The fluid mosaic model describes the cell membrane as a flexible, dynamic phospholipid bilayer containing a varied arrangement of proteins, glycoproteins and cholesterol. It is required for both IB Biology SL and HL under B2.1 Membranes and membrane transport.
How the Model Works
Phospholipids are amphipathic: each has a hydrophilic phosphate head and hydrophobic fatty-acid tails. In water, they form a bilayer with the heads facing the aqueous cytoplasm and extracellular fluid, while the tails face inward, away from water.
This arrangement creates a hydrophobic core that has low permeability to ions, polar molecules and large molecules. Small non-polar molecules such as oxygen and carbon dioxide can cross by simple diffusion, while many other substances require membrane proteins.
| Component | Position and function |
|---|---|
| Phospholipids | Form the bilayer and move laterally, making the membrane fluid and flexible. |
| Integral proteins | Embedded in one or both lipid layers; some act as channels, pumps or receptors. |
| Peripheral proteins | Attached to the inner or outer membrane surface. |
| Glycoproteins | Proteins with carbohydrate chains that contribute to cell recognition, including ABO antigens. |
| Cholesterol | Located between phospholipids in animal cell membranes. |
The model is called fluid because phospholipids and some proteins can move laterally within the membrane. It is called mosaic because different proteins and other components form an irregular, varied pattern within the bilayer.
A common misconception is that the membrane is a rigid wall or that its proteins are fixed in place. Instead, it is a dynamic, selectively permeable boundary whose components can move and perform different functions.
Exam Technique
For an IB diagram, draw two phospholipid layers and clearly label hydrophilic heads, hydrophobic tails, integral proteins, peripheral proteins, glycoproteins and cholesterol. When asked to explain the model, connect structure to function: the hydrophobic core forms a barrier, while specific proteins enable selective transport and signalling.
