The cell membrane, or plasma membrane, is the molecular boundary surrounding a cell, separating its interior from the external environment. It consists mainly of lipids and proteins arranged into a flexible, selectively permeable structure. Besides controlling the movement of substances, it detects signals and participates in interactions with neighboring cells. These functions support homeostasis by allowing the cell to maintain internal conditions distinct from its surroundings. The plasma membrane is distinguished from the membranes enclosing internal compartments of eukaryotic cells. (ncbi.nlm.nih.gov)
Molecular structure
In most organisms, the membrane’s structural foundation is a lipid bilayer. Its principal components include phospholipids, which have water-interacting, hydrophilic head groups and water-avoiding, hydrophobic tails. In an aqueous environment, these molecules assemble with their heads facing the fluids on either side and their tails directed inward. This arrangement creates a hydrophobic interior that inhibits the passage of many water-soluble substances while remaining flexible rather than rigid. (ncbi.nlm.nih.gov)
Membrane composition varies among organisms and cell types. Animal plasma membranes commonly contain substantial amounts of cholesterol, which modifies lipid packing, mechanical properties, and permeability. Its effects depend on temperature and composition: it constrains parts of the lipid chains while also preventing their tight packing into a crystalline state. Archaea have distinctive ether-linked membrane lipids; some possess membrane-spanning tetraether lipids that form a monolayer rather than a conventional bilayer. (ncbi.nlm.nih.gov)
The membrane’s two lipid layers, called leaflets, generally differ in composition. Proteins also have specific orientations, with different regions exposed to the cell interior and exterior. In eukaryotic plasma membranes, carbohydrate chains attached to proteins and lipids occur on the extracellular surface. Together with other surface-associated sugars, they contribute to the glycocalyx, a carbohydrate-rich coat involved in protection and cellular interactions. (ncbi.nlm.nih.gov)
Fluidity and organization
The fluid mosaic model, proposed by S. J. Singer and Garth L. Nicolson in 1972, describes membrane proteins embedded in a fluid lipid matrix. Many lipids and proteins can move laterally within the membrane. “Fluid” does not mean that every component moves freely: associations with other proteins, neighboring cells, or the underlying cytoskeleton can restrict movement and maintain specialized surface regions. The membrane is therefore a dynamic but organized interface, not a uniform molecular sheet. (pubmed.ncbi.nlm.nih.gov)
Integral membrane proteins are embedded in the lipid matrix, and transmembrane proteins extend across it. Peripheral proteins associate with the membrane surface, often through interactions with other proteins. Additional proteins are attached through lipid anchors. These different arrangements position functional regions where they can bind substances, transmit signals, catalyze reactions, or connect the membrane to supporting structures. (ncbi.nlm.nih.gov)
Selective permeability and transport
Small nonpolar molecules can cross the lipid bilayer relatively readily, whereas ions and most large or strongly polar molecules require transport proteins. Passive transport proceeds down a concentration gradient—or, for charged substances, an electrochemical gradient—without direct expenditure of metabolic energy. Simple diffusion occurs through the lipid matrix; facilitated diffusion uses channels or carriers. Channels provide passageways, while carriers bind particular solutes and change conformation to move them across the membrane. (ncbi.nlm.nih.gov)
Ion channels are selective and often regulated by voltage, chemical binding, or other stimuli. Water can cross the bilayer, but aquaporins greatly increase its rate of passage in many cells. The movement of water across a selectively permeable membrane is osmosis, which influences cell volume according to differences in solute concentration between the two sides. (ncbi.nlm.nih.gov)
Active transport moves substances against their electrochemical gradients by coupling movement to an energy source. Primary active transport commonly uses ATP hydrolysis. The sodium–potassium pump, for example, normally exports three sodium ions and imports two potassium ions per ATP molecule hydrolyzed. Secondary active transport instead couples the downhill movement of one substance to the uphill transport of another, using a gradient established by other transport processes. (ncbi.nlm.nih.gov)
Communication and structural connections
Membrane receptors recognize extracellular signals and initiate changes inside the cell. Binding of a ligand can alter a receptor’s activity, allowing information to cross the membrane without requiring the signaling molecule itself to enter. Through these mechanisms, cells adjust their behavior to their chemical environment. (ncbi.nlm.nih.gov)
Other membrane proteins connect cells to adjacent cells or to extracellular supporting material. Connections to the cytoskeleton help coordinate surface organization, cell shape, and mechanical behavior. Thus, membrane proteins serve both informational and structural roles, linking external interactions with intracellular organization. (ncbi.nlm.nih.gov)
Membrane trafficking and renewal
Eukaryotic cells also exchange material through membrane-enclosed vesicles. During endocytosis, regions of the plasma membrane bend inward and separate to carry extracellular material into the cell. Receptor-mediated endocytosis selectively concentrates particular molecules before internalization. Internalized receptors may subsequently return to the surface or be directed toward degradation. (ncbi.nlm.nih.gov)
During exocytosis, intracellular vesicles fuse with the plasma membrane, releasing their contents outside and adding lipids and proteins to the cell surface. Endocytosis, recycling, and exocytosis together continually remodel membrane composition and regulate the abundance of surface proteins. These processes connect the plasma membrane to the cell’s internal membrane-transport system. (ncbi.nlm.nih.gov)