2.3.5Additional lessons
The Fluid Mosaic Model: Skills — IB Biology SL Revision Notes & Topical Questions
5 min read 4 parts Lesson 23 of 142
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.
Part 1 of 4The Model Explained
Fluid
- Fluid Phospholipids and many proteins can move sideways within their own layer, so the membrane is not rigid.
Mosaic
- Mosaic Proteins of different shapes and sizes are scattered through the phospholipid bilayer like pieces in a mosaic.
Main components
- Main components The membrane contains phospholipids, cholesterol, glycoproteins and glycolipids, and integral and peripheral proteins.
What it explains
- What it explains The model accounts for passive and active transport, cell-to-cell interaction and cell signalling.
Evidence
- Evidence Fused cells with labelled surface proteins showed these proteins mixing across the combined membrane, so proteins can move.
Cholesterol
- Cholesterol In animal cells cholesterol sits between phospholipid tails and keeps the membrane fluid at low temperature without becoming too loose when warm.
Part 2 of 4Drawing Skills
Examination drawings should be clear, labelled and accurate rather than artistic.
1
Draw two parallel rows of phospholipids, each with a circular head and two tails, with tails pointing inward and heads outward.
2
Add integral proteins that span the bilayer, such as a channel or carrier, and peripheral proteins attached to the surface.
3
Add glycoproteins with a short branched carbohydrate chain on the outer face, and cholesterol between phospholipid tails.
4
Label each part clearly with a ruled line touching the structure, and avoid arrowheads on label lines.
5
Add the extracellular side and the cytoplasmic side if the question asks, and check that tails face each other.
Part 3 of 4Evaluating the Model
Evaluating the Model
Earlier model
- Earlier model Early models placed a continuous protein layer on both sides of the lipid layer, which could not explain how proteins span membranes.
Evidence for proteins
- Evidence for proteins Freeze-fracture electron microscopy showed proteins embedded in the bilayer, which supported the fluid mosaic arrangement.
Nature of science
- Nature of science Models change as new evidence arrives, and the accepted model is the one that best explains current observations.
Limitations
- Limitations A simple drawing does not show the density of proteins or the variation between membranes.
Part 4 of 4Exam Checklist & Tips
2.3.5 – Key Points for the Exam
- Fluid mosaic model The model of membranes as a fluid phospholipid bilayer containing a mosaic of proteins.
- Cholesterol A lipid in animal membranes that helps regulate fluidity.
- Integral protein A protein embedded in or spanning the bilayer.
- Peripheral protein A protein that attaches to the membrane surface.
Common Mistakes
- Drawing only a single layer of phospholipids rather than a bilayer.
- Putting label lines that end in an arrowhead or cross over each other.
- Placing glycoproteins on the inner surface of the membrane.
- Saying "fluid" means the membrane is a liquid that flows away, when it means parts can move within the layer.
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