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: red blood cells: A red blood cell holds about 30 times more potassium than the plasma around it. The bilayer stops the charged ions from simply leaking out.
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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.
- Oxygen and carbon dioxide (small, non-polar) cross fastest.
- Glucose (large, polar) crosses about 100,000 times more slowly than water.
- Ions (sodium, chloride) are slowest of all: the core is almost a wall to charged particles.
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.
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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 a chart of permeabilities.
Explain why a cell membrane acts as a barrier between the cytoplasm and the tissue fluid around a cell.
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And a two-mark data question:
A bilayer with no proteins lets oxygen through about 10 million million times faster than sodium ions. Explain this difference.
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See the answer broken into clear points, with guidance on how each point earns marks.