2.3.1Additional lessons
Lipid Bilayers — IB Biology SL Revision Notes & Topical Questions
5 min read 4 parts Lesson 20 of 142
Part 1 of 4Formation of the Bilayer
Phospholipids
- Amphipathic molecules with a polar, hydrophilic phosphate head and two non-polar, hydrophobic fatty acid tails.
Orientation in water
- Heads turn towards water while tails turn away, which is the arrangement of lowest energy.
Monolayer first
- At a water surface a single layer of phospholipids forms, with heads in the water and tails in the air.
Bilayer
- At sufficient concentration two sheets form, with tails together in the middle and heads on both outer surfaces.
Stability
- Hydrophobic interactions and attraction of heads to water hold the structure together without covalent bonding between phospholipids.
Hydrophobic effect
- Water molecules form hydrogen bonds with each other and exclude the non-polar tails, which drives the tails together.
Part 2 of 4Bilayer as a Barrier
Bilayer as a Barrier
Hydrophobic core
- The non-polar interior repels ions and large polar molecules, so they cannot diffuse through easily.
Small non-polar molecules
- Oxygen, carbon dioxide and lipid-soluble substances dissolve in the core and pass through by simple diffusion.
Water
- Small polar molecules such as water cross slowly through the bilayer, and faster where channels such as aquaporins are present.
Selective permeability
- Membrane proteins add control, allowing the cell to admit or exclude particular substances.
| Substance | Crosses bilayer directly? | Reason |
|---|---|---|
| Oxygen, CO₂ | Yes | Small and non-polar |
| Water | Slowly | Small and polar |
| Glucose | No | Large and polar, needs a carrier |
| Na⁺, K⁺, Cl⁻ | No | Charged ions repelled by the core |
Part 3 of 4Properties of the Bilayer
1
Phospholipids in each layer move sideways past one another, so the membrane is fluid.
2
Fluidity lets the membrane change shape, form vesicles during endocytosis and exocytosis, and fuse with other membranes.
3
Unsaturated fatty acid tails bend and increase fluidity, while longer saturated tails pack tightly and reduce it.
4
Cholesterol in animal cell membranes restricts movement at high temperature and stops tight packing at low temperature.
5
Embedded proteins add functions that the lipids alone cannot provide, such as transport and signalling.
Part 4 of 4Exam Checklist & Tips
2.3.1 – Key Points for the Exam
- Phospholipid bilayer A double layer of phospholipids that forms the basic structure of cell membranes.
- Selectively permeable Allowing some substances across a membrane while restricting others.
- Simple diffusion Passive movement of particles down a concentration gradient without a protein.
- Cholesterol A lipid in animal membranes that regulates fluidity.
Common Mistakes
- Drawing the hydrophilic heads facing inward toward the tails of the other layer.
- Saying water cannot cross the membrane at all; some does cross, especially through channels.
- Writing that small ions diffuse easily through the lipid core, which repels charged particles.
- Assuming the bilayer is rigid, when its phospholipids move laterally.
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