A bilayer is a biological structure composed of two layers of molecules. This arrangement is found in living organisms and serves as a component for various cellular structures. Its organization, driven by the chemical properties of its molecules, allows it to perform distinct roles.
The Basic Structure of a Bilayer
The primary type of bilayer is the lipid bilayer, which forms the basis of all cell membranes. This structure is built from phospholipid molecules, which have a dual nature. Each phospholipid has a hydrophilic “head” that interacts with water, and two hydrophobic “tails” that repel water.
When phospholipids are in an aqueous environment, they spontaneously arrange into a two-layered sheet. The hydrophilic heads face the watery surroundings. Conversely, the hydrophobic tails cluster inwards, forming a non-aqueous core within the bilayer.
This self-assembly results in a stable structure where the hydrophobic tails are shielded from water. The two opposing layers are often referred to as leaflets, with the interior of the bilayer containing almost no water and excluding water-soluble molecules like sugars or salts. This arrangement is driven by hydrophobic interactions, along with non-covalent forces such as van der Waals forces, electrostatic interactions, and hydrogen bonds that stabilize the bilayer.
Functions and Properties of Bilayers
The lipid bilayer serves as a continuous barrier around all cells and their organelles. One of its primary functions is selective permeability, controlling what substances can pass through it. The hydrophobic core of the bilayer acts as a barrier, preventing most polar molecules and ions from freely crossing without assistance.
This selective barrier allows cells to maintain distinct internal environments, a process known as compartmentalization. For instance, cells can regulate their internal salt concentrations and pH by controlling ion movement across the membrane using specialized proteins. Small, nonpolar molecules, such as oxygen and carbon dioxide, can typically diffuse directly through the hydrophobic interior of the bilayer.
The lipid bilayer is not a rigid structure; it exhibits fluidity, a property described by the fluid mosaic model. This model suggests that the lipid molecules and embedded proteins can move laterally within the plane of the membrane. This dynamic nature is influenced by factors like the saturation of fatty acid chains in the lipid tails and the presence of cholesterol, which can modulate membrane fluidity and decrease its permeability.