Practice question

Plasma membrane lipids assemble through non-covalent forces rather than covalent polymerization. Each phospholipid or cholesterol monomer inserts via hydrophobic effect with tails buried together while polar headgroups contact water, stabilized by van der Waals contacts, hydrogen bonding between phosphate oxygens and water, and electrostatic interactions. This allows lateral diffusion, rotation, and exchange between leaflets catalyzed by flippases, and reversible desorption to cytosolic carriers. Membranes exhibit selective permeability because polar headgroup region impedes hydrophobic solutes while hydrocarbon core impedes polar solutes, requiring transporters. They self-assemble into bilayers spontaneously when amphipathic lipids reach critical micelle concentration, forming vesicles that encapsulate aqueous contents, principle exploited for liposomal drug delivery. If lipids were covalently bonded together like polymers, fluidity would be lost, proteins could not diffuse, and processes like cell fusion, cytokinesis, and endocytosis would be impossible. Consequently fluidity, repair, and remodeling depend on non-covalent nature, with proteins providing scaffold but not crosslinking lipids permanently.