Which type of molecules can pass easily through the membrane?

Small hydrophobic molecules can easily pass through the cell membrane due to the structure of the phospholipid bilayer. The membrane is primarily composed of a double layer of phospholipids, which contain hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails. This structure creates a selective barrier that allows certain types of substances to diffuse freely while restricting others.

Small hydrophobic molecules, such as oxygen and carbon dioxide, can easily dissolve in the lipid bilayer and pass through the membrane without the need for transport proteins. This is because their nonpolar nature allows them to interact favorably with the hydrophobic interior of the membrane. In contrast, larger or hydrophilic molecules struggle to pass through without assistance because their polar characteristics prevent efficient interaction with the lipid bilayer. Similarly, ionic compounds are charged and cannot cross the membrane easily without specialized channels or transporters. Long-chain fatty acids, while hydrophobic, may require carrier proteins for efficient transport due to their size. Thus, small hydrophobic molecules are the most likely to move freely through the membrane, making this option correct.