International A Level Biology / Membranes and transport
The fluid mosaic model revision notes
Edexcel International A Level Biology (YBI11), chapter 4: Membranes and transport, specification point 2.2. Free lesson.
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The fluid mosaic model describes a cell surface membrane as a phospholipid bilayer with proteins and other molecules in it. Its structure explains how the membrane is partially permeable and able to change shape, while evidence explains why this model replaced the Davson-Danielli model.
Why phospholipids form a bilayer
Water is present outside the cell and in the cytoplasm inside it. Each phospholipid has a hydrophilic phosphate head, which is attracted to water, and two hydrophobic fatty acid tails, which are repelled by water. This makes the phospholipids arrange themselves in two layers, with the heads facing the water on each side and the tails pointing towards one another. The tails form a hydrophobic core in the middle of the membrane. Ions and most polar molecules cannot cross this core, so the membrane is partially permeable: some substances can cross, but others cannot.
What is found in the fluid mosaic model
The phospholipid bilayer contains proteins and other molecules. The name mosaic refers to the different molecules scattered through the membrane. The carbohydrate chains on glycoproteins and glycolipids face outwards, away from the cytoplasm. Intrinsic proteins are embedded in the bilayer and often pass right through it. Channel and carrier proteins are examples that move substances across the membrane. Extrinsic proteins attach to just one surface and do not reach through the hydrophobic core. Glycoproteins are proteins with carbohydrate chains. Many act as receptors, for…
How evidence changed the membrane model
A model is an interpretation of the data available at the time. In 1925, lipid extracted from red blood cells covered about twice the cells' surface area when spread on water. This suggested that membranes were two lipid layers thick. In 1935, Davson and Danielli proposed a sandwich model: a phospholipid bilayer coated by continuous protein layers on both surfaces. Electron micrographs showed two dark lines with a pale band between them. This seemed to fit the sandwich model. Freeze fracture splits the membrane between its layers. The exposed inner faces had particles, showing that…
How the models compare
Both models include a phospholipid bilayer, but they differ in how they describe proteins and movement. State both models when making a comparison, so your answer clearly shows the difference. Davson-Danielli has continuous protein layers coating both surfaces; the fluid mosaic model has proteins scattered, with many embedded in or passing through the bilayer. Davson-Danielli is a fixed structure; in the fluid mosaic model, phospholipids and many proteins can move. Freeze fracture supports the fluid mosaic model because it reveals proteins inside the bilayer, which the Davson-Danielli model…
Why the membrane is fluid
The membrane is fluid because phospholipids move relative to each other within their layer. Many proteins can move too. This lets the membrane bend and change shape, such as when a white blood cell engulfs a bacterium. The property that allows the membrane to change shape is fluidity. The fluid mosaic model is the name of the model, not the property.