organelles within the cell, important for helping maintain the cell membrane’s integrity.
The plasma membrane is a phospholipid bilayer barrier that functions as the border of the cell in all living things (it can be reinforced by a surrounding cell wall in some organisms). One of the plasma membrane’s main functions is to regulate the substances that enter or exit the cell, effectively both protecting the cell from harm and sustaining all of its activities. The plasma membrane’s other primary function is to facilitate cell-cell communication and collaboration, often through cell adhesion, an essential process to the function of living tissue in complex eukaryotic organisms. Its composition also makes it flexible and allows many other accessory structures that give the cell a whole new dimension of capabilities. The plasma membrane also participates in maintaining cell membrane-organelle homeostasis by the constant exchange of its parts with the membranes of the cell’s other organelles.