Final answer:
Non-polar molecules diffuse across phospholipid bilayers by interacting with the membrane's hydrophobic interior, while polar substances require assistance from transport proteins.
Step-by-step explanation:
Non-polar molecules are able to be transported across phospholipid bilayers because they can easily interact with the hydrophobic tails of the phospholipids that make up the interior of the membrane. This interaction allows them to diffuse through the membrane without the need for transport proteins.
By contrast, polar molecules and ions, due to their hydrophilic nature, cannot freely pass through the hydrophobic lipid core of the membrane and require assistance from transport proteins, such as channel proteins, gated channel proteins, or carrier proteins, to move across by facilitated diffusion or active transport.
Large molecules and water-soluble materials like glucose, amino acids, and electrolytes, which are repelled by the hydrophobic interior, also require these transport mechanisms to enter or exit the cell.