B1.1 Crossing the Lipid Bilayer
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What determines if a molecule can cross the lipid bilayer?
Size, polarity, and charge.
Which molecules diffuse freely through the bilayer?
Small non-polar molecules (O₂, CO₂) and lipid-soluble molecules (steroids).
Which molecules cross slowly or need help?
Small polar molecules (H₂O, ethanol) — may use aquaporins.
Which molecules cannot cross unaided?
Large polar molecules (glucose) and charged ions (Na⁺, Cl⁻).
How do ions cross the membrane?
Via ion channels (facilitated diffusion) or ion pumps (active transport).
Why can steroids like testosterone cross easily?
They are non-polar and lipophilic.
Keywords
diffusion, aquaporins, facilitated diffusion, active transport, lipophilic, non-polar, selective permeability.