BackMembrane Permeability: Diffusion of Molecules Across the Cell Membrane
Study Guide - Smart Notes
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Membrane Permeability
Overview of Diffusion Across the Cell Membrane
The cell membrane is selectively permeable, allowing certain molecules to diffuse freely while restricting others. The ability of a molecule to diffuse across the membrane depends on its size, charge, and polarity.
Factors Affecting Membrane Permeability
Size: Small molecules diffuse more easily than large molecules.
Charge: Uncharged (nonionic) molecules cross more readily than charged (ionic) molecules.
Polarity: Nonpolar (hydrophobic) molecules diffuse more easily than polar (hydrophilic) molecules.
Classification Table: Molecule Permeability
The following table summarizes which types of molecules can or cannot freely diffuse across the cell membrane:
CAN Freely Diffuse | Uncharged | Nonpolar/Hydrophobic |
|---|---|---|
Small molecules (e.g., O2, CO2, N2) | Yes | Yes |
Cannot Freely Diffuse | Large | Polar/Hydrophilic or Charged (+/-) |
Large molecules (e.g., proteins, polysaccharides) | No | No |
Charged ions (e.g., Na+, Cl-) | No | No |
Polar molecules (e.g., glucose, amino acids) | No | No |
Key Points
Small, uncharged, nonpolar (hydrophobic) molecules can freely diffuse through the lipid bilayer of the cell membrane.
Large, charged, or polar (hydrophilic) molecules cannot freely diffuse and require transport proteins or channels.
Examples of molecules that can diffuse freely: Oxygen (O2), carbon dioxide (CO2), nitrogen (N2), and small lipids.
Examples of molecules that cannot diffuse freely: Ions (Na+, K+, Cl-), glucose, amino acids, and large proteins.
Additional Information
Facilitated diffusion and active transport are required for the movement of molecules that cannot freely diffuse across the membrane.
The fluid mosaic model describes the structure of the cell membrane, emphasizing the role of lipids and proteins in selective permeability.
