Membrane Permeability

The cell membrane is not a wall and it is not an open door — it is a filter. Its phospholipid bilayer has a hydrophobic core, an oily interior that welcomes some molecules and blocks others. Small, nonpolar molecules like O₂ and CO₂ slip straight through; small uncharged polar molecules like water squeeze across slowly; but large polar molecules and every ion are stopped cold at that oily middle. The membrane's structure decides who crosses, and that decision is the whole point of a selectively permeable boundary.

What gets shut out is not abandoned — it is handed to a protein. Glucose, amino acids, and ions cannot dissolve through the hydrophobic core, so the membrane studs itself with channel and carrier proteins that open a hydrophilic path across. This is why permeability is a property of the whole membrane, not of the lipid alone: a molecule the bilayer refuses may still cross freely wherever the right transport protein sits. Read a molecule's size, charge, and polarity against the membrane's structure and you can predict its fate — free passage, protein-assisted passage, or no passage at all.

Sort each molecule by how it meets the membrane: free passage through the bilayer, protein-assisted passage through a channel or carrier, or no passage at all. Size, charge, and polarity on one side — the membrane's structure on the other.

The common mistake here is treating the membrane as one uniform gate: assuming that if a molecule is small it must cross, or that anything the bilayer blocks is blocked from the cell entirely. Both readings ignore that selective permeability comes from two structures working together — the lipid core that turns molecules away and the transport proteins that let the turned-away ones through. Every scenario in this topic asks the same thing: say why a molecule can or cannot cross, tracing its fate to the membrane feature it actually meets, not to size alone.