The Fluid Mosaic Membrane
The cell membrane is not a solid wall, it is a moving, jostling sea of phospholipids studded with proteins, like tiles drifting in a liquid floor. This "fluid mosaic" structure is what lets the membrane be a gatekeeper: flexible enough to move, selective enough to decide what gets in. Meet the components, and see how each one's structure gives it a job.
Get oriented
Make a prediction, read the goals and the key terms, then clear the trap before you start.
Before You Read
warm-upCooking oil and water do not mix. The cell membrane is built from molecules with both an oily part and a water-loving part.
Before reading: if you put molecules that are "oil at one end, water-loving at the other" into water, how do you think they would arrange themselves? Sketch or describe it.
Learning Intentions
goalsKnow
- The components of the fluid mosaic model
- The structure of a phospholipid bilayer
- What each membrane protein and molecule is
Understand
- Why "fluid mosaic" describes the membrane
- How each component's structure gives it a function
- Why the membrane is partially (selectively) permeable
Can Do
- Use the model to explain the role of each named component
- Label a fluid mosaic diagram
- Link membrane structure to its gatekeeping function
Scan these before reading
vocabMisconceptions To Fix
watch outBuild the first idea
The core of the dot point, then two checks you can answer straight from the card.
Core Content
The Phospholipid Bilayer
+5 XPWhy phospholipids form the membrane's "fluid" backbone
The membrane self-assembles because each phospholipid has one end that loves water and one that avoids it.
A phospholipid has a hydrophilic (water-loving) phosphate head and two hydrophobic (water-fearing) fatty-acid tails. In water, phospholipids automatically arrange into a bilayer: heads face the watery environment on both sides, tails point inward away from water, forming a hydrophobic core. This core lets small non-polar molecules (O₂, CO₂) and lipids pass easily, but blocks ions and large polar molecules, making the membrane partially (selectively) permeable. The bilayer is "fluid" because the phospholipids drift sideways.
Pause, copy the highlighted description of the bilayer and its selective permeability.
The phosphate head of a phospholipid is water-loving, or _____.
Two truths, one lie. Three of these statements about the phospholipid bilayer are correct. Which one is the lie?
Take it further
The next move, with the diagram to work from and two more checks.
Proteins in the Mosaic
+5 XPIntegral, peripheral, channels and glycoproteins
We just built the lipid backbone. That raises a question: if the core blocks ions, how do they ever cross? This card answers it → embedded proteins act as doorways and the membrane's working parts.
Proteins scattered through the bilayer give the "mosaic" its variety and do the jobs the lipid alone cannot.
Text description
- Phospholipid bilayer: each phospholipid has a hydrophilic head facing the watery outside or cytoplasm and two hydrophobic tails pointing into the core.
- Channel protein (an integral protein): spans the bilayer and has a water-filled pore through the hydrophobic core.
- Cholesterol: sits between the phospholipid tails.
- Glycoprotein: a protein with a branched carbohydrate chain on the outer surface, used for cell recognition.
- Integral protein: spans the whole bilayer.
- Peripheral protein: sits on one surface of the membrane (drawn here on the inner surface).
Integral proteins span the whole bilayer; many act as channels or carriers that transport ions and polar molecules across the membrane. A protein channel is a water-filled pore that lets specific ions or polar molecules pass through the hydrophobic core. Peripheral proteins sit on the surface and support the membrane and assist in signalling or enzyme activity. Glycoproteins (protein + carbohydrate) act in cell recognition and signalling, they are the cell's identity tags.
Pause, copy each protein component next to its function into your book.
Which component spans the bilayer and provides a water-filled pore for ions to cross?
Match each named membrane component to its function in the fluid mosaic model.
- Protein channel
- Integral protein
- Peripheral protein
- Glycoprotein
- Carries a carbohydrate tag used for cell recognition
- Spans the whole bilayer and transports substances across it
- A water-filled pore that lets specific ions and polar molecules cross
- Sits on one surface, supporting the membrane and assisting signalling
Work it yourself
Put the idea on paper in your own book before you read on.
Activity 1
Label and Explain the Model
Pattern A, Structured recall
Draw a labelled fluid mosaic membrane in your book. Then, for each of the six named components, phospholipids, protein channels, integral proteins, peripheral proteins, cholesterol and glycoproteins, write one sentence linking its structure to its function.
Finish the picture
The last piece of the dot point, then a true or false check across all three cards.
Cholesterol and the Working Whole
+5 XPStability, and why the whole model matters
We just placed the proteins. That raises a question: if the membrane is fluid, what stops it falling apart or going too stiff? This card answers it → cholesterol, the membrane's stabiliser.
A membrane that is too fluid would leak and a too-rigid one would crack, so cholesterol tunes its fluidity.
Cholesterol molecules sit between the phospholipid tails and stabilise the membrane and regulate its fluidity, keeping it from becoming too fluid when warm or too rigid when cold. Putting it all together, the fluid mosaic model explains how the membrane is a partially permeable gatekeeper: the lipid bilayer blocks most substances, while the embedded proteins provide selective routes in and out, and glycoproteins handle recognition, structure and function fit at every level.
Add cholesterol's fluidity-regulating role to your notes before the check.
Cholesterol helps regulate the fluidity and stability of the membrane.
Small non-polar molecules such as oxygen must use a protein channel to cross the membrane.
Glycoproteins act in cell recognition and signalling.
See where it goes
Where this shows up outside the classroom, and why the dot point is worth knowing.
Glycoproteins and blood groups
The reason you have a blood type, A, B, AB or O, comes down to glycoproteins on the surface of your red blood cells. The carbohydrate tags on these membrane proteins are exactly what the immune system reads as "self" or "foreign". A mismatched transfusion is dangerous because the recipient's antibodies recognise the donor's glycoprotein tags as foreign. The fluid mosaic model's recognition function is therefore a matter of life and death in transfusion medicine.
Use the fluid mosaic model to explain the structure and function of phospholipids, integral proteins, protein channels, peripheral proteins, cholesterol and glycoproteins in the cell membrane.
Two instructions are stacked here. Use the fluid mosaic model means the model must do work in your answer, not appear as a name; explain means every component needs a structural feature and the function that feature makes possible.
6 marks over six named components. Open with the model in one sentence, then give each component a sentence of the form "because it is … , it can … ".
The fluid mosaic model describes the membrane as a bilayer of phospholipids in which molecules drift laterally, giving it fluidity, with a varied population of proteins and other molecules embedded in or attached to it, giving it the patchwork appearance of a mosaic. It is this combination that makes the membrane both a stable barrier and a selectively permeable one.
Each phospholipid has a hydrophilic phosphate head and two hydrophobic fatty-acid tails, so in an aqueous environment the molecules self-assemble into a bilayer with the heads facing the water on both sides and the tails turned inward. The resulting hydrophobic core allows small non-polar molecules such as oxygen and carbon dioxide to dissolve through it, while blocking ions and large polar molecules, which is the origin of the membrane’s selective permeability.
Integral proteins span the full width of the bilayer, with hydrophobic regions anchoring them among the tails, so they provide a continuous route from one face to the other. Many act as carriers that bind a specific solute and change shape to move it across. A protein channel is an integral protein enclosing a water-filled pore lined with hydrophilic residues, which allows specific ions or polar molecules to pass through the hydrophobic core without ever contacting it.
Peripheral proteins are bound to one surface of the bilayer rather than spanning it, and they support the membrane structurally, act as enzymes, or relay signals inward from the surface. Cholesterol molecules pack between the fatty-acid tails and regulate fluidity, restricting movement at higher temperatures and preventing the tails packing tightly at lower ones, so the membrane stays workably fluid across a range of conditions. Glycoproteins are integral proteins carrying external carbohydrate chains, which project into the surroundings and act as recognition sites, allowing the immune system and signalling molecules to identify the cell.
Taken together the model explains the membrane’s central property: the fluid bilayer provides a self-sealing, impermeable barrier that keeps the cell’s contents in, while the mosaic of proteins provides the specific, regulated routes across it, so the cell controls which substances enter and leave rather than simply excluding them all.
The fluid mosaic model describes the membrane as a bilayer of phospholipids whose molecules drift laterally, giving fluidity, studded with a varied population of proteins and other molecules that gives it the appearance of a mosaic. Each phospholipid has a hydrophilic phosphate head and two hydrophobic fatty-acid tails, so in water the molecules self-assemble into a bilayer with heads outward and tails inward; the hydrophobic core that results lets small non-polar molecules such as oxygen and carbon dioxide dissolve through while blocking ions and large polar molecules, which is the origin of selective permeability. Integral proteins span the bilayer, anchored by hydrophobic regions among the tails, and provide a continuous route across it, many acting as carriers that bind a specific solute and change shape to transfer it. A protein channel is an integral protein enclosing a water-filled pore lined with hydrophilic residues, allowing specific ions or polar molecules to cross without contacting the hydrophobic core. Peripheral proteins are bound to one face only and provide structural support, enzyme activity or signal relay. Cholesterol packs between the fatty-acid tails and buffers fluidity in both directions, restricting movement when warm and preventing tight packing when cool, so the membrane remains workably fluid across a range of temperatures. Glycoproteins are integral proteins bearing external carbohydrate chains that project outward as recognition sites, allowing the immune system and signalling molecules to identify the cell. Together the model explains the membrane’s defining property: the fluid bilayer supplies a self-sealing impermeable barrier, while the protein mosaic supplies the specific regulated routes across it, so the cell controls what enters and leaves rather than excluding everything.
Justify the description of the cell membrane as a "fluid mosaic".
Justify means support a position with evidence and reasoning. The position is already given to you by the question, so every sentence should be evidence for the name — features of the membrane that the words "fluid" and "mosaic" were chosen to capture.
4 marks, two words to justify. Roughly two marks of evidence for "fluid" and two for "mosaic", and where possible say what the alternative would fail to explain.
The membrane is described as fluid because its components are not fixed in place: individual phospholipids drift laterally within their own layer, and many proteins move with them rather than being held in a lattice. This is supported by the membrane’s observed behaviour — it self-seals when punctured, it allows vesicles to fuse with it during exocytosis, and in cell-fusion experiments the surface proteins of two joined cells intermix within minutes, which could not happen in a rigid structure.
The membrane is described as a mosaic because the proteins embedded in and attached to the bilayer differ in size, position and function — integral proteins spanning it, channels enclosing pores, peripheral proteins on one surface, glycoproteins carrying external carbohydrate tags — so the surface is a patchwork of dissimilar pieces rather than a uniform sheet. Their distribution also differs between cell types, which is why a liver cell and a nerve cell are not interchangeable.
The name is justified because the two properties together explain what a membrane does that neither would explain alone: fluidity allows the membrane to bend, fuse, self-repair and let its components redistribute, while the mosaic of proteins provides the specific transport and recognition functions the lipid bilayer cannot perform. Earlier models, which pictured a static lipid sheet coated with a continuous protein layer, could explain neither vesicle fusion nor the selectivity of transport.
The membrane is described as fluid because its components are not fixed: phospholipids drift laterally within their layer and most proteins move with them rather than sitting in a rigid lattice. The evidence is behavioural — the membrane self-seals when punctured, vesicles fuse with it during exocytosis, and in cell-fusion experiments the surface proteins of two joined cells intermix within minutes, none of which a rigid structure permits. It is described as a mosaic because the embedded and attached proteins differ in size, position and function, with integral proteins spanning the bilayer, channels enclosing hydrophilic pores, peripheral proteins bound to one face and glycoproteins carrying external carbohydrate tags, so the surface is a patchwork of dissimilar pieces set in a common lipid background, and their distribution differs between cell types. The description is justified because the two properties together explain what neither explains alone: fluidity allows the membrane to bend, fuse, self-repair and redistribute its components, while the protein mosaic supplies the transport and recognition functions the lipid alone cannot. The earlier model of a static lipid sheet sandwiched between continuous protein layers could account for neither vesicle fusion nor the specificity of transport, which is what the fluid mosaic model was framed to explain.
Practise independently
Answer in your own words first. The model answers are for checking, not for copying.
Multiple Choice
+5 XPA fresh set drawn from this lesson's question bank, feedback shown immediately. +5 XP per correct · +25 XP all correct
Pick your answer, then rate your confidence, that tells the system what to drill next.
Short Answer, 12 marks
+5 XPUnderstandBand 5(6 marks) 1. Use the fluid mosaic model to explain the structure and function of the phospholipid bilayer, protein channels, cholesterol and glycoproteins.
1–2 marks: bilayer structure (heads/tails) and selective permeability · 1–2 marks: protein channels allow ions/polar molecules across · 1 mark: cholesterol regulates fluidity · 1 mark: glycoproteins for recognition
ApplyBand 4(3 marks) 2. Oxygen crosses the membrane straight through the bilayer, but a sodium ion cannot. Use the structure of the phospholipid bilayer to explain this difference.
1 mark: describes the hydrophobic core · 1 mark: explains why a small non-polar molecule passes · 1 mark: explains why a charged ion is blocked and needs a protein
AnalyseBand 5–6(3 marks) 3. Explain how cholesterol contributes to the function of the cell membrane, and predict what would happen to a membrane containing no cholesterol at high and at low temperature.
1 mark: states cholesterol's position and stabilising role · 1 mark: predicts the high-temperature effect · 1 mark: predicts the low-temperature effect
Show all answers
Multiple choice
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set from the lesson bank.
Short Answer Model Responses
Q1 (6 marks): The phospholipid bilayer consists of two layers of phospholipids, each with a hydrophilic phosphate head facing the watery environment and two hydrophobic fatty-acid tails facing inward to form a hydrophobic core. This makes the membrane partially permeable, small non-polar molecules pass through the core, but ions and large polar molecules cannot. Protein channels are integral proteins spanning the bilayer, providing water-filled pores so that specific ions and polar molecules can cross the hydrophobic core. Cholesterol molecules sit between the tails and regulate the membrane's fluidity, keeping it stable across a range of temperatures. Glycoproteins are proteins with attached carbohydrate chains that act as recognition and signalling sites, allowing cells to identify one another. Together these components make the membrane a fluid, partially permeable gatekeeper, with structure suiting function at every point.
Q2 (3 marks): The centre of the bilayer is a hydrophobic core formed by the fatty-acid tails [1]. Oxygen is a small non-polar molecule, so it dissolves in and passes straight through that core [1]. A sodium ion carries a charge and is repelled by the hydrophobic core, so it cannot cross the lipid directly and must instead pass through a protein channel, which provides a water-filled pore [1].
Q3 (3 marks): Cholesterol sits between the phospholipid tails and regulates fluidity, keeping the membrane stable across a range of temperatures [1]. Without it, at high temperature the phospholipids would move more freely and the membrane would become too fluid, so it would leak and lose its partial permeability [1]. At low temperature the tails would pack together and the membrane would become too rigid, so it would be brittle and its transport proteins could not work properly [1].
Retrieve what stuck
Work the spaced-review set first. It mixes this lesson with earlier ones on purpose.
Check what actually stuck
A short retrieval set drawn from this lesson and the ones before it, so the earlier work keeps coming back rather than fading.
Lock it in
bossYou can now use the fluid mosaic model to explain each membrane component's structure and function. Next lesson puts the membrane to work, explaining diffusion, osmosis and facilitated diffusion.
↩ Back to focus-area hubCompare it with where you started
Read your opening prediction again, then mark the lesson complete.
How did your thinking change?
At the start you predicted how "oil-at-one-end" molecules would arrange in water. They form a bilayer: hydrophilic heads facing the water on both sides, hydrophobic tails tucked inward. If you pictured the membrane as a solid sheet, revisit the misconception box, it is fluid, with components drifting within it.