Final answer:
Small, nonpolar molecules like oxygen and carbon dioxide, hydrophobic molecules, and very small polar molecules such as water can pass through the lipid bilayer via passive diffusion. Polar molecules and ions generally require facilitated transport for membrane crossing.
Step-by-step explanation:
Substances that can pass through the lipid bilayer via passive diffusion include small, nonpolar molecules such as oxygen (O₂) and carbon dioxide (CO₂), as well as hydrophobic substances like lipids and alcohol. Moreover, very small polar molecules, including water, can also pass through the cell membrane via simple diffusion due to their diminutive size. However, substances that are larger, polar, or carry a charge typically require specialized transport mechanisms, as the phospholipid bilayer's hydrophobic interior repels them.
Passive transport is characterized by the movement of substances from an area of higher concentration to an area of lower concentration, without the need for cellular energy. It provides the means through which gases like O₂ and CO₂ easily move across the cell membrane. Certain small molecules and water can move freely through the bilayer directly, whereas polar molecules and ions usually need facilitated transport involving protein channels.