Ans. D. Consists of 3 types of unique proteins in it.SurfactantThe low surface tension when the alveoli are small is due to the presence in the fluid lining the alveoli of surfactant, a lipid surface-tension-lowering agent. Surfactant is a mixture of Di palmitoyl phosphatidylcholine (DPPC), other lipids, and proteins If the surface tension is not kept low when the alveoli become smaller during expiration, they collapse in accordance with the law of Laplace. In spherical structures like the alveoli, the distending pressure equals two times the tension divided by the radius (P=2T/r); if T is not reduced as r is reduced, the tension overcomes the distending pressure. Surfactant also helps to prevent pulmonary edema. It has been calculated that if it were not present, the unopposed surface tension in the alveoli would produce a 20 mm Hg force favouring transudation of fluid from the blood into the alveoli.Approximate Composition of Surfactant.ComponentPercentage CompositionDipalmitoyl phosphatidylcholine62Phosphatidylglycerol5Other phospholipids10Neutral lipids13Proteins8Carbohydrate2Surfactant is produced by type II alveolar epithelial cells Typical lamellar bodies, membrane-bound organelles containing whorls of phospholipid, are formed in these cells and secreted into the alveolar lumen by exocytosis. Tubes of lipid called tubular myelin form from the extruded bodies, and the tubular myelin in turn forms the phospholipid film. Following secretion, the phospholipids of surfactant line up in the alveoli with their hydrophobic fatty acid tails facing the alveolar lumen. Surface tension is inversely proportional to their concentration per unit area. The surfactant molecules move further apart as the alveoli enlarge during inspiration, and surface tension increases, whereas it decreases when they move closer together during expiration. Some of the protein-lipid complexes in surfactant are taken up by endocytosis in type II alveolar cells and recycled.Formation of the phospholipid film is greatly facilitated by the proteins in surfactant. This material contains four unique proteins: surfactant protein (SP)-A, SP-B, SP-C, and SP-D. SP-A is a large glycoprotein and has a collagen-like domain within its structure. It has multiple functions, including regulation of the feedback uptake of surfactant by the type II alveolar epithelial cells that secrete it. SP-B and SP-C are smaller proteins, which facilitate formation of the monomolecular film of phospholipid. A mutation of the gene for SP-C has been reported to be associated with familial interstitial lung disease. Like SP-A, SP-D is a glycoprotein. Its full function is uncertain. However, SP-A and SP-D are members of the collecting family of proteins that are involved in innate immunity in the conducting airway as well as in the alveoli.