The Cell Membrane: Structure and Function

The cell surface membrane (also called the plasma membrane) is a fundamental structure in all living cells. Understanding its structure and function is central to Edexcel A-Level Biology (9BI0) Topic 2. The fluid mosaic model proposed by Singer and Nicolson (1972) is the accepted model of membrane structure and is a key concept you must be able to describe and explain.

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The Fluid Mosaic Model

The fluid mosaic model describes the cell membrane as a dynamic structure consisting of a phospholipid bilayer with various proteins embedded in it or attached to its surface.

The name "fluid mosaic" reflects two key properties:

• Fluid — the individual phospholipid molecules and proteins are not fixed in place; they can move laterally within the plane of the membrane (lateral diffusion)
• Mosaic — the membrane contains a diverse array of proteins scattered throughout, creating a mosaic-like pattern when viewed from above

Components of the Cell Membrane

Component	Description	Function
Phospholipids	Molecules with a hydrophilic (polar) phosphate head and two hydrophobic (non-polar) fatty acid tails	Form the bilayer that acts as a barrier to water-soluble substances
Cholesterol	A lipid molecule that sits between phospholipid molecules	Regulates fluidity — prevents the membrane from becoming too rigid at low temperatures or too fluid at high temperatures
Intrinsic (integral) proteins	Proteins that span the entire width of the membrane (transmembrane proteins)	Channel proteins, carrier proteins, receptor proteins
Extrinsic (peripheral) proteins	Proteins attached to the inner or outer surface of the membrane	Enzymes, structural support, cell signalling
Glycoproteins	Proteins with carbohydrate chains attached on the extracellular surface	Cell recognition, cell signalling, immune response (act as antigens)
Glycolipids	Lipids with carbohydrate chains attached on the extracellular surface	Cell recognition, cell adhesion, stability

Exam Tip: When drawing or labelling the fluid mosaic model, always include: phospholipid bilayer, cholesterol, intrinsic proteins (showing them spanning the bilayer), extrinsic proteins, glycoproteins, and glycolipids. Label the hydrophilic heads and hydrophobic tails of the phospholipids.

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Phospholipids in Detail

Phospholipids are the main structural component of all biological membranes. Each phospholipid molecule consists of:

• A glycerol backbone
• Two fatty acid tails (hydrophobic — "water-hating")
• One phosphate group head (hydrophilic — "water-loving")

This gives the phospholipid its characteristic amphipathic nature — one end is attracted to water while the other end repels water.

Why Do Phospholipids Form a Bilayer?

When phospholipids are placed in an aqueous (water-based) environment, they spontaneously arrange themselves into a bilayer:

• The hydrophilic phosphate heads face outwards towards the aqueous environment (both the extracellular fluid and the cytoplasm)
• The hydrophobic fatty acid tails face inwards, away from water, forming a hydrophobic core

This arrangement creates an effective barrier to most water-soluble (polar) molecules and ions, while allowing small non-polar molecules to pass through. The bilayer is held together by hydrophobic interactions between the fatty acid tails, not by covalent bonds, which is why the membrane is fluid.

Saturated vs Unsaturated Fatty Acids

The fluidity of the membrane depends partly on the composition of the fatty acid tails:

Feature	Saturated fatty acids	Unsaturated fatty acids
Structure	No C=C double bonds; straight tails	One or more C=C double bonds; kinked tails
Packing	Pack closely together	Pack less closely (kinks prevent tight packing)
Effect on fluidity	Decrease fluidity (more rigid)	Increase fluidity (more fluid)

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The Role of Cholesterol

Cholesterol is a lipid molecule found in the membranes of animal cells (it is largely absent from plant cell membranes, which instead contain phytosterols). Cholesterol molecules are positioned between the phospholipid molecules with their hydroxyl group interacting with the phospholipid heads.

How Cholesterol Regulates Membrane Fluidity

• At high temperatures: Cholesterol reduces fluidity by restricting the movement of phospholipids. It acts as a buffer, preventing the membrane from becoming too fluid and permeable
• At low temperatures: Cholesterol prevents the fatty acid tails from packing too closely together, which would make the membrane too rigid. It disrupts the regular packing, maintaining fluidity
• Overall, cholesterol acts as a fluidity buffer, maintaining an optimal level of membrane fluidity across a range of temperatures

Cholesterol also reduces the permeability of the membrane to small water-soluble molecules and ions by filling gaps between phospholipids.

Exam Tip: A common 6-mark question asks you to describe the effect of temperature on membrane permeability. Make sure you include the role of cholesterol and explain what happens when proteins denature at very high temperatures, increasing permeability significantly.

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Membrane Proteins

Proteins make up approximately 50% of the mass of the cell membrane (though this varies between cell types). They perform a wide range of essential functions.

Intrinsic (Integral) Proteins

These span the full thickness of the phospholipid bilayer. Examples include:

• Channel proteins — form hydrophilic pores that allow specific ions or small polar molecules to pass through the membrane by facilitated diffusion. Some are gated, meaning they can open or close in response to stimuli (e.g. voltage-gated sodium channels in neurons)
• Carrier proteins — bind to specific molecules on one side of the membrane, change shape, and release the molecule on the other side. They are involved in both facilitated diffusion and active transport
• Receptor proteins — have a specific binding site on the extracellular surface for signalling molecules such as hormones or neurotransmitters. Binding triggers a cellular response (signal transduction)

Extrinsic (Peripheral) Proteins

These are attached to one surface (inner or outer) of the membrane but do not span it. They function as:

• Enzymes catalysing reactions at the membrane surface
• Structural anchors connecting the membrane to the cytoskeleton or extracellular matrix
• Components of cell signalling pathways

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Glycoproteins and Glycolipids

Carbohydrate chains (oligosaccharides) attached to proteins (glycoproteins) and lipids (glycolipids) on the extracellular surface of the membrane form the glycocalyx — a carbohydrate-rich zone on the cell surface.

Functions of the Glycocalyx

1. Cell recognition — allows the immune system to distinguish between "self" and "non-self" cells (this is the basis of the immune response and transplant rejection)
2. Cell signalling — glycoproteins act as receptors for hormones and other signalling molecules
3. Cell adhesion — helps cells stick to each other and to the extracellular matrix, forming tissues
4. Protection — forms a protective layer against mechanical and chemical damage

Exam Tip: ABO blood groups are determined by the specific glycolipids and glycoproteins (antigens) present on the surface of red blood cells. This is a good real-world example to use in exam answers about cell recognition.

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Factors Affecting Membrane Structure and Permeability

Several factors affect the fluidity and permeability of cell membranes:

Temperature

Temperature range	Effect on membrane
Below optimum	Phospholipids have less kinetic energy; membrane becomes more rigid; less permeable
At optimum (~37 degrees C for human cells)	Membrane is selectively permeable; optimal fluidity
Above optimum	Phospholipids have more kinetic energy; membrane becomes more fluid and permeable; at very high temperatures, proteins denature and the membrane loses its selective permeability entirely

Solvents

Organic solvents (e.g. ethanol, acetone) can dissolve the phospholipids in the membrane, disrupting its structure and increasing permeability. This is the principle behind the beetroot cell membrane permeability practical.

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Core Practical: Investigating Membrane Permeability

The Edexcel specification includes a core practical on investigating the effect of temperature (or a named solvent such as ethanol) on membrane permeability, using beetroot as the experimental material.

Principle

Beetroot cells contain a red pigment called betacyanin (or betalain) in their vacuoles. When the cell membrane is damaged or becomes more permeable, betacyanin leaks out into the surrounding solution. The amount of pigment released can be measured using a colorimeter (measuring absorbance or percentage transmission).

Method

1. Cut beetroot into uniform cylinders/discs using a cork borer
2. Rinse thoroughly to remove pigment from cut surfaces
3. Place beetroot pieces in water baths at different temperatures (e.g. 20, 30, 40, 50, 60, 70 degrees C) for a set time
4. Remove the beetroot and measure the absorbance of the surrounding solution using a colorimeter (with a blue-green filter, ~530 nm)
5. Higher absorbance = more pigment released = greater membrane permeability

Expected Results

• At low temperatures, little pigment leaks out (low absorbance)
• As temperature increases, membrane permeability increases gradually (phospholipids become more fluid)
• At high temperatures (above ~60 degrees C), there is a sharp increase in pigment release because membrane proteins denature, creating large gaps in the membrane

Variables

Variable	Details
Independent	Temperature of the water bath
Dependent	Absorbance of the solution (measured by colorimeter)
Control variables	Size of beetroot pieces, volume of water, duration of incubation, same beetroot source

Exam Tip: You must be able to describe this practical and explain the results in terms of membrane structure. Remember: at moderate temperatures, the increase in permeability is due to increased fluidity of the phospholipid bilayer. At high temperatures, the dramatic increase is due to protein denaturation and phospholipid bilayer disruption.

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Cell Signalling and the Membrane

The cell membrane plays a crucial role in cell communication (cell signalling). Receptor proteins on the cell surface bind specific signalling molecules (ligands), triggering a response inside the cell.

Types of Cell Signalling

• Endocrine signalling — hormones travel in the blood to distant target cells (e.g. insulin binding to receptors on liver/muscle cells)
• Paracrine signalling — signalling molecules act on nearby cells (e.g. histamine released during inflammation)
• Autocrine signalling — cells respond to their own signalling molecules
• Synaptic signalling — neurotransmitters cross a synapse to bind to receptors on the postsynaptic membrane

The specificity of cell signalling depends on the complementary shape of the receptor protein and the ligand — only cells with the correct receptor will respond to a particular signalling molecule.

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Summary

• The fluid mosaic model describes the cell membrane as a phospholipid bilayer with embedded proteins
• Phospholipids are amphipathic, forming a bilayer that acts as a selective barrier
• Cholesterol regulates membrane fluidity across different temperatures
• Intrinsic proteins include channel, carrier and receptor proteins
• Glycoproteins and glycolipids on the cell surface are involved in cell recognition and signalling
• Temperature and solvents affect membrane permeability — testable in the beetroot practical
• The membrane is central to cell signalling through specific receptor-ligand interactions

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A-Level Deep Dive: The Cell Membrane: Structure and Function

Spec mapping

The Edexcel 9BI0 specification places membrane structure within Topic 2: Membranes, Proteins, DNA and Gene Expression. The relevant statements require candidates to: describe the fluid mosaic model of cell-membrane structure (Singer–Nicolson, 1972), naming the phospholipid bilayer, intrinsic and extrinsic proteins, cholesterol, glycoproteins and glycolipids; explain how the amphipathic nature of phospholipids drives spontaneous bilayer formation in aqueous environments; describe how membrane fluidity is modulated by temperature, fatty-acid saturation and cholesterol content; and explain the role of membrane proteins in transport, cell recognition and cell signalling. Synoptic threads run forward within the same course to Topic 2 transport mechanisms (the next lesson on diffusion and osmosis, then active and bulk transport), into Topic 4 (Microbiology, Disease and the Immune System) — antibiotic targeting of bacterial membrane components and the host-derived envelopes of budding viruses such as HIV — and into Topic 5 (On the Wild Side) and Topic 7 (Run for Your Life), where mitochondrial cristae and chloroplast thylakoid membranes provide the surface area for the electron-transport chain and the light-dependent reactions (refer to the official Pearson Edexcel 9BI0 specification document for exact wording).

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Worked example with full mark scheme

Question (8 marks):

A student investigates how temperature affects the fluidity of an isolated phospholipid bilayer prepared from animal-cell membranes. Two preparations are made: one from cells grown at $37^\circ C$37∘C and one from cells grown at $10^\circ C$10∘C. The fluidity is measured at temperatures from $0^\circ C$0∘C to $40^\circ C$40∘C.

(a) Using your knowledge of phospholipid structure, explain why a bilayer forms spontaneously when phospholipids are placed in water. (3)

(b) Predict, with reasoning, which of the two preparations is likely to contain a higher proportion of unsaturated fatty acids, and explain why. (3)

(c) Explain one role that cholesterol plays in maintaining membrane fluidity across this temperature range. (2)

Solution with mark scheme:

(a) Step 1 — identify the amphipathic nature of the phospholipid. Each phospholipid has a hydrophilic phosphate head and two hydrophobic fatty-acid tails, making the molecule amphipathic.

M1 (AO1.1) — correct identification of the amphipathic nature, with both hydrophilic and hydrophobic regions named. A common pitfall is to write only "phospholipids have a head and tails" without naming the polarity, which omits the AO1 vocabulary the examiner expects.

Step 2 — explain the thermodynamics. In an aqueous environment, the hydrophobic tails are excluded from contact with water; the lowest-energy arrangement places the tails together and the polar heads facing outwards. A bilayer therefore forms, with both faces hydrated and a hydrophobic interior.

M1 (AO2.1) — correct application: hydrophobic tails minimise contact with water by clustering inwards.

A1 (AO1.2) — explicit statement that the bilayer is held together by hydrophobic interactions (not covalent bonds), which is why the membrane is fluid rather than crystalline. A bare "they line up" with no thermodynamic reasoning scores M1 only.

(b) Step 1 — link unsaturation to fluidity. Unsaturated fatty acids contain C=C double bonds that introduce kinks, preventing the tails from packing closely together. This raises fluidity at any given temperature.

M1 (AO2.1) — correct link between unsaturation, kinking and reduced packing.

Step 2 — apply to the cold-grown preparation. Cells grown at $10^\circ C$10∘C would otherwise be at risk of their membranes solidifying; by inserting more unsaturated phospholipids, they maintain fluidity at low temperature (homeoviscous adaptation).

M1 (AO3.1a) — correct prediction that the $10^\circ C$10∘C-grown preparation has the higher proportion of unsaturated fatty acids.

A1 (AO3.2a) — justification linking the structural feature (kinks) to the functional outcome (preserved fluidity). Many candidates lose marks here by stating the prediction without the mechanistic justification.

(c) M1 (AO1.2) — cholesterol acts as a fluidity buffer: at high temperatures it reduces fluidity by restricting phospholipid movement; at low temperatures it disrupts close packing of fatty-acid tails and prevents the membrane from becoming too rigid.

A1 (AO2.1) — explicit statement that cholesterol therefore raises fluidity at low temperature and lowers fluidity at high temperature, maintaining an optimal range. Stating only that "cholesterol stiffens the membrane" loses A1 because it captures only half of the buffering role.

Total: 8 marks.

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Specimen question modelled on the Edexcel 9BI0 paper format

Question (6 marks): Describe the fluid mosaic model of the cell-surface membrane, and explain how its structure allows the membrane to function in both selective transport and cell recognition.

Mark scheme decomposition by AO:

Marking point	AO	Credit-worthy content
1	AO1.1	States that the membrane is a phospholipid bilayer with hydrophilic heads outwards and hydrophobic tails inwards, with embedded and surface-attached proteins.
2	AO1.2	Names the components: intrinsic (transmembrane) proteins, extrinsic (peripheral) proteins, cholesterol, glycoproteins and glycolipids.
3	AO2.1	Applies structure to selective transport: the hydrophobic core blocks polar/charged solutes; channel and carrier proteins provide selective routes for ions and polar molecules.
4	AO2.1	Applies structure to cell recognition: glycoproteins and glycolipids of the glycocalyx project carbohydrate chains that act as antigens distinguishing self from non-self.
5	AO3.1a	Interprets the fluid aspect: lateral diffusion of components allows receptors to cluster, vesicles to fuse and the membrane to self-seal after damage.
6	AO3.2a	Concludes by linking structure to function — e.g. "the mosaic distribution of selective protein channels and recognition glycoproteins, embedded in a fluid bilayer, allows simultaneous selective permeability and dynamic cell-surface signalling".

Total: 6 marks split AO1 = 2, AO2 = 2, AO3 = 2. This is a classic Section A "describe and explain" question — Edexcel rewards candidates who use the model (AO2/AO3) rather than merely listing components (AO1).

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Synoptic links