Final answer:
Hydrophilic molecules struggle to cross the plasma membrane due to the hydrophobic interior that repels polar substances, requiring specialized transmembrane proteins to facilitate their movement across the membrane.
Step-by-step explanation:
Hydrophilic (polar) molecules have difficulty crossing the plasma membrane because they are not compatible with the hydrophobic interior of the membrane. The plasma membrane is composed of a phospholipid bilayer with hydrophobic tails facing inward, creating a barrier to polar substances. Lipid-soluble materials, however, move through easily as they can interact with the hydrophobic core.
Hydrophilic molecules connect well with the hydrophilic exterior of the membrane, but simple diffusion through the hydrophobic core is not feasible for them. Charged molecules and ions likewise encounter this barrier because their charge prevents them from passing through the hydrophobic interior. To facilitate their transport, transmembrane proteins such as channels and carriers are needed to help these substances enter or exit the cell.
Therefore, without assistance, hydrophilic molecules cannot freely move across the plasma membrane, which is essential for maintaining cellular homeostasis and controlling the internal environment of the cell. The selective permeability of the plasma membrane ensures that essential molecules such as sugars and amino acids are regulated for proper cell function.