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AP Bio



Unit 2

2.5 Membrane Permeability

1 min readjune 8, 2020



Thanks to the structure of the membrane, with the hydrophobic tails and hydrophilic heads, the cellular membrane has selective permeability. This allows some substances to cross easily, while others may not be able to cross or may require a special transport protein to do so.
The membrane acts like a barrier separating the inside of the cell from the external environment of the cell.
Small, non-polar molecules are able to freely cross the cell membrane, while polar or charged molecules require transport proteins to cross. If a molecule is small, polar, and uncharged (like water!) it may be able to pass through the membrane in small quantities but requires a transport protein to move across in any larger quantities.
The hydrophobic fatty acid tails are what controls the movement of substances described above. They repel charged and polar molecules and make it very challenging for them to come across.

Diffusion Across the Plasma Membrane


Image courtesy of WikiMedia Commons.

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