Fluid mosaic membrane structure
BiologyΒ· Unit 4: Cell Membranes and TransportΒ· 15 min read
1. Phospholipids and the bilayerβ β ββββ± 5 min
Phospholipid bilayer
The fundamental structure of all cell membranes, formed by two layers of phospholipid molecules. Hydrophilic phosphate heads orientate outwards towards aqueous environments, while hydrophobic fatty acid tails orientate inwards away from water.
Example:
Most biological bilayers are 7-10 nm thick.
Phospholipids are amphipathic, meaning they have both hydrophilic (water-loving) and hydrophobic (water-hating) regions. This property drives spontaneous bilayer formation in aqueous environments, as this arrangement is the most energetically stable.
Explain why phospholipids form bilayers in aqueous environments.
- 1
- Phospholipids are amphipathic: they have a hydrophilic phosphate head and two hydrophobic fatty acid tails.
- 2
- Any arrangement that exposes hydrophobic tails to water is energetically unfavourable, so monolayers are not stable.
- 3
- Bilayer arrangement solves this problem: all hydrophilic heads interact with water on either side of the membrane, and all hydrophobic tails are protected in the core.
Exam tip:
Always mention the amphipathic property of phospholipids when explaining bilayer formation, this is a required marking point.
2. Key features of the fluid mosaic modelβ β ββββ± 4 min
Fluid mosaic model
The model of cell membrane structure proposed by Singer and Nicolson, describing a dynamic phospholipid bilayer with a scattered mosaic of embedded and surface proteins.
Example:
Most components can move laterally within the bilayer, giving the membrane its fluid character.
Earlier models (such as the Davson-Danielli model) incorrectly proposed a continuous protein coat on both sides of the bilayer. Evidence from electron microscopy and freeze-fracture studies confirmed that proteins are embedded within the bilayer core, leading to the acceptance of the fluid mosaic model.
Explain why the model is described as both 'fluid' and 'mosaic'.
- 1
- 'Fluid' refers to the dynamic nature of the membrane: most phospholipids and many proteins can move laterally within their layer of the bilayer.
- 2
Fluidity depends on temperature, fatty acid saturation and the presence of cholesterol.
- 3
- 'Mosaic' refers to the arrangement of proteins: proteins are scattered throughout the bilayer, varying in size and position, creating a mosaic-like pattern when viewed from above.
3. Membrane proteins: types and rolesβ β β βββ± 5 min
Membrane proteins are classified by their position in the bilayer, and carry out most of the specific functions of cell membranes.
Intrinsic and extrinsic proteins
Intrinsic (integral) proteins are embedded within the hydrophobic core of the bilayer, many spanning the entire membrane (transmembrane proteins). Extrinsic (peripheral) proteins are located on the surface of the bilayer, bound loosely to phospholipid heads or intrinsic proteins.
Transport: Channel and carrier proteins move polar/charged molecules that cannot diffuse through the hydrophobic core.
Enzymes: Catalyse specific metabolic reactions at the membrane surface.
Cell recognition: Glycoproteins act as cell markers/antigens for cell-to-cell recognition.
Receptors: Bind specific signalling molecules to trigger intracellular responses.
Distinguish between intrinsic and extrinsic membrane proteins.
- 1
- Position: Intrinsic proteins are embedded deep within the hydrophobic core of the bilayer, often spanning the entire membrane. Extrinsic proteins are only found on the membrane surface.
- 2
- Interactions: Intrinsic proteins have large hydrophobic regions that interact with fatty acid tails to anchor them in place. Extrinsic proteins only interact via hydrophilic bonds with surface components.
- 3
- Extraction: Intrinsic proteins cannot be removed without disrupting the bilayer, while extrinsic proteins can be easily washed off the membrane surface.
4. Cholesterol, glycoproteins and glycolipidsβ β β βββ± 5 min
In addition to phospholipids and proteins, cell membranes contain other key components with specialised roles.
Component | Structure | Core Function |
|---|---|---|
Cholesterol | Steroid lipid between phospholipid tails | Regulates membrane fluidity and stability |
Glycoproteins | Protein with extracellular carbohydrate chain | Cell-cell recognition, antigen formation |
Glycolipids | Phospholipid with extracellular carbohydrate chain | Cell recognition, membrane stability |
Describe the dual role of cholesterol in mammalian cell membranes.
- 1
- Cholesterol fits between the fatty acid tails of phospholipids in the bilayer core.
- 2
- At high temperatures: Cholesterol restricts phospholipid movement, reducing membrane fluidity and preventing excessive permeability.
- 3
- At low temperatures: Cholesterol stops phospholipid tails from packing tightly together, maintaining fluidity and preventing the membrane from solidifying.
- 4
- It also increases mechanical strength of the membrane, preventing rupture.
5. Common Pitfalls
Wrong move:
Calling phospholipids 'amphoteric' instead of 'amphipathic'.
Why:
Amphoteric refers to acid-base behaviour, not dual hydrophobic/hydrophilic structure.
Correct move:
Always use the term amphipathic to describe phospholipids.
Wrong move:
Reversing the hydrophilic/hydrophobic regions of phospholipids.
Why:
This leads to an incorrect explanation of bilayer arrangement, losing all linked marks.
Correct move:
Remember: phosphate heads (polar) are hydrophilic, fatty acid tails (non-polar) are hydrophobic.
Wrong move:
Claiming all intrinsic proteins span the entire bilayer.
Why:
Some intrinsic proteins are only partially embedded in the core, so this definition is inaccurate.
Correct move:
Define intrinsic proteins as any proteins embedded within the hydrophobic bilayer core.
Wrong move:
Stating cholesterol only reduces membrane fluidity at all temperatures.
Why:
Examiners specifically test for the dual role of cholesterol, so you will lose a mark for missing the low temperature function.
Correct move:
Always mention both roles: reduces fluidity at high temperatures, prevents solidification at low temperatures.
Wrong move:
Confusing the terms 'fluid' and 'mosaic'.
Why:
This is a common basic error that costs easy marks in exams.
Correct move:
Remember: Fluid = movement of components, Mosaic = scattered arrangement of proteins.
6. Quick Reference Cheatsheet
Component | Key Function |
|---|---|
Phospholipid bilayer | Basic membrane structure, barrier to polar molecules |
Intrinsic protein | Transport, enzymes, cell signalling receptors |
Extrinsic protein | Structural support, cell signalling |
Cholesterol | Regulates membrane fluidity and stability |
Glycoprotein | Cell-cell recognition, antigen formation |
Glycolipid | Cell recognition, membrane stability |
When this came up on past exams
AI-estimated based on syllabus patterns β cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2022 Β· 2
Draw and label fluid mosaic model
- 2023 Β· 1
Function of cholesterol in membranes
- 2021 Β· 2
Distinguish intrinsic/extrinsic proteins
What's Next
Understanding fluid mosaic membrane structure is the foundation for all subsequent topics in cell membranes and transport, including diffusion, osmosis, facilitated diffusion and active transport. This model is central to explaining how molecules move across membranes, how cells communicate with each other, and how membrane permeability changes in different physiological conditions. Many CIE A-Level exam questions combine knowledge of membrane structure with questions about transport processes, so a solid grasp of this topic is essential for scoring full marks in both multiple choice and structured paper questions.