The Fluid Mosaic Model

AS · 15 min

Every cell is surrounded by a cell surface membrane, and eukaryotic cells are divided inside by many more membranes around organelles. Membranes control what enters and leaves, hold the receptors that let cells communicate, and carry the markers by which cells recognise each other. The fluid mosaic model describes how a membrane is built from phospholipids, cholesterol, proteins, glycolipids and glycoproteins, and why it behaves as it does. Questions ask you to label the components, explain their arrangement in terms of hydrophilic and hydrophobic interactions, describe their roles, and outline cell signalling.

Why "fluid mosaic"?

Definition

The fluid mosaic model describes a membrane as a phospholipid bilayer in which protein molecules are scattered (like the tiles of a mosaic). The phospholipids and many of the proteins can move about laterally within their layer, so the membrane is fluid. Cholesterol, glycolipids and glycoproteins are also part of the structure.

Fluid: individual phospholipid molecules move sideways within their own layer, exchanging places millions of times a second. Many proteins drift sideways too. The membrane behaves more like a thin film of oil than a rigid wall, which is why it can bend, fuse with vesicles and seal small tears.
Mosaic: seen from above, protein molecules are scattered among the phospholipids in an irregular pattern, varying in size and shape.

A cell surface membrane is about 7 nm (7–10 nm) thick. It is too thin to see with a light microscope. In an electron micrograph it appears as two dark lines (the hydrophilic heads, which stain densely) separated by a pale line (the hydrophobic tails).

The phospholipid bilayer and why it forms

A phospholipid has a hydrophilic (polar) phosphate head and two hydrophobic (non-polar) fatty acid tails (see Lipids).

Membranes exist in a watery environment on both sides: tissue fluid or the external medium outside, cytoplasm inside. In water, phospholipids arrange themselves spontaneously so that:

the hydrophilic heads face outwards, into the water on both sides, where they can form hydrogen bonds with water molecules;
the hydrophobic tails point inwards, towards each other, away from water, held together by hydrophobic interactions.

This produces a bilayer: two layers of phospholipids with a hydrophobic core. Nothing needs to "build" it; it forms because this arrangement is the most stable one in water.

Key result

Arrangement of membrane components and the interactions that cause it

Phospholipids: heads face the aqueous cytoplasm and tissue fluid; tails form the hydrophobic interior.
Proteins: parts with hydrophobic R groups on their surface sit in the hydrophobic core, in contact with the fatty acid tails; parts with hydrophilic R groups project into the aqueous environment on either side.
Cholesterol: sits among the fatty acid tails, with its small hydrophilic –OH group near the phosphate heads.
Glycolipids and glycoproteins: the carbohydrate chains always project outwards from the outer surface into the watery tissue fluid, because carbohydrates are hydrophilic.

Components and their roles

Phospholipids

Form the basic bilayer structure of all membranes.
The hydrophobic core acts as a barrier to water-soluble (polar) substances and ions, such as glucose, amino acids, $\text{Na}^+$ and $\text{Cl}^-$ , which cannot easily pass between the tails. This allows the membrane to be partially permeable and lets the cell control what enters and leaves, using proteins.
Small non-polar molecules (oxygen, carbon dioxide) and lipid-soluble molecules (such as steroid hormones) can diffuse straight through the bilayer.
The type of fatty acids controls fluidity: phospholipids with unsaturated fatty acids have kinks in their tails, so they cannot pack closely, which keeps the membrane more fluid.

Cholesterol

Cholesterol molecules lie between the phospholipids in both layers. They regulate fluidity and add stability.

At normal and high temperatures, cholesterol restricts the movement of phospholipid tails, making the membrane less fluid and giving mechanical stability. Without cholesterol, animal cell membranes would be too fluid to hold their shape at body temperature, and would break up.
At low temperatures, cholesterol prevents the tails packing closely together, so it stops the membrane becoming too rigid (it maintains fluidity).
Cholesterol also reduces the permeability of the membrane to ions and small polar molecules, because it fills gaps between the phospholipid tails in the hydrophobic core.

Cholesterol is found in animal cell membranes; plant cell membranes contain similar sterols, and prokaryotes generally lack cholesterol.

Proteins

Membrane proteins are of two broad kinds:

Intrinsic (integral) proteins are embedded in the bilayer; many span it completely (transmembrane proteins).
Extrinsic (peripheral) proteins lie on the surface of one layer, attached to intrinsic proteins or phospholipid heads.

Their roles:

Protein	Role
Channel proteins	Form water-filled pores through the membrane, lined with hydrophilic R groups, allowing specific ions or small polar molecules to diffuse through (facilitated diffusion). Many are gated: they open or close in response to a signal (e.g. a voltage change or a binding molecule).
Carrier proteins	Bind a specific molecule or ion, then change shape to move it across the membrane. Used in facilitated diffusion (down a concentration gradient) and active transport (against a concentration gradient, using ATP).
Cell surface receptors	Have a binding site complementary to a specific signalling molecule, such as a hormone; binding triggers a response in the cell (cell signalling). Many are glycoproteins.
Enzymes	Some membrane proteins catalyse reactions at the membrane surface, e.g. maltase and other digestive enzymes on the membranes of epithelial cells in the small intestine; ATP synthase in mitochondrial membranes.
Recognition proteins and adhesion molecules	Glycoproteins that act as antigens and attach cells to each other (see below).

Glycolipids and glycoproteins

A glycolipid is a lipid with a carbohydrate chain attached; a glycoprotein is a protein with a carbohydrate chain attached. The carbohydrate chains project from the outer surface and together form a coat called the glycocalyx.

Roles of glycolipids and glycoproteins:

Receptor molecules: they bind specific signalling molecules such as hormones and neurotransmitters, as part of cell signalling. (For example, the receptor for insulin is a glycoprotein.)
Cell recognition: they act as antigens, molecular markers that identify a cell as "self" or "non-self" and identify cell types. The ABO blood group antigens are glycolipids and glycoproteins on red blood cells. Lymphocytes recognise foreign antigens on pathogens.
Cell adhesion: they bind cells together to form tissues, for example in the attachment of epithelial cells to each other.
Stabilising the membrane: the hydrophilic carbohydrate chains form hydrogen bonds with water molecules around the cell.

Key result

Component	Main role(s)
Phospholipids	form bilayer; barrier to water-soluble substances and ions; affect fluidity
Cholesterol	regulates fluidity; mechanical stability; reduces permeability to ions and polar molecules
Channel proteins	facilitated diffusion of specific ions/polar molecules through a hydrophilic pore
Carrier proteins	facilitated diffusion and active transport; change shape to move specific molecules
Receptor proteins (often glycoproteins)	bind specific signalling molecules: cell signalling
Glycolipids and glycoproteins	receptors; cell recognition (antigens); cell adhesion; hydrogen bond with water

Factors affecting membrane fluidity and permeability

Temperature. As temperature rises, phospholipids gain kinetic energy and move faster, so the membrane becomes more fluid and more permeable. At high temperatures (above about $45$ – $50\ ^\circ\text{C}$ ), membrane proteins denature (their tertiary structure changes, leaving gaps), and the bilayer becomes very fluid and disordered, so permeability rises sharply. At low temperatures, phospholipids pack closely and the membrane becomes rigid.
Fatty acid saturation. More unsaturated fatty acids (kinked tails) make the membrane more fluid; organisms living in cold places have more unsaturated fatty acids in their membranes.
Cholesterol buffers fluidity, as described above.
Organic solvents such as ethanol dissolve lipids and disrupt the bilayer, increasing permeability.

Practical skills

Investigating the effect of temperature on membrane permeability using beetroot

Beetroot cells contain a red pigment, betalain, in their vacuoles. The pigment is kept in by the tonoplast (vacuole membrane) and the cell surface membrane. If the membranes are damaged, the pigment leaks out into the surrounding water.

Method: cut equal-sized pieces (cylinders) of beetroot with a cork borer and knife, e.g. $1\ \text{cm}$ long. Rinse them thoroughly in distilled water to remove pigment released from cells cut during preparation. Place one piece in each of several tubes containing the same volume of distilled water (e.g. $5\ \text{cm}^3$ ) in water baths at a range of temperatures (e.g. $20$ – $80\ ^\circ\text{C}$ ) for a fixed time (e.g. 30 minutes). Remove the beetroot, shake the tube and measure the absorbance of the liquid with a colorimeter using a blue-green filter (complementary to red).
Independent variable: temperature. Dependent variable: absorbance (intensity of red colour) of the surrounding water.
Standardised variables: size and surface area of beetroot pieces; same beetroot (or same variety and age); volume of water; time in water bath; rinsing time.
Results: little leakage at low temperatures; a steep increase above about $40$ – $50\ ^\circ\text{C}$ .
Explanation: higher temperatures increase the kinetic energy and fluidity of the phospholipids; above about $45\ ^\circ\text{C}$ membrane proteins denature, leaving gaps, and the phospholipid bilayer becomes disrupted, so betalain leaks out.
Sources of error: pieces not all the same size; incomplete rinsing; colour judged by eye if no colorimeter (use colour standards); temperature fluctuations.
Variations: use different concentrations of ethanol at a fixed temperature (ethanol dissolves phospholipids).

Roles of membrane components (routine, 4 marks)

Describe the roles of cholesterol and glycoproteins in a cell surface membrane.

Solution

Cholesterol:

regulates fluidity: reduces fluidity at higher temperatures by restricting movement of phospholipid tails;
prevents the membrane becoming too rigid at low temperatures (stops tails packing closely);
gives mechanical stability; reduces permeability to ions/polar molecules.

Glycoproteins (any two):

act as receptors for signalling molecules such as hormones (cell signalling);
act as antigens for cell recognition (self/non-self);
involved in cell adhesion (binding cells together);
form hydrogen bonds with water to stabilise the membrane.

Membrane thickness from an electron micrograph (moderate)

In an electron micrograph at a magnification of $\times 300\,000$ , a cell surface membrane appears as a band $2.1\ \text{mm}$ wide. Calculate the actual thickness of the membrane in nm.

Solution

\text{actual size} = \frac{\text{image size}}{\text{magnification}}

Image size $= 2.1\ \text{mm} = 2.1 \times 10^{6}\ \text{nm}$ (since $1\ \text{mm} = 10^{6}\ \text{nm}$ ).

\text{actual thickness} = \frac{2.1 \times 10^{6}}{300\,000} = 7.0\ \text{nm}

This matches the typical thickness of a membrane, which is why membranes cannot be seen with a light microscope (resolution about $200\ \text{nm}$ ). See Microscopy for converting units.

Beetroot data (moderate)

The table shows the absorbance of the water surrounding beetroot pieces after 30 minutes at each temperature.

Temperature / °C	20	30	40	50	60	70	80
Absorbance	0.05	0.06	0.08	0.15	0.62	0.88	0.92

(a) Describe the results. (b) Explain the change between $50\ ^\circ\text{C}$ and $70\ ^\circ\text{C}$ . (3 marks) (c) Suggest why the pieces were rinsed in distilled water before the experiment.

Solution

(a) Between $20$ and $40\ ^\circ\text{C}$ the absorbance increases only slightly (0.05 to 0.08). It rises steeply between $50$ and $70\ ^\circ\text{C}$ (0.15 to 0.88), then levels off (0.92 at $80\ ^\circ\text{C}$ ).

(b)

Phospholipids gain kinetic energy and move more, so the bilayer becomes more fluid / gaps appear between phospholipids;
membrane proteins denature (hydrogen and ionic bonds break; their tertiary structure changes), leaving gaps / channels in the membrane;
the cell surface membrane and tonoplast become more permeable, so more betalain (red pigment) leaks out, giving a higher absorbance.

(c) To wash away pigment released from cells that were cut open during preparation, so that the colour measured comes only from leakage through the membranes during the experiment.

Cell signalling

Cells in a multicellular organism must coordinate their activities. They do this by cell signalling: one cell releases a chemical that is detected by another cell (the target cell), which responds.

Outline of cell signalling

A cell releases a signalling molecule (a ligand), such as a hormone (e.g. glucagon, insulin) or a neurotransmitter, often by exocytosis.
The signal is transported to the target cell: by diffusion over short distances (e.g. across a synapse) or in the blood over long distances (hormones).
The signalling molecule binds to a specific receptor on the cell surface membrane of the target cell. The receptor (a protein or glycoprotein) has a binding site with a shape complementary to the signalling molecule, so only cells with that receptor respond.
Binding causes the receptor to change shape.
This change transmits the signal across the membrane into the cell. It may activate an enzyme on the inside of the membrane, which produces a second messenger (for example cyclic AMP), or it may open an ion channel.
The second messenger activates a cascade of reactions (often activating enzymes in turn), which amplifies the signal.
The cell makes a specific response, e.g. activating enzymes, changing membrane permeability, secreting a substance or changing which genes are expressed.

For example, the hormone glucagon binds to receptors on liver cells; inside the cell, a cascade involving cyclic AMP activates enzymes that break down glycogen to glucose, which is released into the blood. The full mechanism is covered at A Level; at AS you need the outline above.

Lipid-soluble signalling molecules, such as steroid hormones (e.g. oestrogen), can diffuse through the phospholipid bilayer and bind to receptors inside the cell (in the cytoplasm or nucleus), rather than to cell surface receptors.

Explaining specificity of signalling (moderate, 3 marks)

Insulin travels in the blood to every part of the body, yet only certain cells respond to it. Explain why.

Solution

Only target cells have receptors for insulin on their cell surface membranes.
The receptor has a binding site with a shape complementary to insulin.
Insulin binds only to these receptors, causing a change in the receptor shape and a response inside those cells; cells without the receptor cannot bind insulin and do not respond.

Exam-hard: explaining the arrangement (5 marks)

Explain how hydrophobic and hydrophilic interactions determine the arrangement of phospholipids and proteins in a cell surface membrane.

Solution

Phospholipids have hydrophilic phosphate heads and hydrophobic fatty acid tails.
Both sides of the membrane are aqueous (cytoplasm and tissue fluid), so heads face outwards towards water, forming hydrogen bonds with it.
Tails point inwards, away from water, held together by hydrophobic interactions, forming a bilayer with a hydrophobic core.
Intrinsic proteins have hydrophobic R groups on the part of their surface in contact with the fatty acid tails, so this region sits in the core of the bilayer.
Hydrophilic R groups are on the parts of the proteins that project into the aqueous cytoplasm or tissue fluid (and line the pores of channel proteins).
Carbohydrate chains of glycoproteins/glycolipids are hydrophilic and project outwards into the tissue fluid.

Watch out

The heads are hydrophilic and the tails hydrophobic. Reversing these is a common slip.
"Cholesterol makes the membrane more fluid" is only half the story. It reduces fluidity at high temperatures and prevents rigidity at low temperatures: it regulates fluidity.
The bilayer is a barrier to water-soluble (polar) substances and ions, not to all substances. Oxygen, carbon dioxide and lipid-soluble molecules cross it easily.
Carbohydrate chains are on the outer surface only; do not draw them on the cytoplasmic side.
A receptor is complementary to its signalling molecule, not "the same shape".

Exam tip

On a membrane diagram, labels examiners look for: phospholipid bilayer, hydrophilic head, hydrophobic tail, intrinsic (transmembrane) protein, extrinsic protein, channel protein, carrier protein, cholesterol, glycoprotein, glycolipid, carbohydrate chain (glycocalyx).
"Explain why the model is called fluid mosaic": fluid because phospholipids (and proteins) move laterally; mosaic because proteins are scattered among phospholipids.
In signalling questions, use the sequence signal released → transported → binds complementary receptor → receptor changes shape → second messenger / cascade (amplification) → specific response.
In the beetroot practical, name both membranes (cell surface membrane and tonoplast) and both mechanisms (proteins denature; phospholipids more fluid).

Summary

Fluid mosaic model: phospholipid bilayer with scattered proteins; components move laterally; about 7 nm thick.
Bilayer forms because hydrophilic heads face the water on both sides and hydrophobic tails face inwards.
Phospholipids: barrier to water-soluble substances and ions; fluidity depends on fatty acid saturation.
Cholesterol: regulates fluidity, gives mechanical stability, reduces permeability to ions.
Channel and carrier proteins: facilitated diffusion; carrier proteins also active transport.
Glycoproteins and glycolipids: receptors, cell recognition (antigens), cell adhesion.
Cell signalling: ligand released, binds a complementary receptor on the target cell, receptor changes shape, signal relayed (second messenger, cascade, amplification), specific response.
Higher temperature increases permeability; above about 45 °C membrane proteins denature (beetroot practical).

Practice questions

Question

Explain why the structure of the cell surface membrane is described as a fluid mosaic. (2 marks)
Explain why phospholipids form a bilayer in a cell surface membrane. (3 marks)
State two roles of proteins in a cell surface membrane.
Distinguish between a channel protein and a carrier protein.
State three roles of glycoproteins.
Explain why ions such as $\text{Na}^+$ cannot diffuse directly through the phospholipid bilayer.
A cell membrane appears $5.6\ \text{mm}$ wide in an electron micrograph at a magnification of $\times 800\,000$ . Calculate its actual width in nm.
Fish living in cold water have a higher proportion of unsaturated fatty acids in their membrane phospholipids than fish living in warm water. Suggest the advantage of this. (3 marks)
Outline how a hormone such as glucagon brings about a response in a target cell. (5 marks)
A student investigated the effect of ethanol concentration on the permeability of beetroot membranes. At 0% ethanol the absorbance was 0.04; at 20%, 0.10; at 40%, 0.45; at 60%, 0.81. (a) Calculate the percentage increase in absorbance between 20% and 40% ethanol. (b) Explain the results. (c) State three variables that should be controlled. (6 marks)

Answers

Fluid: the phospholipids (and many proteins) can move laterally / change places within their layer. Mosaic: protein molecules are scattered among the phospholipids in an irregular pattern.
Phospholipids have hydrophilic (polar) phosphate heads and hydrophobic (non-polar) fatty acid tails; both sides of the membrane are aqueous; heads face the water on each side and tails face inwards, away from water (held by hydrophobic interactions), so two layers form.
Any two: channel proteins for facilitated diffusion; carrier proteins for facilitated diffusion or active transport; receptors for cell signalling; enzymes; antigens for cell recognition; cell adhesion.
A channel protein forms a fixed, water-filled (hydrophilic) pore through which specific ions or small polar molecules diffuse (it may be gated); a carrier protein binds a specific molecule and changes shape to move it across; carrier proteins can be used for active transport, channel proteins cannot.
Receptors for signalling molecules (e.g. hormones); antigens for cell recognition (self/non-self, blood groups); cell adhesion; hydrogen bonding with water to stabilise the membrane.
Ions are charged (and surrounded by water molecules), so they are not soluble in lipid; they cannot pass through the hydrophobic core formed by the fatty acid tails; they need channel or carrier proteins.
$5.6\ \text{mm} = 5.6 \times 10^{6}\ \text{nm}$ ; $5.6 \times 10^{6} / 800\,000 = 7.0\ \text{nm}$ .
Unsaturated fatty acids have kinks (C=C double bonds) in their tails, so phospholipids cannot pack closely together; this keeps the membrane fluid at low temperatures (prevents it becoming rigid); so membrane proteins can still move/function, vesicles can fuse, and transport across the membrane continues.
Glucagon is secreted into the blood and carried to target (liver) cells; it binds to specific receptors on the cell surface membrane with a complementary binding site; the receptor changes shape; this activates an enzyme/G protein on the inner side, producing a second messenger (cyclic AMP); the second messenger activates a cascade of enzymes (amplification); producing a specific response, e.g. activation of enzymes that break down glycogen to glucose.
(a) $(0.45 - 0.10)/0.10 \times 100 = 350\%$ . (b) Ethanol is an organic solvent that dissolves phospholipids / disrupts the bilayer (and can denature membrane proteins); this increases membrane permeability; so more betalain leaks out of the vacuoles through the tonoplast and cell surface membrane; the higher the ethanol concentration, the more disruption, so the higher the absorbance. (c) Any three: temperature; size/surface area of beetroot pieces; time in solution; volume of ethanol solution; rinsing before the experiment; same beetroot.

Why "fluid mosaic"?

The phospholipid bilayer and why it forms

Components and their roles

Phospholipids

Cholesterol

Proteins

Glycolipids and glycoproteins

Factors affecting membrane fluidity and permeability

Cell signalling

Practice questions

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