Practice question

Asymmetry generation and maintenance underlie plasma membrane functional identity. Compositional asymmetry means outer leaflet enriched phosphatidylcholine sphingomyelin contributing saturated chains stability and interaction with extracellular proteins, inner leaflet enriched phosphatidylethanolamine phosphatidylserine phosphatidylinositol providing anionic surface negative charge minus one for PS PI at neutral pH recruiting peripheral proteins with polybasic patches K-Ras, C2 domains of protein kinase C, annexins bridging via calcium. Glycolipids glycosphingolipids exclusively exoplasmic presenting ABO antigens and gangliosides for recognition. Origin during synthesis requires selective transport: P4-ATPase flippases inward ATP dependent, ABC floppases outward ATP dependent, scramblases randomization calcium activated during apoptosis externalizing PS as eat-me signal for phagocytes and procoagulant surface for tenase prothrombinase assembly. Equal distribution would abolish signaling potential and lectin recognition. Cholesterol flips rapidly but does not generate asymmetry alone. Different lipid compositions between leaflets therefore cause asymmetry essential for curvature, budding, apoptosis recognition and signal transduction polarity. Maintenance involves continuous ATP consumption counteracting spontaneous scrambling tendency, highlighting energy dependence of plasma membrane identity and signaling competence.