Explain why hydrophobic molecules can easily cross the plasma membrane, while hydrophilic molecules cannot.

Membranes are semipermeable lipid bilayers. Permeability refers to the ease with which molecules cross biological membranes. Because of the chemical and structural nature of the phospholipid bilayer (hydrophobic core), only lipid-soluble molecules are able to freely pass through the lipid bilayer because it attracts these non polar molecules.

Water soluble or hydrophilic molecules cannot pass through these lipid membranes because hydrophobic bilayer repels hydrophilic molecules. Ions and large polar molecules cannot pass through the lipid bilayer.  

But more specifically, whether a molecule can pass through the membrane depends on its size and its electrical nature.

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