Final answer:
Hydrophilic molecules cannot easily pass through the plasma membrane due to its hydrophobic interior. Hydrophobic molecules and small uncharged polar molecules like water can pass more easily. Charged ions and larger polar molecules require special mechanisms to cross the membrane.
Step-by-step explanation:
Most molecules, however, can't get through plasma membranes on their own. They may be hydrophilic and, hence, are repelled by the hydrophobic middle region of the plasma membrane. Molecules that are hydrophobic can easily pass through the plasma membrane if they are small enough, because they are water-hating like the interior of the membrane. However, molecules that are hydrophilic, on the other hand, cannot pass through the plasma membrane without help because they are water-loving like the exterior of the membrane, and are therefore excluded from the hydrophobic interior.
Polar substances and charged particles such as ions also face challenges when trying to penetrate plasma membranes. Substances like sodium, potassium, calcium, and chloride need special transport mechanisms because their charge prevents them from passing through the hydrophobic interior of the membrane. Some small polar molecules, like water, can still cross by simple diffusion due to their small size.