Answer:
small molecules
Explanation:
Molecules that are small and non-polar, such as oxygen, carbon dioxide, and water, can diffuse across the cell membrane through the lipid bilayer. These molecules are able to pass through the hydrophobic region of the bilayer because they are small enough to fit between the lipid molecules and because they are non-polar, which means they do not have a charged or polar region that would interfere with their passage through the bilayer.
Some other types of molecules, such as glucose and amino acids, can also diffuse across the cell membrane, but they require the use of special proteins called transport proteins.