4.1 Fluid Mosaic Membranes
- Syllabus
- 9700–2028–2029
- Topic
- 4.1
- Level
- AS
Learning objectives
4.1.1Fluid mosaic membrane model• Fluid mosaic model: hydrophobic/hydrophilic interactions in phospholipid bilayer and protein arrangement4.1.2Membrane component arrangement• Arrangement of cholesterol, glycolipids, glycoproteins4.1.3Role of membrane components• Roles of membrane components:- Phospholipids, cholesterol: stability, fluidity, permeability- Proteins: transport (carrier/channel proteins), cell signalling (receptors), cell recognition (antigens)- Glycolipids and glycoproteins4.1.4Cell signalling process• Cell signalling process:- Secretion of ligands from cells- Transport to target cells- Binding to cell surface receptors
The fluid mosaic model is a moving selective bilayer
The fluid mosaic model describes a cell membrane as a phospholipid bilayer containing different proteins. It is a flexible, moving interface rather than a static wall: the bilayer forms the basic boundary and the components together control exchange and communication.
- Bilayer arrangement: Hydrophilic phosphate heads face the aqueous environments on either side, while hydrophobic fatty-acid tails face inward to form the membrane core.
- Fluid: Phospholipids and many membrane proteins can move laterally within the membrane, although some proteins may be held in position.
- Mosaic: Different proteins are distributed through the bilayer in a scattered pattern, giving a mosaic appearance; their hydrophilic and hydrophobic regions determine where they sit.
- Selective barrier: The hydrophobic core makes passage difficult for many polar substances and ions, while membrane proteins can provide controlled routes for exchange or communication.
- Compartmentalisation: Bilayers can enclose the cell or internal organelles, separating reaction environments without making the membrane immovable.
Amphipathic phospholipids self-organise into a bilayer, the hydrophobic core limits unassisted passage, and embedded or surface-associated proteins add selective functions. Because components can move and be arranged differently, the membrane can remain continuous while adapting its exchange and communication roles.
“Fluid” does not mean that the membrane dissolves or that every component moves freely in every direction. “Mosaic” does not mean that the arrangement is meaningless or wholly random. Selective permeability is produced by lipid chemistry plus the position and function of membrane proteins.
Cell-surface membrane components connect structure to function
A cell-surface membrane contains several components whose positions and chemical properties give the membrane its barrier, transport, stability and recognition functions. The phospholipid bilayer is the base; cholesterol, proteins and carbohydrate-containing molecules add specialised roles.
- Phospholipids: Their hydrophilic heads face the aqueous surroundings and hydrophobic tails face inward, forming the bilayer and a barrier to many water-soluble substances. Individual molecules can move within their monolayer.
- Cholesterol: It fits between phospholipids with its hydrophilic region near the surface and hydrophobic region among the tails. This helps regulate fluidity, limits excessive permeability and adds stability.
- Membrane proteins: Intrinsic proteins are embedded in one or both monolayers; many span the membrane as transmembrane proteins. Extrinsic proteins associate with the inner or outer surface. Transport proteins provide selective routes for particular ions or polar molecules.
- Glycolipids and glycoproteins: Carbohydrate chains project from the outer surface. They can act as receptors and cell markers, supporting specific binding and cell-to-cell recognition.
The hydrophobic phospholipid core blocks many polar substances, while specific transmembrane proteins provide controlled hydrophilic routes for substances that cannot cross the core unaided. Cholesterol tunes how tightly the lipid region behaves, and outward-facing carbohydrate chains expose molecular identities for recognition and receptor binding.
Not every membrane protein is a transport protein, and carbohydrate chains are not distributed equally on both sides: glycolipids and glycoproteins have their carbohydrate chains facing the extracellular side. This component card explains the parts and their jobs; the overall fluid-mosaic definition belongs to the neighbouring model card.
Each membrane component contributes a specific role
The cell-surface membrane combines phospholipids, cholesterol, glycolipids, proteins and glycoproteins. Their positions and chemistry create stability, controlled fluidity and permeability, selective transport, signalling and cell recognition.
| Component | Required roles linked to structure |
|---|---|
| phospholipids | form the bilayer; hydrophobic core limits passage of ions and many polar molecules while allowing some lipid-soluble molecules through |
| cholesterol | fits between phospholipid tails; stabilises the membrane, limits excessive permeability and buffers fluidity across temperature changes |
| channel proteins | provide selective hydrophilic pores for particular ions or polar molecules |
| carrier proteins | bind specific solutes and change shape to move them across the membrane |
| receptor proteins | bind specific extracellular ligands and initiate cell signalling |
| glycolipids and glycoproteins | expose carbohydrate chains as cell-surface antigens and recognition markers; some glycoproteins also act as receptors |
The lipid core supplies the default permeability barrier. Channels provide continuous hydrophilic routes, while carriers alternate access after specific binding. These proteins therefore allow selective transport without making the bilayer freely permeable.
Do not reduce every membrane protein to transport: receptors signal, and glycoproteins can support recognition. Cholesterol does not simply make the membrane rigid; it stabilises and buffers fluidity while also reducing permeability.
Cell signalling runs from ligand secretion to a specific response
Cell signalling links a source cell to a target cell: a specific chemical ligand is secreted, transported to cells, binds only to a complementary receptor on a target cell and triggers a specific response.
- A signalling cell secretes a specific chemical ligand.
- The ligand is transported away from the source cell through an extracellular fluid or other transport medium.
- Only target cells with a complementary cell-surface receptor bind that ligand.
- Binding changes the receptor and initiates an intracellular signalling change; the ligand need not cross the bilayer.
- The target cell produces a specific response, such as changing enzyme activity, membrane transport or gene expression.
Secretion makes the signal available, transport brings it to potential targets, and receptor complementarity selects which cells respond. The receptor then transduces extracellular binding into an internal change, explaining why a neighbouring cell without the receptor does not respond.
A receptor is not automatically a channel or carrier, and the ligand does not need to enter the cell in this surface-receptor pathway. Do not omit secretion or transport: receptor binding is the third major stage, not the beginning of the full signalling sequence.