Which of the following components of the pulmonary surfactant mainly contribute to its physiological function?

Pulmonary surfactant, composed of lecithin and myelin, is secreted on a continuous basis by type II alveolar cells and Clara cells beginning at approximately week 20 of gestation. Pulmonary surfactant has both a lipid (approximately 90% of total) and a protein (approximately 10% of total) component. About half of the lipids are dipalmitoylphosphatidylcholine. The remaining lipids include phosphatidylglycerol, which modulates the fluidity of the surfactant as well as cholesterol and other lipids. About half of the proteins are apoproteins, which have both hydrophobic and hydrophilic poions; the other half is composed of proteins normally found in blood plasma. These lipids and proteins all have the capability of interacting with hydrophilic or hydrophobic environments and it is this quality that leads to their specialized function in the lung. Ref: Huang H.J. (2012). Chapter 17. The Respiratory System. In L.W. Janson, M.E. Tischler (Eds), The Big Picture: Medical Biochemistry.