0.0625
m/s
6.25
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0.0625
m/s
6.25
cm/s
The Membrane Permeability Calculator determines the permeability coefficient of a biological membrane for a given substance. Membrane permeability quantifies how easily a molecule can pass through a lipid bilayer and is determined by the molecule's ability to dissolve in the membrane (partition coefficient) and its mobility within the membrane (diffusion coefficient), relative to the membrane thickness.
This concept is central to pharmacology (drug absorption), physiology (ion and gas transport), and cell biology (nutrient uptake).
The permeability coefficient is calculated as:
P = D × K / Δx
A higher P means faster passive transport. Lipophilic (fat-soluble) molecules have high K values and permeate membranes easily, while charged or large polar molecules have low K and permeate poorly.
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Oxygen has high membrane solubility and diffuses rapidly through the 8 nm lipid bilayer.
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Glucose has a very low partition coefficient, making passive membrane permeation extremely slow. This is why cells need glucose transporters.
The partition coefficient (K) is the ratio of a substance's solubility in the lipid membrane to its solubility in water. A high K (greater than 1) means the substance dissolves readily in lipids and crosses membranes easily. A low K means the substance prefers water and has difficulty penetrating the hydrophobic core of the membrane.
Molecules with very low permeability coefficients (charged ions, large polar molecules like glucose and amino acids) cannot cross membranes fast enough by passive diffusion to meet cellular needs. Transport proteins (channels and carriers) provide alternative pathways that are faster and can be regulated by the cell.
Permeability is inversely proportional to membrane thickness. A thicker membrane presents a longer diffusion path, reducing the rate of transport. Biological membranes are typically 7-8 nm thick, but myelin sheaths and other specialized barriers are much thicker, greatly reducing permeability.
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