Acritical 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 the plasma membrane makes it selectively permeable, enabling it to regulate the transport substance into and out of the cell, such as
- small, non polar molecules are hydrophobic
- Polar molecules and ions are hydrophilic
- Polar molecules and ion usually pass with the help of protein

hope this helps