The phospholipid bilayer, which forms the basis of cell membranes, consists of two layers of phospholipid molecules. Each phospholipid molecule has a polar phosphate head and two hydrophobic (nonpolar) fatty acid tails. The behavior of polar and nonpolar molecules in the phospholipid bilayer can be explained by the principles of polarity and the hydrophobic effect.
Polar molecules have an uneven distribution of charge, with regions of partial positive and partial negative charges. In contrast, nonpolar molecules have no significant charge separation and are electrically neutral.
The polar phosphate heads of phospholipids in the bilayer can interact with water molecules due to their partial charges. This interaction between the polar heads and water is known as hydrophilic (water-loving) interaction. Since water is a polar molecule, it tends to interact favorably with other polar molecules. As a result, polar molecules can dissolve in water but have limited solubility in the hydrophobic interior of the phospholipid bilayer.
On the other hand, the hydrophobic fatty acid tails of phospholipids repel water due to their nonpolar nature. This repulsion is known as the hydrophobic effect. To minimize contact with water, hydrophobic substances tend to cluster together or associate with other hydrophobic regions. When nonpolar or hydrophobic molecules are present in the vicinity of the phospholipid bilayer, they can interact with the hydrophobic tails and become incorporated into the bilayer, thus dissolving within the hydrophobic interior.
In summary, polar molecules have limited solubility in the hydrophobic interior of the phospholipid bilayer because they cannot form favorable interactions with the nonpolar region. On the other hand, hydrophobic substances dissolve in the hydrophobic interior due to the favorable interactions between their nonpolar nature and the hydrophobic tails of phospholipids.