4.1.14 · Cell Membranes and Transport
The Fluid Mosaic Model — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions
5 min read 8 parts Lesson 3 of 20 Unit quiz
Lesson summary
4.1.1 The Fluid Mosaic Model — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe the fluid mosaic model and explain why membranes are called fluid mosaics.Presence — membranes are found in all cells.Cell surface membrane — separates the internal environment fro…
- Intracellular membranes
- Phospholipid bilayer
- First outlined
- Cell boundary
- Organelles
- Cholesterol
Part 1 of 6Role of membranes
Presence
- Presence membranes are found in all cells.
Cell surface membrane
- Cell surface membrane separates the internal environment from the external environment.
Intracellular membranes
- Intracellular membranes form compartments such as the nucleus, mitochondria and RER.
Control
- Control membranes are selectively permeable and control exchange and communication.
Part 2 of 6The model
Phospholipid bilayer
- Phospholipid bilayer roughly 7 nm wide, just visible under an electron microscope at very high magnification.
First outlined
- First outlined in 1972.
Explains
- Explains passive and active movement, cell-to-cell interactions and cell signalling.
Part 3 of 6Phospholipid arrangements in water
Monolayer
- Monolayer a single layer at the surface, with heads in the water and tails up.
Micelle
- Micelle a sphere with heads facing out and tails facing in.
Bilayer
- Bilayer two layers in sheets, with tails facing inwards, forming the cell membrane.
Part 4 of 6Compartmentalisation
Compartmentalisation
Cell boundary
- Cell boundary the bilayer forms the cell surface membrane.
Organelles
- Organelles membrane-bound compartments allow specialised processes.
Lysosome
- Lysosome keeps hydrolytic enzymes separate, otherwise they would break down cellular components.
Part 5 of 6Why fluid and why mosaic
Fluid
- Fluid phospholipids and proteins move by diffusion, mainly sideways within their own layer.
Icebergs
- Icebergs proteins move within the bilayer a bit like icebergs in the sea, though some are fixed.
Mosaic
- Mosaic the scattered pattern of proteins looks like a mosaic from above.
Proteins
- Proteins intrinsic proteins are embedded and extrinsic proteins lie on the surfaces.
Part 6 of 6More from the specification
Amphipathic lipid bilayer polar heads
- Amphipathic lipid bilayer polar heads hydrophilic polar phosphate head groups orient toward intra- and extracellular aqueous environments, flanking hydrophobic fatty acid tails.
Ligand
- A signalling molecule that binds a specific receptor.
Receptor
- A cell that detects a stimulus and generates an impulse.
Cholesterol
- Cholesterol a small molecule with a dual hydrophilic and hydrophobic character that sits between phospholipids and regulates how fluid the membrane is.
Glycoproteins and glycolipids
- Glycoproteins and glycolipids proteins and lipids with carbohydrate chains on the outer surface, used in cell recognition and signalling.
Key terms
| Term | Meaning |
|---|---|
| Fluid mosaic model | A model of the membrane as a fluid bilayer with scattered proteins. |
| Selectively permeable | Allowing only some substances to pass. |
| Monolayer | A single layer of phospholipids at a water surface. |
| Micelle | A sphere of phospholipids in water. |
| Intrinsic protein | A protein embedded in the bilayer. |
| Extrinsic protein | A protein on the bilayer surface. |
Avoid these mistakes
1
Saying membranes are static — phospholipids and proteins move.
2
Saying phospholipids flip frequently between layers — they mainly move sideways within a layer.
3
Mixing up the monolayer and bilayer — a monolayer forms at the water surface.
4
Saying the model was proposed recently — it dates from 1972.
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