Biology › Membranes and transport across cells › The fluid mosaic: what a membrane is actually made of
The fluid mosaic: what a membrane is actually made of
Every cell is wrapped in a sheet about 7 nm thick that no bond holds together. Two layers of phospholipid, proteins drifting about in them, cholesterol wedged between the tails, sugars on the outside — and almost everything a cell does to control its contents happens there.
Before this Phospholipids and triglycerides · Protein tertiary structure and denaturation
Before you start
Something must be holding the bilayer together — bonds between the phospholipids, or a frame of protein underneath. Almost everyone assumes this, and it is wrong. No covalent bond joins one phospholipid to the next. Each molecule is free to slide past its neighbours, and swaps places with one of them millions of times a second. What keeps the sheet intact is not attachment but exclusion: the tails have nowhere better to go than next to each other, because the alternative is sitting in water.
What you should be able to do
- Explain why a phospholipid bilayer forms without anything assembling it.
- Label a fluid mosaic membrane and give the job of every component on it.
- Distinguish intrinsic from extrinsic proteins, and channel from carrier proteins.
- Describe what cholesterol does at high temperature and at low temperature, and say why those two effects are not contradictory.
- Explain how temperature and organic solvents change permeability, and describe the beetroot investigation that measures it.
Why the bilayer builds itself
A phospholipid is a molecule with a split personality. One end is a phosphate group attached to glycerol, it carries charge, and it mixes happily with water. The other end is two fatty acid chains, uncharged hydrocarbon, and water will have nothing to do with it. Chemists call such a molecule amphipathic, and the word is worth knowing because the entire structure of a membrane follows from it.
Put enough phospholipids in water and they arrange into a double layer with the heads facing the water on both faces and the tails buried between them. Nobody builds it. The arrangement simply has the lowest energy, because it hides the greatest number of hydrophobic tails from the greatest amount of water. Shake a membrane apart and the fragments reseal into closed vesicles for the same reason: an open edge would leave tails exposed.
The finished sheet is about 7 nm thick, which is far too small to resolve in a light microscope and is why membranes stayed hypothetical until electron microscopy arrived. It is also why so much of what you know about membranes is inferred from how they behave rather than from pictures of them.
- Phospholipid
- A lipid in which one of the three fatty acids of a triglyceride is replaced by a phosphate group, giving a hydrophilic head and two hydrophobic tails.
- Hydrophilic
- Attracted to water; polar or charged.
- Hydrophobic
- Repelled by water; non-polar.
- Bilayer
- Two layers of phospholipid with the hydrophobic tails facing inwards and the hydrophilic heads facing the watery solutions on each side.
That hydrophobic core is the reason a membrane is selective at all. Small non-polar molecules — oxygen, carbon dioxide, steroid hormones — dissolve in it and pass straight through. Anything charged or large and polar cannot: a sodium ion carries a shell of water molecules around it, and dragging that into a layer of fatty acid tails costs far more energy than it gains. Ions and glucose need help, and the help is protein.
The mosaic: what is set into the bilayer
Singer and Nicolson named the fluid mosaic model in 1972, and both halves of the name earn their keep. Fluid: the phospholipids and most of the proteins move sideways within their own layer. Mosaic: the proteins are scattered through the phospholipids in an irregular pattern, like tiles set into a floor, rather than forming a continuous layer of their own.
Proteins come in two positions. Intrinsic (or integral) proteins are embedded in the bilayer, and many span it completely; their surfaces that face the core are hydrophobic, which is what holds them there. Extrinsic (or peripheral) proteins sit on one surface, bound to the heads or to an intrinsic protein, and do not enter the core at all. Enzymes, receptors and the proteins that anchor the cytoskeleton are often extrinsic.
Two kinds of intrinsic protein carry substances across, and exam questions separate them constantly.
| Channel protein | Carrier protein | |
|---|---|---|
| Shape | A pore lined with hydrophilic amino acids | A binding site that fits one kind of molecule |
| How it works | The substance passes through the open pore | The protein changes shape to move the substance across |
| Carries | Water-soluble ions: Na⁺, K⁺, Cl⁻ | Larger polar molecules: glucose, amino acids |
| Specific? | Yes, usually to one ion | Yes, to one molecule or a small group |
| Used in | Facilitated diffusion only | Facilitated diffusion and active transport |
Many channels are gated: they open in response to a voltage change or to a molecule binding, which is how a nerve impulse and a synapse work. That comes later, but it is worth registering now that a channel is not simply a hole.
On the outer face, some proteins and some phospholipids carry short branched carbohydrate chains. A protein with sugars attached is a glycoprotein; a phospholipid with sugars attached is a glycolipid. Both stick out into the surroundings, and both are how one cell reads another: they act as antigens and as receptor sites, they hold cells together in tissues, and they help hold a layer of water at the surface. The ABO blood groups are differences in these chains, which is why the wrong transfusion is recognised as foreign within minutes.
- Intrinsic protein
- A protein embedded within the bilayer, often spanning it completely.
- Extrinsic protein
- A protein on one surface of the bilayer that does not enter the hydrophobic core.
- Glycoprotein
- A membrane protein with a carbohydrate chain attached, acting in cell recognition, adhesion or as a receptor.
- Glycolipid
- A phospholipid with a carbohydrate chain attached, acting in cell recognition and adhesion.
Cholesterol does two opposite-sounding jobs
Cholesterol is a short, rigid, mostly hydrophobic molecule with one small hydroxyl group at the end. That hydroxyl sits among the phosphate heads and the rest of the molecule wedges between the fatty acid tails. In an animal cell surface membrane, up to about a quarter of the lipid molecules are cholesterol.
Cholesterol is usually summed up as making a membrane 'more stable', which is not enough for a question worth several marks. What it actually does depends on the temperature.
At high temperature the phospholipids have more kinetic energy and would move apart. Cholesterol binds to the tails around it and restricts that movement, so the membrane stays less fluid than it otherwise would and does not become leaky.
At low temperature the danger is the opposite. The tails would pack tightly together and the membrane would set solid, cracking rather than bending. Cholesterol sits between the tails and stops them packing that closely, so the membrane keeps some fluidity.
The two effects sound contradictory only if you describe cholesterol as making a membrane more or less fluid. Describe it instead as reducing the change in fluidity when the temperature changes, and both follow from the same physical fact: a rigid molecule sitting between the tails gets in the way of whatever they were about to do.
Answering the cholesterol question properly
A fish that lives under Antarctic ice at −1.8 °C has membranes with a high proportion of unsaturated fatty acids in its phospholipids. Suggest why, and explain what cholesterol contributes at that temperature.
Unsaturated fatty acids contain C=C double bonds, and each double bond puts a kink in the tail. Kinked tails cannot pack closely, so the bilayer stays fluid at temperatures where straight saturated tails would have set.
A membrane that has set solid cannot let its proteins move or change shape, and it fractures rather than bending, so the cell would leak. Fluidity at −1.8 °C is not a luxury for this fish.
Cholesterol works alongside that. Sitting between the tails, it keeps them from packing tightly even where they could, so it supports the same outcome by a different mechanism — a shape that gets in the way rather than a kink in the chain itself.
What makes a membrane leak
Permeability is not fixed. Two factors are examined regularly, and both damage the same two things: the bilayer's packing and the proteins' shapes.
Temperature. Below about 40 °C, warming a membrane raises permeability gently. Phospholipids gain kinetic energy and move more, gaps open and close more often, and the molecules trying to cross are moving faster too. Above roughly 45 to 50 °C the graph turns sharply upward, because the intrinsic proteins begin to denature: hydrogen and ionic bonds break, tertiary structure is lost, and the protein no longer fits the space it occupied. Gaps open around it that nothing controls.
Note what is happening to what. The phospholipids do not denature — they are not proteins, and 'the membrane denatures' loses the mark. The phospholipids move more; the proteins denature.
Organic solvents. Ethanol, propanone and methylbenzene dissolve lipids, which is exactly what a bilayer is made of. Increasing the concentration of ethanol around a cell dissolves phospholipids out of the membrane and disrupts what remains, so permeability rises with concentration. It is also why alcohol-based hand gel works: it takes apart the lipid envelope of viruses that have one.
- Permeability
- How readily a membrane allows a substance to cross it.
- Denaturation
- The loss of a protein's tertiary structure when hydrogen and ionic bonds break, so its shape and therefore its function is lost.
The beetroot investigation
The standard practical uses beetroot because its vacuoles are full of betalain, a red pigment that is water-soluble and easy to measure. The pigment is inside the vacuole, so it has to cross two membranes to escape: the tonoplast and the cell surface membrane. If the water around a beetroot disc turns red, those membranes have failed.
Cut discs of equal size with a cork borer, wash them thoroughly to remove pigment released by the cutting itself, then hold each one in a water bath at a set temperature for a fixed time — five minutes is common. Transfer to a fixed volume of distilled water, wait, and measure the colour with a colorimeter using a green filter of around 530 nm. Higher absorbance means more pigment escaped means more permeable membranes.
The washing step is the one candidates forget, and it matters: cutting ruptures cells at the surface, and unwashed discs release pigment that has nothing to do with the temperature being tested. Control variables are the disc size and number, the volume of water, the time in the bath, the time before reading, and the age and variety of the beetroot, since older tissue leaks more.
TRY IT — Reading the graph as an examiner does
A student reports that absorbance rises slowly from 20 °C to 40 °C and then rises steeply between 50 °C and 60 °C. Explain both parts of that result.
Check your answer
Between 20 °C and 40 °C the rise is caused by the phospholipids gaining kinetic energy. They move more, so the bilayer is slightly more open and slightly more pigment gets through — but the proteins still hold their shape, so the membrane remains largely intact.
Between 50 °C and 60 °C the intrinsic proteins denature. Hydrogen and ionic bonds break, tertiary structure is lost, and the proteins no longer fill the space they occupied, so gaps appear. The phospholipids are also moving far more by now. Pigment leaves the vacuole freely and absorbance climbs steeply.
A full answer names the change at each stage rather than saying 'the membrane is damaged' twice. Two different things are happening, and the graph has two different gradients because of it.
In the exam
- Say what a component does, not what it is. 'Cholesterol is a lipid' scores nothing; 'cholesterol restricts the movement of the phospholipids at high temperature, so the membrane is less fluid and less permeable' scores.
- Proteins denature; phospholipids do not. Write that the membrane 'denatures' and you have thrown away the mark that distinguishes the two halves of the temperature graph.
- Fluid mosaic questions want both words explained. Fluid because the phospholipids and proteins move within the layer; mosaic because the proteins are scattered irregularly among them.
- Intrinsic against extrinsic is about position, not size. Intrinsic proteins are embedded in the bilayer, often spanning it; extrinsic proteins sit on one surface.
- For the beetroot practical, mention washing the discs after cutting. It is a standard marking point and it is the step candidates skip.
- Carbohydrate chains are on the outer surface only. A diagram question that shows sugars on the cytoplasmic face is asking you to notice they are on the wrong side.
Check yourself
A student places identical discs of beetroot in a range of ethanol concentrations from 0% to 100% at room temperature and measures the absorbance of the surrounding liquid after 30 minutes. Absorbance increases as ethanol concentration increases. Explain this result, and explain why the same student sees almost no pigment released when the discs are left in water at 20 °C.
Answer
Ethanol is an organic solvent and the bilayer is lipid, so ethanol dissolves phospholipids out of the membrane. The more concentrated the ethanol, the more phospholipid is removed and the more disrupted the remaining arrangement is.
Once the bilayer is incomplete, the hydrophobic core no longer forms a continuous barrier. Betalain is water-soluble and would normally be held in the vacuole; with the tonoplast and the cell surface membrane both breached, it diffuses out into the surrounding liquid and the absorbance rises.
In water at 20 °C neither thing has happened. The phospholipids have relatively little kinetic energy and stay closely packed, and the intrinsic proteins are well below the temperature at which they denature, so their tertiary structure is intact and no gaps open around them.
The membrane is therefore still selectively permeable at 20 °C. A large polar molecule like betalain cannot cross the hydrophobic core unaided, and nothing in the membrane transports it, so it stays inside and the water stays clear.
Questions
Question 14 marks
Explain why phospholipids arrange themselves into a bilayer when they are placed in water, and why a torn fragment of membrane reseals itself into a closed vesicle.
Mark scheme
- B1 a phospholipid is amphipathic: the phosphate head is hydrophilic and the two fatty acid tails are hydrophobic
- B1 the heads face the water on both surfaces while the tails are buried between them, facing each other
- B1 no bonds join one phospholipid to the next; this arrangement is simply the one of lowest energy, because it hides the greatest number of tails from water
- B1 an open edge would leave hydrophobic tails exposed to water, so a fragment closes on itself rather than remaining as a sheet
Question 24 marks
Describe what cholesterol does to a cell surface membrane at high temperature and at low temperature, and describe the effect it has on permeability in each case.
Mark scheme
- B1 at high temperature cholesterol binds to the fatty acid tails around it and restricts their movement
- B1 so the membrane is less fluid than it would otherwise be and does not become leaky
- B1 at low temperature cholesterol sits between the tails and stops them packing closely together
- B1 so the membrane keeps some fluidity instead of setting solid and cracking, and stays intact
Question 34 marks
Compare a channel protein with a carrier protein as routes across a cell surface membrane.
Mark scheme
- B1 a channel protein is a pore lined with hydrophilic amino acids, whereas a carrier protein has a binding site shaped to fit one kind of molecule
- B1 a substance passes through the open channel, whereas a carrier protein changes shape to move the substance across
- B1 channels carry water-soluble ions such as sodium, potassium and chloride, whereas carriers take larger polar molecules such as glucose and amino acids
- B1 channels are used in facilitated diffusion only, whereas carrier proteins are used in both facilitated diffusion and active transport
Question 44 marks
A student cuts discs of beetroot with a cork borer and places them directly into water baths at a range of temperatures without washing them first. Suggest how this affects her absorbance readings, and suggest one control variable she must still keep the same.
Mark scheme
- B1 cutting ruptures the cells at the cut surfaces, which releases pigment that has nothing to do with the temperature being tested
- B1 absorbance is therefore raised at every temperature, including those at which the membranes are undamaged
- B1 the readings at low temperature overstate the permeability, so the rise caused by temperature itself is masked
- B1 any one control variable named: size and number of discs, volume of distilled water, time in the water bath, time before the reading is taken, or the age and variety of the beetroot
Question 52 marks
Name the two kinds of molecule found on the outer surface of a cell surface membrane that carry short branched carbohydrate chains.
Mark scheme
- B1 glycoprotein: a membrane protein with a carbohydrate chain attached
- B1 glycolipid: a phospholipid with a carbohydrate chain attached
Worth remembering
- Heads are hydrophilic and face the water; tails are hydrophobic and face each other.
- Nothing bonds the bilayer together — it is the arrangement that hides the most tails from water.
- Intrinsic proteins are embedded in the bilayer; extrinsic proteins rest on one surface.
- Cholesterol reduces fluidity at high temperature and prevents tight packing at low temperature.
- Above about 45 °C, permeability rises sharply because the proteins denature.