01What passes and what doesn't
Small nonpolar molecules (O₂, CO₂, N₂) and lipid-soluble molecules (steroid hormones) slip directly through the bilayer.
Small polar molecules like water pass slowly; cells speed it up with aquaporins.
Ions (Na⁺, K⁺, Cl⁻) and large polar molecules (glucose) cannot cross the hydrophobic core on their own — they need transport proteins.
02The cell wall is different
Plants (cellulose), fungi (chitin) and most bacteria (peptidoglycan) have a cell wall outside the membrane. It is porous and does not control what enters — its job is structural support and preventing bursting in hypotonic water.
03Notebook box
Permeability ranking: small nonpolar > small uncharged polar > large polar > ions.
Worked example: A new drug is a small, nonpolar molecule. Will cells need a transporter to take it up? → No; it can diffuse straight through the phospholipid core.