Lipid bilayer

A lipid bilayer, also known as a phospholipid bilayer, is a thin, polar membrane consisting of two layers of lipid molecules. This structure serves as a continuous, essential barrier for almost all living organisms and many viruses. It forms the outer plasma membrane of cells, the nuclear membrane surrounding the cell nucleus, and the membranes of various membrane-bound organelles. The primary role of this bilayer is to maintain cellular order by keeping ions, proteins, and other molecules in their necessary locations. By acting as a barrier, it prevents these vital substances from diffusing into areas where they could disrupt cellular functions.

The mechanism of the bilayer is driven by the unique properties of phospholipids. These molecules are amphiphilic, meaning they possess two distinct parts: a hydrophilic phosphate head and a hydrophobic tail. The hydrophilic head is attracted to water, while the hydrophobic tail, made of two fatty acid chains, repels it. When exposed to water, these molecules self-assemble into a two-layered sheet through the hydrophobic effect. In this arrangement, the tails point toward the center to avoid water, while the heads face outward to interact with the aqueous environment. This creates a central core that is almost entirely devoid of water, effectively excluding water-soluble molecules like sugars or salts.

Across its cross-section, the bilayer is organized into several distinct chemical regions. The first region is the hydrophilic headgroup, which is completely hydrated and typically measures between 0.8 and 0.9 nanometers in thickness. Within this region, the phosphate group is located approximately 0.5 nanometers outside the hydrophobic core. Next is an intermediate boundary layer about 0.3 nanometers thick, where water concentration drops rapidly from 2M to nearly zero. Finally, the hydrophobic core is typically 3 to 4 nanometers thick. This thickness can vary based on the chemistry and length of the lipid tails, as well as the temperature.

Many biological bilayers exhibit asymmetry, meaning the composition of the inner and outer leaflets differs. In human red blood cells, the inner cytoplasmic leaflet is mostly composed of phosphatidylethanolamine, phosphatidylserine, and phosphatidylinositol. In contrast, the outer extracellular leaflet consists of phosphatidylcholine, sphingomyelin, and various glycolipids. This asymmetry is maintained by specialized enzymes. Flippases transport lipids from the inner to the outer monolayer, while floppases move them in the opposite direction. Scramblases can randomize this distribution, such as during apoptosis, when phosphatidylserine moves to the outer surface to signal macrophages to scavenge the dying cell.

The physical state of the bilayer is highly sensitive to temperature and molecular structure. A bilayer can exist in either a solid gel phase or a fluid liquid phase. At lower temperatures, the lipids are less mobile in a gel phase. At higher temperatures, the bilayer undergoes a phase transition into a fluid state. In this liquid phase, lipid molecules can perform a "random walk," exchanging locations with neighbors millions of times per second. The transition temperature is influenced by the length of the lipid tails and their degree of unsaturation. For example, an unsaturated double bond can create a kink in the tail, disrupting tight packing and increasing fluidity.

Various components work together to regulate the bilayer's mechanical and chemical properties. Cholesterol is a vital molecule in animal cells that strengthens the bilayer and decreases its permeability. It also helps regulate the activity of integral membrane proteins, which are proteins embedded within the bilayer. These proteins are held in place by an annular lipid shell and are essential for cellular signaling. Some membrane proteins even facilitate the fusion of two bilayers, a process seen during viral entry into a cell or during the acrosome reaction in fertilization.

Because lipid bilayers are extremely thin and fragile, they are difficult to study using traditional microscopes. Scientists must use advanced techniques such as electron microscopy, atomic force microscopy, and x-ray reflectometry to observe them. To understand these structures, researchers often create artificial "model" bilayers in laboratory settings. These model systems allow scientists to study how different lipids affect membrane properties like stretching and bending. Furthermore, vesicles made from these model bilayers have found clinical applications in the delivery of drugs to the body.