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[Biochemistry](https://durgical.com/en/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

- [Cystinuria](https://durgical.com/en/conditions/cystinuria/)
- [Hartnup disease](https://durgical.com/en/conditions/hartnup-disease/)

## Related

- [Phospholipids](https://durgical.com/en/biochemistry/topics/phospholipids/)
- [Phosphatidylcholine (lecithin)](https://durgical.com/en/biochemistry/molecules/phosphatidylcholine/)
- [Sphingomyelin](https://durgical.com/en/biochemistry/molecules/sphingomyelin/)
- [Cholesterol](https://durgical.com/en/biochemistry/molecules/cholesterol/)
- [Membrane transport](https://durgical.com/en/biochemistry/topics/membrane-transport/)
- [Cell organelles and their metabolism](https://durgical.com/en/biochemistry/topics/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.

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Educational information, not medical advice. For any symptom, see a health professional.

Sources: [Phospholipid metabolism](https://reactome.org/PathwayBrowser/#/R-HSA-1483257) (Reactome, 2026); [Membrane Trafficking](https://reactome.org/PathwayBrowser/#/R-HSA-199991) (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.

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