Small, relatively nonpolar molecules and small uncharged polar molecules pass directly through the cell membrane by simple diffusion. This includes gases like oxygen (O2) and carbon dioxide (CO2), as well as small polar molecules like water (H2O) and ethanol. Larger uncharged polar molecules (like glucose) and charged molecules (ions) cannot pass directly through the membrane and require transport proteins.
Molecules Passing Directly Through Membrane
- Small nonpolar molecules (e.g., O2, CO2)
- Small uncharged polar molecules (e.g., H2O, ethanol)
- Hydrophobic molecules that dissolve in the phospholipid bilayer
Molecules Not Passing Directly Through Membrane
- Larger uncharged polar molecules (e.g., glucose)
- Charged molecules and ions (e.g., Na+, K+, Cl-)
The mechanism is passive diffusion, driven by concentration gradients, without involvement of membrane proteins for the molecules that can dissolve in the lipid bilayer.
