Final answer:
Small nonpolar or hydrophobic molecules like oxygen, carbon dioxide, and lipids can easily pass through the cell membrane without aquaporins. Polar molecules and ions need facilitated transport via transport proteins because of their hydrophilic nature and charge.
Step-by-step explanation:
The types of molecules that can pass easily through the cell membrane without an aquaporin are small, nonpolar, or hydrophobic molecules. This is because the cell membrane is composed primarily of a lipid bilayer which is hydrophobic in nature. Thus, small, nonpolar molecules such as oxygen, carbon dioxide, and lipids are able to diffuse directly through the lipid bilayer by simple diffusion. Simple diffusion does not require energy as it involves the movement of molecules from an area of higher concentration to an area of lower concentration. On the other hand, polar molecules and ions, due to their charge and hydrophilicity, cannot pass directly through the lipid bilayer. They typically require facilitated transport through specific transport proteins, such as channels or carriers, to move across the cell membrane.