Lesson 1 of 4 / Membranes and transport
The fluid mosaic membrane
How can a membrane be a barrier and a communication surface?
I can explain membrane constituents and surface/internal membrane functions.
I can explain the fluid mosaic membrane and constituent molecules.Syllabus
Syllabus 9477, Core 1(j). Explain the fluid mosaic membrane and constituent molecules.
I can explain surface and internal membrane functions.Syllabus
Syllabus 9477, Core 1(k). Explain surface and internal membrane functions.
About 6 min
Make a guess. You are not marked.
The cell membrane is called "fluid". Does that mean substances can flow freely across it?
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Molecules move sideways within it; crossing stays selective.
Fluid describes lateral movement of lipids and proteins. The hydrophobic core still restricts ions and polar molecules.
The key idea
A mobile phospholipid bilayer forms the barrier; embedded molecules provide selective transport, recognition and signalling.
Explore the idea
Find the molecule, explain its role
Polar heads face water on each side; non-polar tails face inward. This hydrophobic interior restricts ions and many polar molecules. Fluidity means lateral movement within the membrane, not unrestricted passage across it.
Original schematic. Molecular sizes and distances are illustrative, not to scale.
Explanation
Phospholipids arrange with hydrophilic heads towards water and hydrophobic tails inward. The non-polar interior restricts ions and many polar substances, allowing different compositions on each side. Many lipids and proteins can move laterally, so the membrane is fluid rather than a rigid wall.
Integral proteins extend into or across the bilayer; peripheral proteins associate with its surface. Channels and carriers enable selective transport, receptors bind signals, and enzymes catalyse reactions. Different membranes contain different proportions and types of proteins because they carry out different functions.
Glycoproteins and glycolipids have carbohydrate chains on the extracellular face of the cell surface membrane. They contribute to recognition, adhesion and receptor functions. Cholesterol fits between phospholipids in animal membranes, restraining excessive movement at higher temperatures and preventing overly tight packing at lower temperatures; it buffers fluidity rather than simply always increasing it.
Internal membranes create compartments with suitable conditions and concentrate reaction components. For example, lysosomes contain hydrolytic enzymes, while mitochondrial inner membranes provide surfaces for electron carriers and ATP synthase. A membrane is therefore both a separating boundary and an organised site of biological activity.
Step by step
- 1
Locate the molecule
Place polar heads, non-polar tails and proteins correctly.
- 2
Name its property
Distinguish a hydrophobic barrier from a selective binding site.
- 3
Connect to a function
Use an example of surface exchange or internal compartmentalisation.
Worked example
Work through the evidence
Why does removing membrane proteins reduce uptake of a polar solute while oxygen still crosses?
One way to explain it
The polar solute may require a specific carrier or channel, whereas oxygen can dissolve in and diffuse through the hydrophobic bilayer.
Why this answer works
- Different substances use different routes.
- A transport observation does not show that every protein has the same function.
Watch out for this
A student says: "Fluid means all molecules freely cross the membrane." What is wrong with this?
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Fluid describes lateral movement within the membrane; permeability across it remains selective.
Check your understanding
Which part generally faces the cytosol?
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Hydrophilic phospholipid heads
Heads face the aqueous cytosol and extracellular fluid; tails point inward.
Exam-style questions
Check your understanding
Which statements about the fluid mosaic model are correct? 1: Phospholipids can move sideways within each layer. 2: Glycoproteins face the cytoplasm. 3: Cholesterol helps regulate fluidity. 4: Channel proteins give a hydrophilic route for ions.
Show the answer
1, 3 and 4 only
Lateral movement, cholesterol and channel proteins are all part of the model. Carbohydrate chains of glycoproteins face outwards, so 2 is false.
Quick recall
How can a membrane be a barrier and a communication surface?