What you need to know
- 2.2(i–ii)Membrane structure, properties and the interpretation of evidence through models.
Core Practical 3 (2.3), testing effects of alcohol and temperature on membrane permeability, remains a separate practical lesson.
By the end, you should be able to…
- Relate phospholipidA lipid made of glycerol, two fatty acids and a phosphate group. It has a hydrophilic head and two hydrophobic tails. and protein structure to membrane properties.
- Explain the fluid mosaic modelThe model of membrane structure: proteins are scattered through a phospholipid bilayer, and both can move sideways..
- Evaluate evidence for a membrane model.
Build your understanding
A bilayer with two different environments
A phospholipid has a polarHaving an uneven distribution of charge, so the molecule has partially charged ends. Polar molecules dissolve in water., hydrophilic phosphate-containing head and non-polarHaving no separated charges, so the molecule does not attract water and does not dissolve well in it., hydrophobic fatty acid tails. In water, phospholipidA lipid made of glycerol, two fatty acids and a phosphate group. It has a hydrophilic head and two hydrophobic tails. arrange in a bilayer: heads face the aqueous environments on both sides, while tails face inward, away from water.
The hydrophobic interior restricts the passage of ions and many polar molecules. Small non-polar molecules, including oxygen and carbon dioxide, cross relatively readily. This creates selective permeability rather than a barrier that is either completely open or completely closed.
A fluid mosaic
Many phospholipids and some proteins can move laterally within the membrane, giving it fluidity. Different proteins are embedded in or associated with the bilayer, forming a mosaic. Some proteins span it; others lie on one surface. Movement can be restricted by interactions with other structures, so not every protein moves freely.
Channel and carrier proteinA membrane protein that binds a specific molecule and changes shape to move it across the membrane, in facilitated diffusion or active transport. provide selective transport routes. Receptors bind particular signalling molecules. Carbohydrate chains on glycoproteinA protein with carbohydrate chains attached. In membranes, glycoproteins act as receptors and as antigens for cell recognition. and glycolipidA lipid with a carbohydrate chain attached, found on the outer surface of membranes and used in cell recognition. project from the outer surface and contribute to recognition and interactions between cells.
cholesterolA lipid found in cell membranes. High levels of LDL cholesterol in the blood are a risk factor for atheroma. lies among phospholipids in animal cell surface membraneThe partially permeable membrane around a cell that controls which substances enter and leave.. It affects packing, permeability and fluidity: it restrains movement at higher temperatures and helps prevent close packing at lower temperatures. Its effect should not be reduced to “always increases fluidity” or “always decreases fluidity”.
NOT: cholesterol does not always increase fluidity, and the membrane is not full of open holes.
- Heads face water on both sides; tails face inward
- Fluid: sideways movement. Mosaic: scattered proteins
- Ions need a channel or carrier; charge blocks the hydrophobic core
- Glycoproteins on the outer surface help recognition
Two traps this diagram blocks
- Do not draw a single layer of phospholipids for the cell-surface membrane.
- Recovery of fluorescence shows that some labelled molecules can move. It does not show that every protein is freely mobile.
From the diagram alone
Question
Explain why a sodium ion does not cross a phospholipid bilayer readily, and how a membrane protein can allow it. [3]
Show a model answer
The ion is charged, so entering the hydrophobic fatty-acid core is unfavourable. A suitable channel or carrier provides an alternative route through the membrane. The bilayer itself is not a set of holes open to every small substance.
Make your reasoning visible
Label both sides correctly
In a bilayer diagram, hydrophilic heads face water on both surfaces; hydrophobic tails face the interior. Do not draw a single layer for a cell surface membrane.
Connect evidence to the model
Describe the observation, then explain which feature it supports or challenges. Do not claim that one experiment proves every detail of a model.
Key explanations
- The hydrophobic core makes it difficult for ions and many polarHaving an uneven distribution of charge, so the molecule has partially charged ends. Polar molecules dissolve in water. molecules to cross.
- The membrane is fluid because many phospholipidA lipid made of glycerol, two fatty acids and a phosphate group. It has a hydrophilic head and two hydrophobic tails. and some proteins can move sideways.
Avoid this misconception
The claim: Membranes contain holes that let any small substance through.
The correction: The hydrophobic bilayer and selective proteins determine passage. Charge and lipid solubility matter as well as size.
Bleach a patch: how we know membranes are fluid
Membrane molecules are far too small to watch one at a time, so how do we know they move? One neat experiment is called FRAP, short for fluorescence recovery after photobleaching. You can run it yourself in the membrane model below.
- Label the membrane. Scientists attach a fluorescent label to some of the molecules in a cell’s membrane. The label is a small tag that glows when light shines on it, so under a fluorescence microscope the membrane glows evenly. Real experiments often label proteins. The model below labels lipids, but the idea is the same.
- Bleach a patch. A short, intense laser pulse is aimed at one small patch. It permanently destroys the glow of the labels there. This is called bleaching. The molecules and the membrane are still there: they just no longer glow, so the patch looks dark.
- Watch the patch. If the molecules stayed put, the patch would stay dark. Instead it slowly brightens, because glowing molecules from the surrounding membrane drift sideways into it while bleached ones drift out. In the model you’ll see the dark patch refill with green, and the graph plots the patch’s brightness, as a percentage of its starting value, against time.
What this tells us
Recovery means that lipids, and many proteins, can move sideways within the membrane. This is the “fluid” part of the fluid mosaic model: the membrane is a moving sheet, not a rigid wall.
Be careful what you claim
Recovery does not always reach 100%. Some labelled molecules may be anchored, for example tied to the cytoskeleton inside the cell, so they cannot move into the patch. If 40% are anchored, the brightness levels off at about 60%. That shows the membrane has a mobile part and a fixed part. It does not mean the model is wrong.
One FRAP result also has limits. It shows that the labelled molecules in the tested patch can move. It does not prove that every molecule moves, or that every part of the membrane has the same make-up.
Try it in the model: switch to the FRAP view at 37 °C and press Bleach patch. Then set 40% of the labelled molecules as anchored, bleach again and see where the curve levels off this time.
Explore a fluid mosaic
A teaching model of an animal cell-surface membrane, not a micrograph. Build the bilayer, then test which molecules cross the lipid core and which need a specific protein. Change temperature and cholesterol, and bleach a patch to see the evidence for a fluid membrane. Counts and rates are illustrative.
What you see
- A slice through an animal cell-surface membrane: tissue fluid at the top, cytoplasm at the bottom.
- Phospholipids (heads facing water, tails inward), cholesterol among the tails, an aquaporin, a gated Na⁺ channel, a glucose carrier and a glycoprotein whose carbohydrate faces outside.
- Molecules moving at random. Counters show crossings per second for each route.
How to use it
- Drag a molecule from the toolbox into either side, or click it to add twenty to the tissue fluid (forty for water).
- Open or close the Na⁺ channel. Change temperature, cholesterol and the share of saturated tails, and watch lipid mobility.
- Click a protein or lipid in the scene to read what it does. In the FRAP view, bleach a patch and follow recovery on the graph.
What it shows
- Selective permeability: small non-polar molecules cross the core; ions, water and glucose rely mostly on specific proteins.
- Net movement runs from more to fewer molecules, but individual molecules cross both ways.
- Fluidity depends on temperature, tail saturation and cholesterol, and lateral mobility is the evidence behind “fluid”.
Limits of the model
- Molecules, proteins and lipids are drawn far larger and fewer than in a real membrane, and the speeds are slowed down.
- Mobility, permeability and FRAP numbers are illustrative, not measurements. The FRAP view labels lipids; real experiments often label proteins.
- Only one example of each protein is shown, and the membrane is a flat 2D slice of a 3D sheet.
Guided tour · step 1 of 11
- In Proteins and time, just below these steps, press Assemble bilayer.
- Watch the scattered phospholipids in the simulator settle into two layers.
Hydrophilic heads face the water on both sides and hydrophobic tails hide in the middle: a bilayer forms without any energy input.
Molecules · drag into the scene or click
Proteins and time
Membrane conditions
Mobility is relative to 37 °C with half the tails saturated and no cholesterol (illustrative).
FRAP: bleach a patch
A laser switches off the glow in one patch. If labelled lipids can move, glowing ones drift back in. How FRAP works
Crossings
| Molecule | Outside | Inside | Crossed in | Crossed out | Main route |
|---|---|---|---|---|---|
| O₂ | 0 | 0 | 0 | 0 | Lipid core |
| CO₂ | 0 | 0 | 0 | 0 | Lipid core |
| H₂O | 0 | 0 | 0 | 0 | Aquaporin (some lipid core) |
| Na⁺ | 0 | 0 | 0 | 0 | Na⁺ channel (when open) |
| Glucose | 0 | 0 | 0 | 0 | Carrier |
Selected part
A slice through the membrane
Click a lipid or protein in the scene to read what it does. Add molecules from the toolbox and watch which route each one uses.
Explain
Interpret the evidence
Models are explanations of evidence
A model represents an interpretation of observations, not a literal photograph. Evidence of proteins within the bilayer challenges models placing all protein as continuous layers on its surfaces. Redistribution of labelled membrane proteins supports lateral mobility. The FRAP experiment introduced above the membrane model is one example of that evidence.
Evidence in historical order · the fluid mosaic model is the one used in this lesson
1 of 7
Much of the membrane is lipid
Charles Overton
Lipid-soluble substances entered cells more readily than charged or strongly polar ones. Overton concluded that a large part of the membrane itself must be lipid.
What this supports
The barrier behaves like a lipid, not like a watery gap.
What this does not show
It does not show whether that lipid is one layer or two.
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
Why does a sodium ion need a suitable membrane protein to cross readily?
Show suggested answer
Its charge makes passage through the hydrophobic bilayer interior energetically unfavourable. A suitable channel or carrier provides an alternative route.
Practice question
Retrieval and explanation · suggested answer
Fluorescence returns to a bleached membrane patch. What does this support?
Show suggested answer
Unbleached labelled molecules can move into the patch, supporting lateral mobility. It does not show that all proteins move equally freely.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
Only 60% of fluorescence returns after bleaching, despite a long observation period. Suggest an explanation without assuming that the whole model is wrong.
Show a hint
Consider whether all labelled molecules must be mobile.
Explore the reasoning
Some labelled molecules may be anchored or confined, leaving only a mobile fraction able to redistribute. Other explanations, including experimental limitations, should be considered. Partial recovery is compatible with a membrane containing both mobile and restricted components.
Recall this lesson
Lesson recall questions
Question 1 of 5: Describe the arrangement of phospholipids in a cell-surface membrane.
Membrane structure and models
Practice question
Retrieval and explanation · suggested answer
Describe the arrangement of phospholipids in a cell-surface membrane.
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
A channel with a measurable job
2A · Cell MembranesThe CFTR protein forms an ion channel in cell membranes. Investigating its function links a molecular component to transport across a cell surface. A researcher can separate several…Specification 2.2(i) · 2.5(ii)Read & use the scienceClose article
Why this matters
The CFTR protein forms an ion channel in cell membranes. Investigating its function links a molecular component to transport across a cell surface. A researcher can separate several questions: was CFTR synthesised, did it reach the cell surface, and does the channel open and conduct ions? Different experiments are needed for each. A fluorescent signal somewhere inside a cell might establish the presence of a tagged protein without establishing normal transport. This extends the fluid-mosaic model into a measurable problem: membranes are organised working systems, and the location and activity of an embedded protein matter as much as its presence.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 1, pp. 15–18. The application extends beyond required recall.
- 2.2(i) know the structure and properties of cell membranes
- 2.5(ii) understand the involvement of carrier and channel proteins in membrane transport
Use the science
Why would knowing that a cell makes a protein be insufficient to show that it functions as a surface channel?
Explore a possible response
It must also reach the correct membrane, adopt an appropriate structure and permit transport under the relevant conditions. Protein production alone does not establish location or function.
Explore the source
- Cystic Fibrosis Foundation: basics of CFTRCystic Fibrosis Foundation
Getting a message through a membrane
2A · Cell MembranesAn RNA medicine must reach the part of a cell where it can work. Large, charged RNA molecules do not simply diffuse through the hydrophobic interior of a…Specification 2.2(i) · 2.5(i)Read & use the scienceClose article
Why this matters
An RNA medicine must reach the part of a cell where it can work. Large, charged RNA molecules do not simply diffuse through the hydrophobic interior of a membrane. Lipid nanoparticles help carry the cargo into cells, but uptake is only one hurdle: RNA can remain trapped in an endosome, a membrane-bound compartment formed during entry. A 2025 imaging study investigated several barriers between uptake and release into the cytosol. It found that membrane damage did not automatically mean useful RNA delivery. This makes membrane structure a practical research problem rather than just a diagram to label.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 1, pp. 15–18. The application extends beyond required recall.
- 2.2(i) know the structure and properties of cell membranes
- 2.5(i) understand what is meant by passive transport (diffusion, facilitated diffusion), active transport (including the role of ATP as an immediate source of energy), endocytosis and exocytosis
Use the science
A fluorescent signal shows nanoparticles inside a cell. Why does that not demonstrate that their RNA has reached ribosomes?
Explore a possible response
The particles or RNA may remain in endosomes; an intracellular location is not necessarily the cytosol. Cargo-specific localisation or functional translation measurements are needed.
Explore the source
- Cellular and biophysical barriers to lipid nanoparticle mediated delivery of RNA to the cytosolNature Communications (2025)
Why “stronger alcohol” is not the whole answer
2A · Cell MembranesAlcohol changes the behaviour of biological materials, which is why it appears in your membrane-permeability practical and in disinfectant research. But more concentrated does not automatically mean more…Specification 2.3 · 2.6(iii)Read & use the scienceClose article
Why this matters
Alcohol changes the behaviour of biological materials, which is why it appears in your membrane-permeability practical and in disinfectant research. But more concentrated does not automatically mean more effective. The CDC describes protein denaturation as a major explanation for alcohol’s antimicrobial action and notes the importance of water. This prevents a common overinterpretation of a school experiment: pigment leaking from beetroot demonstrates a change in permeability under those conditions, not that a particular preparation kills every microorganism. Testing a real disinfectant requires a defined organism, exposure time and measure of survival as well as a plausible molecular mechanism.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 1, pp. 15–18. The application extends beyond required recall.
- 2.3 CORE PRACTICAL 3 Investigate membrane properties including the effect of alcohol and temperature on membrane permeability.
- 2.6(iii) understand the significance of a protein’s primary structure in determining its secondary structure, three-dimensional structure and properties (globular and fibrous proteins and the types of bonds involved in its three-dimensional structure)
Use the science
Why would greater pigment leakage from beetroot be insufficient evidence that a disinfectant is more effective at killing bacteria?
Explore a possible response
Pigment leakage measures plant-tissue permeability, not bacterial survival. Cell type, target structures, exposure conditions and the measured endpoint differ.
Explore the source
- Chemical Disinfectants: AlcoholCDC, Guideline for Disinfection and Sterilization