Cell membranes: the fluid mosaic model

Phospholipid bilayer

A phospholipid has a hydrophilic (water-loving) phosphate head and two hydrophobic (water-hating) fatty acid tails. In water they form a bilayer: heads face the watery cytoplasm and tissue fluid, tails point inwards.

The hydrophobic core stops ions and polar molecules passing freely, but small non-polar molecules (O2, CO2) and lipid-soluble substances can diffuse through.

The fluid mosaic model

Proposed by Singer and Nicolson in 1972:

• Fluid: phospholipids and many proteins move about within the layer.
• Mosaic: proteins are scattered through it. Intrinsic proteins span the bilayer (channels and carriers); extrinsic proteins sit on the surface.
• Cholesterol sits between phospholipids and regulates fluidity and stability.
• Glycoproteins and glycolipids (with carbohydrate chains) act as receptors and in cell recognition.

Building models from evidence

Earlier models (such as a protein-lipid-protein sandwich) were replaced as new evidence appeared: freeze-fracture electron micrographs showed proteins embedded within the bilayer, and experiments fusing two cells with differently labelled membrane proteins showed the proteins mixed within minutes, so the membrane must be fluid. Scientific models change when evidence does not fit them.

Worked example

Explain why oxygen can cross the membrane freely but sodium ions cannot.

1. Oxygen is small and non-polar, so it passes through the hydrophobic core.
2. Na+ is charged, so it is repelled by the hydrophobic tails and needs a channel or carrier protein.

Answer: Small non-polar oxygen dissolves through the tails; charged ions need proteins.

Check yourself

1. Which part of a phospholipid faces the watery cytoplasm?
   • The hydrophilic phosphate head
   • The hydrophobic fatty acid tails
   • The glycerol only
   • The cholesterol
   Show answer

   The hydrophilic phosphate head. Heads are attracted to water.

2. What does 'fluid' mean in the fluid mosaic model?
   • The membrane is made of water
   • The membrane is liquid at all temperatures
   • Phospholipids and proteins can move within the membrane
   • Proteins are fixed in place
   Show answer

   Phospholipids and proteins can move within the membrane. Components move laterally within the bilayer.

3. What is the role of cholesterol in the cell membrane?
   • It regulates fluidity and stability
   • It acts as a channel for ions
   • It is a receptor for hormones
   • It carries out active transport
   Show answer

   It regulates fluidity and stability. Cholesterol sits between phospholipids, controlling fluidity.

4. Which membrane components are involved in cell recognition?
   • Fatty acid tails
   • Phosphate heads only
   • Cholesterol only
   • Glycoproteins and glycolipids
   Show answer

   Glycoproteins and glycolipids. Carbohydrate chains act as recognition sites and receptors.

5. Which substance diffuses most easily through the phospholipid bilayer?
   • Glucose
   • Carbon dioxide
   • Sodium ions
   • Amino acids
   Show answer

   Carbon dioxide. CO2 is small and non-polar.

6. Two cells with differently labelled membrane proteins are fused, and the labels mix within minutes. What does this show?
   • The membrane has no proteins
   • Proteins are fixed in a rigid layer
   • Phospholipids are hydrophobic
   • Membrane proteins can move, so the membrane is fluid
   Show answer

   Membrane proteins can move, so the membrane is fluid. Mixing shows lateral movement of proteins.

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Sources

• Pearson Edexcel International Advanced Level in Biology specification (XBI11/YBI11, first teaching 2018)

Last updated 7 November 2026. Spotted a mistake? Tell us.