Biological membrane

Biological membranes, also known as biomembranes, are semipermeable structures that enclose the interior of cells or define compartments within them. They separate the intracellular environment from the outside world and form boundaries between organelles, enabling cells to maintain distinct biochemical conditions essential for life. In eukaryotic cells, the defining structural feature of biological membranes is the phospholipid bilayer, into which various integral and peripheral proteins are embedded. These components collectively support functions such as signalling, transport, energy conversion and cellular recognition.

Structural Organisation and Membrane Asymmetry

The phospholipid bilayer consists of two distinct layers: the outer leaflet and the inner leaflet, which differ markedly in chemical composition. This asymmetry is fundamental to membrane function, influencing signalling pathways, interactions with the extracellular matrix and intracellular trafficking. Distinct proteins and lipids are confined to specific leaflets, contributing to the orientation of membrane-bound processes.
Cytosolic and exoplasmic faces are maintained through vesicular transport pathways. Lipids and glycoconjugates initially inserted into the lumen of the endoplasmic reticulum or the Golgi apparatus ultimately appear on the extracellular surface of the plasma membrane. New phospholipids are synthesised by cytosol-facing enzymes of the endoplasmic reticulum. To achieve balanced membrane growth across both monolayers, half of the newly formed phospholipids are transferred to the opposite leaflet by flippases, which selectively translocate particular lipid species. Glycolipids, which display consistent asymmetry, follow a different mechanism and become localised almost exclusively on the outer surface of animal cell membranes.

Lipid Components

Membrane lipids possess hydrophilic headgroups and hydrophobic hydrocarbon tails. Their chain length and degree of saturation influence membrane fluidity and physical behaviour. Specific lipid domains, known as lipid rafts, form when certain lipids and proteins cluster, creating microenvironments that organise signalling components and transport machinery.
Erythrocytes offer a characteristic example of specialised lipid composition. Their membranes contain cholesterol and phospholipids in roughly equal proportions by weight, contributing to membrane rigidity and mechanical stability. Phosphatidylserine, normally confined to the cytoplasmic leaflet, can be translocated to the outer surface during blood clotting, where it provides a platform for coagulation factors.

Membrane Proteins

The bilayer hosts diverse proteins with structural, enzymatic and regulatory functions:

• Integral membrane proteins span the lipid bilayer and rely on strong hydrophobic interactions with lipids. They contain domains exposed on both sides of the membrane and require chemical treatments to detach.
• Peripheral membrane proteins bind loosely to the membrane surface through non-covalent interactions and can readily dissociate. Their attachment to only one leaflet contributes to membrane asymmetry.

Together, membrane proteins mediate communication with the environment, catalyse reactions, transport molecules, maintain ion gradients and organise cytoskeletal attachments.

Oligosaccharides, Glycolipids and Glycoproteins

Oligosaccharides can be covalently attached to lipids, forming glycolipids, or to proteins, forming glycoproteins. Glycolipids exhibit strong asymmetry, with their sugar moieties exposed exclusively on the external face of the membrane. These carbohydrate groups participate in cell recognition, cell–cell adhesion and immune interactions.
Glycoproteins, mostly integral in nature, contribute to protective mechanisms and the immune response. Their carbohydrate chains extend into the extracellular environment, forming important components of the cell surface.

Formation of the Phospholipid Bilayer

Bilayers assemble spontaneously in aqueous environments due to the hydrophobic effect, which drives the aggregation of lipid tails away from water while exposing hydrophilic headgroups. This arrangement maximises favourable hydrogen bonding with water and increases system entropy. The resulting structure is stable, self-sealing and fluid.

Functions and Properties of Biomembranes

Biological membranes are amphiphilic, containing both hydrophilic and hydrophobic regions. This duality underpins several critical functions:

• Compartmentalisation: Membranes define enclosed spaces such as organelles, which maintain distinct chemical conditions. For example, peroxisomal membranes isolate oxidative reactions that generate potentially harmful peroxides.
• Selective permeability: Membranes permit some molecules to cross freely while restricting others. Small hydrophobic molecules diffuse easily, whereas charged or large molecules require transport proteins. Larger particles may enter cells via endocytosis.
• Mechanical flexibility: Membranes possess elastic properties enabling shape changes, vesicle formation, motility and interactions with the cytoskeleton.

Various specialised membranes exist in different contexts, such as apical and basolateral domains of epithelial cells, synaptic membranes, myelin sheaths, and the membranes of cilia and flagella. Numerous supramembrane structures—including caveolae, desmosomes, focal adhesions and postsynaptic densities—arise from the interactions between membranes and associated proteins.
Within cells, membranes define organelles such as endosomes, the endoplasmic reticulum, Golgi apparatus, lysosomes, mitochondria, nuclei and transport vesicles. Differences in lipid and protein composition grant each membrane distinct physical and functional characteristics. Some membrane-associated components, such as efflux pumps, are of medical significance due to their roles in drug transport.

Membrane Fluidity

The lipid bilayer is dynamic. Individual lipid molecules rotate around their bonds, and hydrophobic tails bend and interact. These movements contribute to the fluid character of the membrane. Hydrophilic headgroups, however, experience constraints due to hydrogen bonding with water, leading to less rotational freedom and increased viscosity near the membrane surface.
Membrane fluidity is temperature-dependent. Below a specific transition temperature, lipids adopt a more ordered, gel-like state. Above this temperature, lipids shift into a fluid phase, allowing increased lateral movement. This fluidity is essential for membrane protein function, vesicle formation, and many cellular processes requiring membrane rearrangement.