Summary
Highlights
The cell membrane acts as a vital barrier between intracellular and extracellular fluids. It is composed of three main elements: membrane lipids, membrane proteins, and the glycocalyx.
Lipids form the bilayer structure. Phosphate and sphingosine heads are hydrophilic and negatively charged, while the internal fatty acid tails are hydrophobic. Cholesterol is also embedded to help maintain membrane stability and control fluidity.
Proteins include integral/transmembrane proteins for transport and peripheral proteins for weaker linkages. The glycocalyx, composed of glycoproteins and glycolipids, serves as a crucial network for water regulation and antigenic recognition, helping the immune system distinguish host cells from foreign bodies.
Fluidity is regulated by temperature, cholesterol concentration, and the saturation levels of fatty acids. Higher temperatures and unsaturated fatty acids (which contain kinks) increase fluidity, whereas cholesterol acts as glue to stabilize and potentially decrease fluidity depending on concentration.
Simple diffusion allows small, lipid-soluble molecules like oxygen and CO2 to pass through the lipid core. Larger or polar molecules require transmembrane proteins to act as channels or carriers. Proteins also enable exocytosis, endocytosis, cellular communication, and enzymatic activity.
The membrane is described as a 'fluid mosaic' because it is a flexible, dynamic structure composed of varied components that move laterally and enable controlled passage of substances, supporting overall cell homeostasis.