One of the reasons the plasma membrane can have<span> selective permeability</span><span> it's because it's composed by a bi-layer of phospholipids.
The interaction between them and other molecules will influence how these molecules will cross the membrane. If the molecule it's </span>hydrophobic it can easily cross, if it's polar molecules and ions( hydrophilic), they cannot cross easily.
The Human Respiratory system.
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The heads of the phospholipids, which are positioned outwards on both layers, are hydrophilic. The trails of both layers are hydrophobic and face inwards, and protected by the outer layers.
