Cell Biology
Lipid Bilayer
Foundation of cell membranes — phospholipids self-assemble into two-layer structure
The lipid bilayer is the basic structure of cell membranes. Composed of phospholipids — molecules with hydrophilic (water-loving) heads and hydrophobic (water-fearing) tails. In water, they self-assemble: heads face water on both surfaces; tails point inward forming hydrophobic interior. Selectively permeable: small/nonpolar molecules pass; charged/large molecules need transporters. Plus: cholesterol (modulates fluidity), proteins (~50% by mass, do most functions), carbohydrates (cell recognition). Fluid mosaic model: dynamic, lateral movement. Foundation of all cell membranes — plasma, organelle.
- CompositionPhospholipids (most), cholesterol, proteins, carbs
- Width~5-10 nm
- PhospholipidHydrophilic head + 2 hydrophobic tails
- Self-assemblyHeads to water; tails inward (hydrophobic effect)
- Fluid mosaicDynamic; lateral movement; flip-flop rare
- PermeabilitySelective; small/nonpolar pass; charged/large need transporters
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Why bilayer matters
- Cell biology. Foundation of all membranes.
- Pharmacology. Drug crossing membranes.
- Vaccines. Lipid nanoparticles for mRNA delivery.
- Membrane transport. Selective permeability.
- Signaling. Receptors at membrane.
- Origin of life. Bilayer enabled compartmentalization.
- Disease. Membrane disorders (cystic fibrosis).
Common misconceptions
- Bilayer is solid. Fluid, dynamic.
- Bilayer is purely lipid. Many proteins, cholesterol.
- Both leaflets identical. Asymmetric — different lipids on each side.
- Anything passes through. Selective permeability.
- Lipids are inert. Active in signaling, structure.
- Membrane is just a barrier. Site of much cellular activity.
Frequently asked questions
How does the bilayer form?
Self-assembly via hydrophobic effect. Phospholipids in water: hydrophilic heads attracted to water; hydrophobic tails repelled. Tails cluster together; heads face water. Bilayer is most stable arrangement (vs micelles, vesicles). Spontaneous — given right conditions, lipids self-organize. Drives all biological membrane structure.
What's a phospholipid?
Molecule with: glycerol backbone + 2 fatty acid tails + phosphate head group. Tails: hydrocarbon chains (typically 14-22 C); hydrophobic. Head: phosphate + variable group (choline, ethanolamine, serine, inositol); hydrophilic. Charge: negative phosphate; some heads also charged. Different phospholipids: different membrane properties.
What's the fluid mosaic model?
Singer and Nicolson (1972). Membrane is fluid (lateral diffusion of lipids and proteins) and mosaic (heterogeneous protein distribution). Lipids: rapid lateral movement (~1 µm/s). Flip-flop (across bilayer): rare. Proteins: lateral diffusion + some restricted regions. Modern: more complex with lipid rafts, asymmetry, microdomains.
How does cholesterol affect membranes?
Cholesterol embedded in bilayer. At normal T: increases packing → less fluid; reduces permeability. At low T: prevents membrane rigidification; maintains fluidity. Animal cells: 25-50% of membrane lipid. Plant cells: phytosterols (sitosterol, stigmasterol). Bacteria: hopanoids. Cholesterol provides flexibility across temperatures.
What about membrane proteins?
~30-50% of membrane mass. Many functions. (1) Transporters: bring molecules across (channels, pumps). (2) Receptors: signaling (G-protein coupled, growth factor, etc.). (3) Enzymes: catalysis on membrane surface. (4) Anchors: attach cytoskeleton. (5) Cell adhesion: integrins. Proteins do most actual membrane functions; lipids provide barrier.
What's selective permeability?
Small nonpolar molecules (O₂, CO₂, N₂) pass freely (slow). Small polar uncharged (H₂O, urea) pass slowly. Large or charged molecules: don't pass without transporters. Selective: transporters specific to molecules (glucose transporters for glucose). Active transport: uses energy. Foundation of cellular control over what enters/leaves.
What happens at very low temperatures?
Bilayer becomes rigid (gel state). Lipid tails crystalline; less fluid. Functions (transport, signaling) impaired. Cells with cholesterol: protected somewhat. Cold-adapted organisms: more unsaturated fatty acids in membranes (kinks prevent close packing → fluid at low T). Adaptation strategy.