Which of the following is true for phosphatidylcholine?
Phosphatidylcholine dominates eukaryotic membranes, constituting 40-50% of total phospholipid, existing as zwitterion at neutral pH with positively charged trimethylammonium and negatively charged phosphate, yielding net neutral molecule that stabilizes lamellar bilayer phase and prevents excessive electrostatic repulsion between headgroups. Its cylindrical molecular geometry with roughly equal cross-section of headgroup and two acyl chains favors planar bilayer formation, contrasting cone-shaped PE that induces curvature. Physiologically outstanding example is lung surfactant synthesized by type II alveolar cells in lamellar bodies containing dipalmitoylphosphatidylcholine DPPC with two saturated palmitate chains plus surfactant proteins SP-B and SP-C; DPPC reduces alveolar surface tension to near zero at end-expiration, preventing alveolar collapse and decreasing work of breathing. Premature infants deficient in surfactant develop respiratory distress syndrome treated with exogenous surfactant replacement therapy. In plasma membrane outer leaflet, PC serves as reservoir for signaling lipid phosphatidic acid via phospholipase D and as substrate for sphingomyelin synthesis, illustrating structural and metabolic versatility beyond membrane structure.
Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 10: Phosphatidylcholine and Pulmonary Surfactant.