Molecules that can easily diffuse across a plasma membrane are primarily small, nonpolar molecules such as oxygen (O2) and carbon dioxide (CO2). Small polar molecules like water (H2O) can also diffuse through, but more slowly and often with the help of specialized channels called aquaporins. Larger nonpolar molecules diffuse slower, while large polar molecules, charged molecules, and ions generally cannot diffuse easily through the plasma membrane without the assistance of transport proteins or active transport mechanisms.
Molecules That Easily Diffuse
- Small nonpolar gases: Oxygen (O2), Carbon dioxide (CO2)
- Small polar molecules: Water (H2O), though less readily and often via channel proteins
Molecules That Diffuse Slowly or With Assistance
- Larger nonpolar molecules: Benzene, Ethylene (diffuse slower due to size)
- Small polar molecules: Ethanol (diffuse but slower than nonpolar gases)
Molecules That Do Not Easily Diffuse
- Large polar molecules: Glucose
- Charged molecules and ions: Na+, K+, Cl−, H+ (require active transport or facilitated diffusion)
Thus, simple diffusion across the plasma membrane favors small, nonpolar molecules and to a lesser extent, small polar molecules without charges.
