What is the major component of surfactant and the principal surface-active lipid?

• A. Cholesterol
• B. Protein
• C. Saturated phosphatidylcholine
• D. Phosphatidylinositol

Answer: (C)

The major component is a saturated phosphatidylcholine known as dipalmitoyl phosphatidylcholine (DPPC). This lipid is the principal surface-active component of pulmonary surfactant, and its saturated fatty acyl chains pack tightly at the air–liquid interface in the alveoli. That tight packing dramatically lowers surface tension as the lungs inflate and deflate, preventing alveolar collapse at end expiration and maintaining lung compliance. Surfactant also contains cholesterol and specific surfactant proteins, which aid spreading, stability, and immune functions, but they are not the primary lipid responsible for the surface-active properties.