Introduction
Cell Membrane, Glycocalyx, and Cell Membrane Models
The cell membrane (also called the plasma membrane or plasmalemma) is a lipid bilayer that surrounds every eukaryotic cell, separating the cytoplasm from the extracellular environment. It is only visible with a transmission electron microscope (TEM).
The cell membrane acts as a selective barrier, regulating the passage of materials into and out of the cell, and facilitating the transport of specific molecules. It also keeps the ion content of the cytoplasm constant, which is different from the ion content of the extracellular fluid. Additionally, it carries out specific recognition and signaling functions, playing a key role in the interactions of the cell with its environment.
Cell membranes are composed of:
Phospholipids: amphipathic molecules with a hydrophilic (polar) head that bears a phosphate group, and two nonpolar (hydrophobic) long-chain fatty acids.
Cholesterol: a sterol lipid that inserts among the phospholipid fatty acids, restricting their movement and modulating the fluidity and movement of all membrane components.
Proteins: integral proteins are embedded in the lipid layers, while peripheral proteins are associated with the membrane surface.
Transmembrane proteins are integral proteins that completely span the bilayer.
Oligosaccharide chains: these are covalently linked to phospholipid and protein molecules and extend outward from the cell surface.
The fluid mosaic model
describes the structure of the cell membrane. In this model, membrane proteins comprise a moveable mosaic within the fluid lipid bilayer.
Glycocalyx is a delicate cell surface coating found on the external surface of the plasma membrane. It is composed of oligosaccharides from glycolipids and glycoproteins. It contributes to important antigenic and functional properties of the cell surface.
Under a TEM, the cell membrane and other organellar membranes exhibit a trilaminar appearance after being fixed in osmium tetroxide. This is because osmium binds to the polar heads of the phospholipids, the outer sugar chains, and associated membrane proteins, producing two dark outer lines enclosing a light band of osmium-free fatty acids.
Freeze-fracture electron microscopy studies show that many integral proteins protrude from both the outer and inner membrane surfaces. When the lipid bilayer is cleaved along the hydrophobic center, more particles are found attached to the half of the membrane adjacent to the cytoplasm (protoplasmic face), compared to the outer half of the membrane (extracellular face).
Functions of the glycocalyx:
Protection: provides a physical barrier.