Biochemistry · Topic
Membrane structure !
The cell membrane is a lipid bilayer with embedded proteins that move within it, like a fluid mosaic.
Updated 2026-11-01
Key facts
- Fluid mosaic model (Singer and Nicolson): a phospholipid bilayer with integral and peripheral proteins that diffuse sideways.
- Rough composition: lipids and proteins in similar amounts, plus carbohydrates on the outer face (glycocalyx).
- Fluidity rises with short, unsaturated fatty acids and with temperature; cholesterol buffers it (less fluid when warm, prevents stiffening when cold).
- Asymmetry: glycolipids and sugars face outward only; phosphatidylserine faces inward. Flippases keep this layout using ATP.
- Lipid rafts: microdomains rich in cholesterol and sphingolipids that cluster receptors.
In detail
- 1. Non-polar tails hide from water in the middle (≈ 3 nm hydrophobic core); polar heads face water on each side.
- 2. Lipids move sideways quickly and almost never from one leaflet to the other (flip-flop), except with enzymes.
- 3. Integral proteins span the bilayer with hydrophobic α-helices (receptors, channels, carriers). Peripheral proteins attach on the surface by ionic bonds.
- 4. Some proteins are anchored by lipids (GPI, myristoyl, prenyl groups).
- 5. The glycocalyx (glycoproteins and glycolipids) defines the ABO blood groups and takes part in cell recognition.
- 6. Organelle membranes differ: the inner mitochondrial membrane is rich in cardiolipin and impermeable to protons; myelin is rich in sphingolipids and cholesterol.
- Also called
- Fluid mosaic model
Clinical notes
- In hereditary spherocytosis, red cell membrane skeleton proteins (spectrin, ankyrin) fail and the cell loses membrane.
- In paroxysmal nocturnal haemoglobinuria, GPI-anchored proteins (CD55, CD59) are missing and complement lyses red cells.
- General anaesthetics and alcohol change the fluidity of neuronal membranes.
Related conditions
Related
- Phospholipids
- Phosphatidylcholine (lecithin)
- Sphingomyelin
- Cholesterol
- Membrane transport
- Cell organelles and their metabolism
Test yourself
Think of the answer, then tap the question to see it.
What does cholesterol do to membrane fluidity?
It buffers it: lowers fluidity when warm and keeps the bilayer from stiffening when cold.
On which face of the membrane are the carbohydrates?
Only the outer face, as glycoproteins and glycolipids of the glycocalyx.
What is missing in paroxysmal nocturnal haemoglobinuria?
The GPI anchor: without CD55 and CD59 on the membrane, complement destroys red cells.
Educational information, not medical advice. For any symptom, see a health professional.
Sources: Phospholipid metabolism (Reactome, 2026); Membrane Trafficking (Reactome, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).
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Representative model, for educational purposes only.