Which molecules can diffuse directly through the phospholipid bilayer without transport proteins?

Which molecules can diffuse directly through the phospholipid bilayer without transport proteins?

• A. Glucose.
• B. Water.
• C. Small nonpolar molecules such as O2 and CO2.
• D. Ions such as Na+.] ,

Answer: (C)

Membrane permeability depends on a molecule’s polarity and size. The hydrophobic interior of the phospholipid bilayer allows nonpolar, small molecules to dissolve in and diffuse directly down their concentration gradient without help from transport proteins. Oxygen and carbon dioxide are quintessential examples: they are small and nonpolar, so they cross the membrane readily on their own. Glucose is large and polar, so it doesn’t dissolve well in the lipid interior and requires a transporter to cross. Water is small and polar; it can cross to some extent, but its rapid movement across membranes is mainly through water channels (aquaporins), not by free diffusion through the bilayer. Ions are charged and cannot pass the hydrophobic core without specific channels or carriers. So, the molecules that can diffuse directly through the bilayer without transport proteins are small nonpolar ones like O2 and CO2.

Membrane permeability depends on a molecule’s polarity and size. The hydrophobic interior of the phospholipid bilayer allows nonpolar, small molecules to dissolve in and diffuse directly down their concentration gradient without help from transport proteins. Oxygen and carbon dioxide are quintessential examples: they are small and nonpolar, so they cross the membrane readily on their own.

Glucose is large and polar, so it doesn’t dissolve well in the lipid interior and requires a transporter to cross. Water is small and polar; it can cross to some extent, but its rapid movement across membranes is mainly through water channels (aquaporins), not by free diffusion through the bilayer. Ions are charged and cannot pass the hydrophobic core without specific channels or carriers.

So, the molecules that can diffuse directly through the bilayer without transport proteins are small nonpolar ones like O2 and CO2.