Models of membrane structure and the evidence for them

Membranes, Proteins, DNA and Gene Expression · Gas exchange, membranes and transport · note 4 of 8

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Models of membrane structure and the evidence for themSpec 2.2

In short

The fluid mosaic model is a scientific model of the membrane: a flexible phospholipid bilayer in which protein molecules are scattered like tiles in a mosaic and can move sideways. Like any model it is an interpretation of data, and it replaced earlier ideas when new evidence was found.

Scientists cannot see molecules in a membrane directly. A model is an explanation built from the data available. When new data do not fit, the model is changed or replaced. This is how the fluid mosaic model came to be accepted.

How data have been interpreted
Evidence	What it showed	How it was interpreted
Early electron micrographs of membranes	Two dark lines with a lighter band between them.	Taken to show a layer of protein on each side of a lipid layer, an early 'sandwich' model with continuous protein layers.
Freeze-fracture electron microscopy	Bumps (protein molecules) scattered within the lipid layers, which split apart between the two layers.	Proteins are embedded within the bilayer, and not in continuous layers on the surface. This supports the mosaic.
Biochemical analysis of membrane proteins	Many proteins have large hydrophobic regions and are not soluble in water.	Hard to explain if all proteins lay on the surface; consistent with proteins spanning the hydrophobic interior.
Cell fusion experiment: a mouse cell and a human cell fused and their surface proteins labelled with different fluorescent markers	At 37 °C the two labels mixed completely over about 40 minutes. At low temperature, mixing was greatly slowed.	Proteins can move sideways in the membrane, so the membrane is fluid. Slower movement at low temperature fits with the lipid becoming more rigid.

The model is named for these two ideas: the bilayer is fluid because the phospholipids and many proteins move, and mosaic because the proteins are scattered through the bilayer in a pattern like the tiles of a mosaic.

Exam tip:

In 'how does the data support the model' questions: first state what the data show (with a value if the data allow), then say what that means about the membrane, then name the feature of the model it supports.

Two membrane models side by side, the earlier sandwich model with continuous protein layers around a lipid bilayer and the fluid mosaic model with proteins embedded in or spanning the bilayer, plus a cell-fusion experiment in which fluorescent labels on human and mouse proteins are separate at time 0 and mixed after about 40 minutes at 37 degrees Celsius. — The sandwich model against the **fluid mosaic model**. In the cell-fusion experiment the labels mix, so proteins can move within the bilayer.

Written and checked against the Edexcel International AS Level Biology (XBI11) specification · Updated October 2026

Frequently asked questions

What are the adaptations of the alveoli for gas exchange?

Alveoli give a large surface area to volume ratio through their huge number. Their walls and the capillary walls are one cell thick, so diffusion distance is short. A dense capillary network and ventilation maintain steep concentration gradients, and elastic fibres help air to be breathed out.

What is Fick's law of diffusion?

Fick's law says the rate of diffusion is proportional to the surface area multiplied by the concentration difference, divided by the thickness of the exchange surface. A large, thin surface with a steep gradient therefore gives rapid diffusion, which is why gas exchange surfaces have all three features.

What is the fluid mosaic model of the cell membrane?

The fluid mosaic model describes a membrane as a phospholipid bilayer in which proteins are scattered like a mosaic. It is fluid because the phospholipids and many proteins can move sideways. It is a model built from experimental evidence, such as freeze-fracture images and cell-fusion experiments.

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