Fluid‑Mosaic Model – Cambridge AS & A Level Biology (9700)
Five functional roles required by the syllabus
| Function | Phospholipids | Cholesterol | Glycolipids | Integral proteins | Peripheral proteins | Glycoproteins |
|---|
| Stability | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ |
| Fluidity | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ |
| Permeability | ✓ | ✓ | ✓ | ✓ | – | – |
| Transport | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ |
| Signalling / Recognition | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ |
1. Syllabus definition (Topic 4.1)
- The fluid‑mosaic model describes a phospholipid bilayer in which the hydrophobic tails face inward and the hydrophilic (polar) heads face the aqueous interior and exterior.
- The bilayer is asymmetric: the outer leaflet contains more glycolipids and sphingolipids, whereas the inner leaflet is richer in phosphatidylserine and phosphatidylethanolamine.
- Proteins, cholesterol, glycolipids and glycoproteins are interspersed, giving the membrane a “fluid” character and a “mosaic” of different molecules.
2. Core description of the model
- Each phospholipid is amphipathic – a polar head‑group (e.g., choline, serine) and two non‑polar fatty‑acid tails.
- Lateral movement of lipids and proteins within the plane of the membrane provides fluidity; occasional flip‑flop of lipids is rare and requires enzymes (flippases, floppases).
- The “mosaic” arises from the heterogeneous distribution of integral, peripheral and anchored molecules.
3. Molecular components and their five syllabus functions
| Component | Structural role | Functions (stability · fluidity · permeability · transport · signalling/recognition) |
|---|
| Phospholipids | Form the basic bilayer matrix; amphipathic heads outward, tails inward. | - Stability: Hydrophobic core holds the membrane together.
- Fluidity: Degree of tail saturation (cis‑double bonds = kinks → more fluid; saturated = tighter packing → rigid).
- Permeability: Allows passive diffusion of small non‑polar molecules (O₂, CO₂); excludes ions and large polar solutes.
- Transport: Provides the matrix in which carrier and channel proteins operate.
- Signalling/recognition: Specific head‑groups (e.g., phosphatidylserine) act as docking sites for peripheral signalling proteins.
|
| Cholesterol | Intercalates between phospholipid tails, predominantly in the inner leaflet but also present in the outer leaflet. | - Stability: Reduces brittleness; prevents membrane rupture under mechanical stress.
- Fluidity: “Fluidity‑buffer”: at high temperature it restrains phospholipid movement; at low temperature it prevents tight packing.
- Permeability: Decreases passive leakiness to small water‑soluble molecules.
- Transport: Organises lipid‑raft micro‑domains that concentrate particular transport proteins.
- Signalling/recognition: Lipid rafts host many receptors and signalling complexes.
|
| Glycolipids | Located mainly in the outer leaflet; a lipid anchor covalently linked to one or more carbohydrate chains. | - Stability: Form hydrogen bonds with extracellular water, stabilising the outer surface.
- Fluidity: Bulky carbohydrate heads hinder tight packing, giving a modest increase in fluidity.
- Permeability: Contribute to the barrier against desiccation.
- Transport: Frequently reside in lipid‑raft platforms that aid localisation of carrier proteins.
- Signalling/recognition: Provide cell‑cell recognition sites (e.g., ABO blood‑group antigens) and act as receptors for bacterial toxins.
|
| Integral (intrinsic) proteins | Span the bilayer, usually as one or more α‑helical transmembrane segments; may be single‑pass or multi‑pass. | - Stability: Anchor the bilayer to the cytoskeleton or extracellular matrix.
- Fluidity: Their lateral movement is modulated by cholesterol and cytoskeletal interactions.
- Permeability: Form pores or channels that selectively allow ions, water or small molecules to cross.
- Transport: Function as channel proteins (e.g., ion channels) or carrier (transport) proteins (e.g., GLUT).
- Signalling/recognition: Act as receptors (GPCRs, RTKs, ion‑channel‑linked receptors) and as adhesion molecules (integrins, selectins).
|
| Peripheral (extrinsic) proteins | Bind loosely to the membrane surface via interactions with integral proteins or phospholipid head‑groups. | - Stability: Link the membrane to the cytoskeleton or extracellular matrix.
- Fluidity: Association is dynamic, allowing rapid assembly/disassembly.
- Permeability: Do not form pores but can modulate the activity of nearby channel proteins.
- Transport: Often act as enzymes (e.g., ATPases) that drive active transport.
- Signalling/recognition: Serve as scaffolds for intracellular signalling cascades (e.g., adaptor proteins, kinases).
|
| Glycoproteins | Proteins (integral or peripheral) covalently linked to one or more carbohydrate chains. | - Stability: Carbohydrate chains increase hydration, protecting the protein core from proteolysis.
- Fluidity: Bulky glycans can hinder lateral diffusion of the protein.
- Permeability: No direct effect, but large extracellular glycans can sterically block access to underlying lipids.
- Transport: Many act as receptor‑mediated transporters (e.g., GLUT glucose transporters, insulin receptor).
- Signalling/recognition: Form the extracellular domain of most surface receptors; provide antigenic determinants for immune recognition.
|
4. Membrane stability & fluidity
- Temperature: Higher temperature increases kinetic energy, reducing van der Waals forces and increasing fluidity.
- Fatty‑acid composition: Cis‑double bonds create kinks → less tight packing → greater fluidity; saturated chains pack tightly → rigidity.
- Cholesterol content: Acts as a fluidity‑buffer (see above).
5. Permeability
Passive diffusion across the lipid bilayer depends on:
- Size: Molecules ≤ 500 Da cross more readily.
- Polarity: Non‑polar gases (O₂, CO₂) diffuse rapidly; polar solutes (glucose, amino acids) require transport proteins.
- Charge: Ions are excluded unless a specific channel is present.
6. Transport mechanisms
6.1 Simple diffusion
Movement of small non‑polar molecules down their concentration gradient directly through the phospholipid core (e.g., O₂, CO₂).
6.2 Facilitated diffusion (carrier proteins)
- Binding site on one side of the membrane.
- Conformational change transports the solute.
- No energy input; moves down a concentration gradient.
- Example: GLUT1 glucose transporter.
6.3 Osmosis
Diffusion of water through specialised channel proteins (aquaporins) or directly through the bilayer, driven by an osmotic gradient.
6.4 Active transport (carrier proteins)
- Requires energy (ATP hydrolysis or coupling to an ion gradient).
- Moves solutes against their concentration gradient.
- Example: Na⁺/K⁺‑ATPase (3 Na⁺ out, 2 K⁺ in per ATP).
6.5 Bulk transport
- Endocytosis – invagination of the plasma membrane to form a vesicle that enters the cytoplasm.
- Pinocytosis – “cell drinking”, uptake of fluid and dissolved solutes.
- Phagocytosis – “cell eating”, engulfment of large particles (e.g., bacteria).
- Receptor‑mediated endocytosis – ligand binds a surface receptor → clathrin‑coated pit formation.
- Exocytosis – fusion of intracellular vesicles with the plasma membrane to release contents outside the cell or to insert membrane proteins/lipids.
7. Cell‑signalling pathway (Ligand → Receptor → Response)
- Ligand release: Hormone, neurotransmitter or growth factor is secreted by a signalling cell.
- Transport to target: Diffuses through extracellular fluid; polar ligands may travel bound to carrier proteins.
- Receptor binding: Ligand binds the extracellular domain of a membrane glycoprotein receptor (GPCR, RTK or ion‑channel‑linked receptor).
- Signal transduction: Conformational change activates intracellular partners (G‑protein, kinase cascade, or ion flux).
- Cellular response: Alters gene expression, enzyme activity, ion concentrations or cytoskeletal arrangement.
Glycoprotein receptors provide specificity of ligand recognition and protect the protein core from proteolysis.
8. Cell‑recognition & antigens
- Carbohydrate chains on glycolipids and glycoproteins constitute the major cell‑surface antigens.
- Functions:
- Self‑non‑self discrimination by the immune system.
- Blood‑group determination (ABO antigens).
- Cell‑cell adhesion during tissue formation (selectins, integrins).
- The high variability of glycoconjugates provides a vast repertoire of recognition patterns.
9. Summary table – functional themes (Cambridge syllabus)
| Theme | Key components | Representative syllabus functions |
|---|
| Stability | Phospholipids, Cholesterol, Glycolipids, Integral & Peripheral proteins, Glycoproteins | Maintain structural integrity; prevent rupture under mechanical stress. |
| Fluidity | Phospholipid tail saturation, Cholesterol, Glycolipids, Integral proteins | Allow lateral movement of proteins; act as a fluidity‑buffer at varying temperatures. |
| Permeability | Phospholipid bilayer, Channel proteins, Aquaporins | Regulate passive diffusion of gases, water and small non‑polar molecules. |
| Transport | Channel proteins, Carrier proteins, Endocytosis, Exocytosis, Glycoprotein receptors | Simple diffusion, facilitated diffusion, osmosis, active transport, bulk uptake/release of material. |
| Cell signalling | Glycoprotein receptors (GPCR, RTK, ion‑channel), Peripheral signalling proteins | Detect extracellular ligands, trigger intracellular cascades, produce a physiological response. |
| Cell recognition | Glycolipids, Glycoproteins (antigens) | Immune identification, blood‑group antigens, tissue‑specific adhesion. |
Exam tip (AO1 & AO2)
- AO1 (knowledge): Memorise the five functional roles and be able to list which membrane components contribute to each.
- AO2 (application/evaluation): When answering a question on membrane function, first name the component, then explicitly link it to the relevant function(s) using the tick‑mark table as a quick reference. For example: “Cholesterol stabilises the membrane and buffers fluidity; it also forms lipid‑raft platforms that concentrate signalling receptors (AO2).”