2.3.5Inquiry Process
The Fluid Mosaic Model: Skills — IB Biology HL Revision Notes & Topical Questions
5 min read 6 parts Lesson 51 of 160
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 6Components of the Model
The model was proposed by Singer and Nicolson in 1972.
Phospholipids
- Phospholipids The bilayer forms the basic fabric and gives the membrane its fluidity.
Cholesterol
- Cholesterol Sits between phospholipids in animal membranes and modifies their fluidity and permeability.
Proteins
- Proteins Integral and peripheral proteins are scattered through and across the bilayer, giving a mosaic pattern.
Carbohydrates
- Carbohydrates Glycolipids and glycoproteins project from the outer surface.
| Term | Meaning in the model |
|---|---|
| Fluid | Phospholipids and many proteins can move sideways |
| Mosaic | Proteins are scattered like tiles among phospholipids |
Section 2 of 6Drawing a Membrane
Diagrams are expected to be clear, labelled and correctly oriented.
1
Draw two parallel rows of phospholipids with round heads on the outside and tails pointing inward.
2
Add integral proteins that cross the bilayer and peripheral proteins on the surface.
3
Add cholesterol molecules between phospholipids in the hydrophobic region.
4
Add carbohydrate chains on the outer surface attached to proteins and lipids.
5
Label the extracellular and cytoplasmic sides and each component.
Section 3 of 6Evidence Supporting the Model
Evidence Supporting the Model
Protein mobility
- Protein mobility In 1970 Frye and Edidin fused mouse and human cells and found that labelled proteins mixed across the hybrid cell surface within about 40 minutes.
Freeze-fracture
- Freeze-fracture Electron micrographs of split membranes show proteins as particles embedded in the bilayer, not as a continuous layer.
Membrane proteins vary
- Membrane proteins vary Proteins differ in size and shape and have hydrophobic regions, which fits being embedded.
Fluorescence recovery
- Fluorescence recovery Bleached patches of fluorescently labelled membrane recover as unbleached molecules diffuse in.
Section 4 of 6Earlier Model
Davson-Danielli model
- Davson-Danielli model Proposed a phospholipid bilayer sandwiched between continuous layers of protein.
Problem
- Problem Many membrane proteins are hydrophobic in part, so they would not sit on the surface in water.
Electron microscope images
- Electron microscope images Freeze-fracture showed globular proteins within the membrane, contradicting the sandwich structure.
Science process
- Science process The newer model replaced the older one because it better explained the evidence.
Section 5 of 6Worked Example
Question
- Mouse and human cells were fused and their surface proteins labelled with different fluorescent markers.
- At the start the colours were separate, but after 40 minutes at 37 °C they had mixed.
- What does this show?
- The proteins moved through the membrane from their starting positions.
- Movement in the membrane requires the lipid layer to be fluid, not fixed.
- This supports the fluid mosaic model and contradicts the idea of rigid protein layers.
Answer Membrane proteins can diffuse laterally, so the membrane is fluid.
Section 6 of 6Exam Checklist & Tips
2.3.5 – Key Points for the Exam
- Fluid mosaic model A model of the membrane as a flexible bilayer with proteins scattered within it.
- Cholesterol A lipid in animal membranes that regulates fluidity.
- Freeze-fracture An electron microscopy technique that splits membranes to show internal structure.
- Integral protein A protein embedded in the bilayer.
Common Mistakes
- Saying the membrane is a fixed layer of protein and fat.
- Drawing cholesterol on the surface rather than between tails.
- Mixing the order of the two models, thinking Davson-Danielli is the current one.
- Forgetting to label which side of the membrane is outside.
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