Bilayers as barriers at Higher Level: The same statement as SL, used all through HL transport: gated channels, pumps and cotransporters exist because the bilayer holds ions back. HL data uses liposomes, the bubbles of bilayer used to carry drugs.
Practise this as you read
- Explain why ions and polar molecules hardly cross a bilayer.
- Read a log-scale chart of leakage or permeability.
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The big idea: The middle of a membrane is oily: hydrophobic hydrocarbon chains. Particles that are large, charged or polar cannot get through it, so the membrane keeps two watery solutions apart.
The points to remember
- The core of the bilayer is made of the hydrocarbon chains of the fatty acid tails: it is hydrophobic.
- It has a low permeability to large molecules, such as glucose and proteins.
- It has a low permeability to hydrophilic particles: ions (Na⁺, K⁺, Cl⁻) and polar molecules.
- Small, non-polar molecules, such as oxygen and carbon dioxide, pass through easily.
- So a membrane is an effective barrier between two aqueous (watery) solutions: the cytoplasm and the fluid outside the cell.
Remember it as: Small and non-polar slips through; large, polar or charged is held back.
Real example: myelin: Myelin is many layers of membrane wrapped round a nerve fibre. Its stacked hydrophobic cores stop ions leaking across, so it insulates the fibre.
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The big idea: Data questions show how fast different substances cross a bilayer with no proteins. Read the pattern: size first, then polarity and charge.
How to read it
- The scale is logarithmic: each grid line is 100 times more than the last.
- Small or non-polar molecules (ethanol, water) leak out in less than a second.
- Larger polar molecules (glucose, sucrose) take months; ions take thousands of years.
- So the longer the time, the lower the permeability: the bilayer is a barrier to ions above all.
Do not say water cannot cross: Water is polar, but it is very small, so a little does cross the bilayer. Say it crosses slowly.
How it is asked: This idea is the reason behind most transport questions: why a particle needs a protein to cross. It is asked as a short Explain, or with data on liposomes or artificial bilayers.
Myelin consists of many layers of membrane. Explain why myelin stops ions from leaking across the membrane of a nerve fibre.
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And a two-mark data question:
Half the urea in a liposome leaks out in about 10 hours, but half the potassium ions take millions of years. Explain this difference.
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See the answer broken into clear points, with guidance on how each point earns marks.