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Proteins

Each SP protein has distinct functions, which act synergistically to keep an interface rich in DPPC during lung's expansion and contraction. Changes in the surfactant mixture composition alter the pressure and temperature conditions for phase changes and the phospholipids' crystal shape as well. Only the liquid phase can freely spread on the surface to form a monolayer. Nevertheless, it has been observed that if a lung region is abruptly expanded the floating crystals crack like " icebergs ". Then the SP proteins selectively attract more DPPC to the interface than other phospholipids or cholesterol, whose surfactant properties are worse than DPPC's. The SP also fastens the DPPC on the interface to prevent the DPPC from being squeezed out when the surface area decreases This also reduces the interface compressibility.