How does phospholipid structure relate to the selective permeability of the plasma membrane? a critical feature of the plasma membrane is that it is selectively permeable. this allows the plasma membrane to regulate transport across cellular boundaries--a function essential to any cell's existence. how does phospholipid structure prevent certain molecules from crossing the plasma membrane freely?

The structure of plasma membrane makes it selectively permeable allowing it to regulate the passage of substances into and out of the cell. Small nonpolar molecules can easily across the phospholipid bilayer of plasma membrane because it is hydrophobic. Polar molecules and ion cannot easily across the hydrophobic portion of plasma membrane because it is hydrophilic. Polar molecules and ions usually across the plasma membrane with the help of transport proteins.

Air enters through the nose (and sometimes the mouth), moves through the nasal cavity, the  pharynx, the  larynx, enters the  trachea, moves through the  bronchi  andbronchioles  till the alveoli.