Membrane permeability

The lipid bilayer of cell membrane is semi-permeable, which means that some molecules can pass through the membrane through diffusion. The rate of diffusion varies depending on the molecule size, polarity, charge and concentration on the inside of the membrane versus the concentration on the outside of the membrane. Below are some molecules permeabilities through a lipid bylayer in the absence of all transmembrane proteins. In other words, this is permeability through the lipid bylayer itself without any help from pumps or ion channels:

Hydrophobic Molecules - highly permeable

- O2 - Oxygen
- N2 - Nitrogen

Small uncharged Polar Molecules - less permeable

- H2O - Water
- urea
- glycerol

Large Uncharged Polar Molecules - even less permeable

- Glucose
- Sucrose

Ions - non-permeable

- H+ - Hydrogen ion
- Na+ - Sodium ion
- K+ - Potassium ion
- Ca2+ - Calcium ion
- Cl- - Chloride ion


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