Quick Scoop

Small, nonpolar molecules like oxygen (O₂) and carbon dioxide (CO₂) can easily diffuse across a plasma membrane, along with very small polar molecules like water and ethanol. These molecules slip through the phospholipid bilayer without needing any special transport proteins because they can dissolve in the hydrophobic (water-repelling) interior of the membrane.

How Membrane Permeability Works

The plasma membrane acts as a selective barrier that controls what enters and exits the cell. Think of it like a velvet rope at an exclusive club—only certain molecules get past easily, and the decision is based on two main factors: size and polarity.

The phospholipid bilayer that forms the membrane has a hydrophobic (fatty) core, which creates a natural barrier against polar and charged molecules. Only molecules that can dissolve in this oily interior can pass through freely via a process called passive diffusion.

Molecules That Diffuse Easily

The membrane is most welcoming to these types of molecules:

  • Gases : Oxygen (O₂), carbon dioxide (CO₂), and nitrogen (N₂) cross rapidly due to their small size and nonpolar nature
  • Small hydrophobic molecules : Benzene and other nonpolar organic compounds slip through effortlessly
  • Very small polar molecules : Water (H₂O) and ethanol can diffuse through despite being polar, thanks to their tiny size

These molecules move down their concentration gradient—from areas of high concentration to areas of low concentration—without requiring any energy or assistance from membrane proteins.

Molecules That Cannot Diffuse Easily

On the flip side, the membrane blocks several types of molecules that need specialized transport proteins to cross:

  • Large polar molecules : Glucose and other larger uncharged polar molecules cannot dissolve in the hydrophobic bilayer
  • Ions and charged molecules : Sodium (Na⁺), potassium (K⁺), chloride (Cl⁻), and hydrogen ions (H⁺) have extremely low permeability despite their small size
  • Macromolecules : Proteins, RNA, and other large biological molecules cannot pass through at all

Interestingly, even slightly larger polar molecules like urea and glycerol have significantly lower permeability compared to gases. Ions are particularly restricted—sodium and potassium ions have permeability coefficients around 10⁻¹⁴ cm/s, which is astronomically low compared to oxygen at 10¹ cm/s.

The Bottom Line (TLDR)

Small, nonpolar gases (O₂, CO₂) and very small polar molecules (water, ethanol) easily diffuse across plasma membranes through passive diffusion. Larger polar molecules like glucose and all charged molecules (ions) cannot cross freely and require specialized transport proteins to enter or exit cells. The key determinants are molecular size and polarity—smaller and less polar means easier passage through the membrane's hydrophobic core.

Information gathered from public forums or data available on the internet and portrayed here.