Cell Membranes & Transport
The fluid mosaic model from 1972 onwards: how phospholipids, cholesterol, glycolipids and proteins assemble into selectively permeable barriers; cell signalling; and the five main routes by which substances move into and out of cells — passive and active.
Fluid mosaic membranes
The fluid mosaic model, proposed by Singer and Nicolson in 1972, describes the structure of cell surface membranes. The membrane is a phospholipid bilayer (the “fluid” component) studded with diverse proteins and other molecules (the “mosaic” component). The model is the foundation for understanding selective permeability, transport, signalling, and cell recognition.
The phospholipid bilayer
Membrane structure follows directly from the amphipathic nature of phospholipids (Topic 2.2). In water, phospholipid molecules spontaneously arrange themselves so that:
- Hydrophilic phosphate heads face the aqueous environment outside the cell and the aqueous cytoplasm inside
- Hydrophobic fatty acid tails face inwards, away from water, forming the membrane core
Individual phospholipids are not fixed in place — they diffuse laterally within their own monolayer (transferring from one leaflet to the other is rare). The bilayer therefore behaves like a two-dimensional liquid: cohesive enough to hold its shape, fluid enough to let proteins drift across the membrane and to reseal small punctures spontaneously.
Hydrophobic interactions are the main driving force for bilayer assembly: the hydrophobic tails are pushed together because water molecules prefer to hydrogen-bond with each other rather than surround non-polar tails. Hydrogen bonds between water and the phosphate heads stabilise the outer surfaces.
Other membrane components
Cholesterol is a steroid lipid found inserted between phospholipids in animal cell membranes. Its small hydroxyl group sits near the membrane surface; its rigid hydrocarbon ring system sits within the hydrophobic core.
Effects:
- At higher temperatures: restricts movement of phospholipids, reducing fluidity and preventing the membrane from becoming too leaky
- At lower temperatures: keeps phospholipids apart, preventing tight packing — maintaining fluidity
- Increases overall mechanical stability
A glycolipid is a phospholipid (or other lipid) with a short carbohydrate chain attached to its head. The carbohydrate always faces the extracellular side of the membrane.
Roles: cell recognition (the carbohydrate chain acts as an antigen identifying the cell type, including blood group antigens), receptors for some signals, and stabilisation of membrane structure on the outer surface.
A glycoprotein is a membrane protein with a carbohydrate chain covalently attached, projecting into the extracellular space. Often more elaborate than the glycolipid carbohydrates.
Roles: cell-cell recognition (e.g. ABO blood group antigens), self vs non-self recognition by the immune system (Topic 11), receptors for hormones and other signals, and contributing to the glycocalyx.
Membrane proteins fall into two structural classes:
- Intrinsic / integral: span the entire bilayer; have hydrophobic regions matching the membrane core. Include channels, carriers, and many receptors.
- Extrinsic / peripheral: attached to one face of the membrane only; usually held by ionic or hydrogen bonds to phospholipid heads or to integral proteins.
Roles of membrane components
| Component | Stability | Fluidity | Permeability | Transport | Signalling | Recognition |
|---|---|---|---|---|---|---|
| Phospholipids | ● | ● | ● | — | — | — |
| Cholesterol | ● | ● | ● | — | — | — |
| Glycolipids | ● | — | — | — | ● | ● |
| Channel/Carrier proteins | — | — | ● | ● | — | — |
| Receptor proteins | — | — | — | — | ● | — |
| Glycoproteins | — | — | — | ● | ● | ● |
Glycoproteins and glycolipids on the outer membrane act as cell surface antigens, allowing the immune system to identify cells as “self” or “non-self”. This is developed in Topic 11.1 (immunology). When you write about these molecules in Topic 4, mention that they enable cell recognition; full mechanism details belong with immunology.
Cell signalling
Cells communicate with each other by releasing chemical signals (ligands — e.g. hormones, neurotransmitters, growth factors) that bind to specific receptors on target cells. The fluid mosaic membrane is central to this process: receptors are integral membrane proteins (often glycoproteins) on the cell surface.
- Signal release: a signalling cell secretes a specific ligand (e.g. an endocrine cell releases a hormone into the blood)
- Signal transport: the ligand travels to its target cell (through bloodstream, extracellular fluid, or synaptic gap)
- Receptor binding: the ligand binds to a complementary receptor on the target cell membrane — this is highly specific (only cells with the right receptor respond)
- Signal transduction: receptor binding triggers a cascade of events inside the cell (often via second messengers and a series of protein modifications)
- Cellular response: the cascade produces a specific outcome — gene expression change, enzyme activation, channel opening, etc.
A hormone in the blood reaches every cell, but only those with a complementary receptor respond. Different target cells often have different receptors or different downstream cascades, so the same hormone (e.g. adrenaline) can produce different effects in different tissues (heart rate up, blood vessel contraction in skin, glucose release from liver). Specificity comes from the receptor + downstream pathway, not the ligand alone.
Which feature of the fluid mosaic model is most directly responsible for membrane self-repair after small punctures?
- A. The presence of cholesterol between phospholipids
- B. Carbohydrate chains on glycoproteins facing outward
- C. The lateral fluidity of the phospholipid bilayer, allowing molecules to move and fill gaps
- D. Covalent bonds between phospholipid heads
Describe the structure of a cell surface membrane according to the fluid mosaic model. Refer to phospholipids, cholesterol, proteins, and carbohydrates in your answer. [6]
Six creditable points (any six):
- The membrane consists of a phospholipid bilayer with hydrophilic heads facing outwards (toward water on both sides) and hydrophobic tails facing inwards (toward each other) [1]
- Cholesterol molecules are inserted between phospholipids, modulating membrane fluidity at different temperatures [1]
- Membrane proteins are scattered throughout the bilayer in a mosaic pattern [1]
- Intrinsic (integral) proteins span the whole bilayer; extrinsic (peripheral) proteins are attached to only one face [1]
- Carbohydrates are attached to some proteins (glycoproteins) and to some lipids (glycolipids), always on the extracellular face [1]
- The membrane is fluid because phospholipids and many proteins can diffuse laterally within the bilayer [1]
- The structure provides a selectively permeable barrier — small non-polar molecules can pass directly through the bilayer; polar molecules and ions need protein channels or carriers [1]
Mark scheme guidance: Points must mention each named component to access full marks. Stating “mosaic” without explaining what makes the mosaic (proteins of varied position) is insufficient.
Movement of substances into and out of cells
Substances cross cell membranes by five main routes. Three are passive (no metabolic energy required): simple diffusion, facilitated diffusion, and osmosis. Two are active (requiring ATP from respiration): active transport, and bulk transport via endo/exocytosis.
Overview — the five routes
| Route | Substance type | Direction | Energy | Membrane proteins involved |
|---|---|---|---|---|
| Simple diffusion | Small non-polar molecules (O2, CO2, steroid hormones) | Down concentration gradient | None (kinetic energy of molecules) | None — through phospholipid bilayer |
| Facilitated diffusion | Polar molecules and ions (glucose, amino acids, Na+) | Down concentration gradient | None | Channel proteins, carrier proteins |
| Osmosis | Water | Higher water potential → lower water potential | None | Aquaporins (channels) and through bilayer |
| Active transport | Specific ions and molecules | Against concentration gradient | ATP required | Carrier proteins (often called pumps) |
| Endo/exocytosis | Bulk: large molecules, particles | Bulk movement via vesicles | ATP required | Membrane is itself bent and fused |
Diffusion (simple)
Diffusion is the net movement of molecules or ions from a region of higher concentration to a region of lower concentration, down a concentration gradient. The energy comes from the kinetic (thermal) energy of the molecules themselves — not from the cell.
Only substances that can pass through the hydrophobic core: small (high diffusion coefficient) and non-polar (interact well with hydrocarbon tails). Examples include O2, CO2, N2, and lipid-soluble molecules such as steroid hormones (testosterone, oestrogen). Polar molecules (glucose, amino acids) and ions (Na+, K+, Cl−) cannot cross the bilayer directly.
Factors affecting rate of diffusion (Fick's law in plain language):
| Factor | Effect on rate | Reason |
|---|---|---|
| Concentration gradient | Steeper gradient → higher rate | More net flux per unit time |
| Surface area | Larger SA → higher rate | More molecules can cross at the same time |
| Diffusion distance | Shorter distance → higher rate | Less time required to cross |
| Temperature | Higher T → higher rate | Greater kinetic energy of molecules |
| Molecule size | Smaller molecule → higher rate | Lower mass → higher speed at given KE |
| Molecule polarity | Less polar → higher rate (across bilayer) | Less interaction with water; more compatible with hydrophobic core |
Surface area to volume ratio & diffusion
The surface area to volume ratio (SA:V) is a critical factor determining how efficiently a cell or organism can exchange substances with its environment by diffusion. As an organism or cell grows larger, its volume increases faster than its surface area — reducing the SA:V ratio and making diffusion progressively less effective.
For a cube of side length s:
- Surface area = 6s2
- Volume = s3
- SA:V ratio = 6s2 / s3 = 6/s
As s increases, SA:V decreases. This means a larger cube has proportionally less surface available per unit of volume. This principle explains why cells cannot simply grow indefinitely — diffusion cannot supply materials fast enough to a large volume through a relatively small surface.
| Side length (s) / mm | Surface area / mm² | Volume / mm³ | SA:V ratio |
|---|---|---|---|
| 1 | 6 | 1 | 6.0 |
| 2 | 24 | 8 | 3.0 |
| 3 | 54 | 27 | 2.0 |
| 4 | 96 | 64 | 1.5 |
As size doubles, SA increases by 4× but volume increases by 8× — so SA:V halves. The pattern: larger size = lower SA:V = slower relative rate of diffusion into the interior.
The effect of SA:V ratio on diffusion can be investigated using agar blocks of different sizes:
- Prepare coloured agar (e.g. agar with dilute NaOH and universal indicator — purple/pink at alkaline pH; or agar with phenolphthalein)
- Cut agar into cubes of different sizes (e.g. 0.5 cm, 1 cm, 1.5 cm, 2 cm per side); calculate SA:V for each
- Place all cubes simultaneously into dilute acid (e.g. HCl); the acid diffuses in and causes a colour change
- Measure the time for complete colour change (or measure depth of colour change after a fixed time)
- Calculate rate = 1/time (or depth/time); plot rate vs SA:V ratio
Expected result: cubes with higher SA:V (smaller cubes) show a faster rate of complete colour change because a greater proportion of the volume is close to the surface; diffusion distance to the centre is smaller. Larger cubes take longer because diffusion must penetrate further into a greater volume.
Control variable: ensure cubes go into acid simultaneously; use the same acid concentration for all cubes; use the same type and concentration of indicator in all agar blocks.
- Cells must divide before they grow too large — beyond a critical size, diffusion cannot supply materials to the centre or remove waste fast enough; this limits maximum cell size
- Structures that increase SA:V: microvilli in the intestine, root hair cells, alveoli, gill lamellae — all maximise exchange surface relative to volume
- Large organisms overcome low SA:V by developing specialised transport and exchange systems (blood circulatory systems, lungs, gills)
Facilitated diffusion
Facilitated diffusion is the passive transport of polar molecules or ions through specific membrane proteins. It is still diffusion (down the gradient, no ATP) but requires a protein because the substance cannot cross the bilayer alone.
An integral protein with a water-filled hydrophilic channel through the middle. Allows specific ions or small polar molecules to pass through.
Most channels are gated: they open in response to a stimulus (voltage change, ligand binding, mechanical pressure) and close again, providing fine regulation.
Examples: voltage-gated Na+ channels in nerve membranes (Topic 15); aquaporins for water (osmosis).
An integral protein that binds a specific molecule on one side, then changes shape to release it on the other side. For facilitated diffusion, this still requires no ATP — the conformational change is driven by the binding of the substrate moving down its gradient.
Examples: GLUT family transporters for glucose; amino acid transporters in gut and kidney.
Osmosis & water potential
Water moves across selectively permeable membranes by a special form of diffusion called osmosis. The 9700 syllabus uses the formal language of water potential, denoted by the Greek letter Ψ (psi). Be aware: examiner reports specifically penalise candidates for using “water concentration” instead of “water potential”.
Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane.
Water potential measures the tendency of water to move from one place to another. Pure water has a water potential of zero (the highest possible value under normal conditions). Adding solute to water lowers its water potential (makes it more negative).
Common units: kilopascals (kPa). Typical values: pure water = 0 kPa; mammalian blood plasma ≈ −7000 kPa; concentrated sucrose solution might be −2000 kPa or more negative.
Three traps that lose marks — the 9700 examiner reports specifically call these out:
- “Water moves from higher to lower water concentration” — WRONG. Use “water potential”.
- “Water moves from a high water potential gradient to a low water potential gradient” — WRONG. Water moves down a water potential gradient, not from a high gradient to a low gradient.
- “Water diffuses through the membrane” — acceptable as a description of mechanism, but specifically the membrane must be described as partially permeable.
Effect of osmosis on cells
Animal and plant cells respond very differently to osmotic stress, because plant cells have a strong cell wall and animal cells do not.
| External solution | Animal cell | Plant cell |
|---|---|---|
| Higher Ψ than cell (hypotonic) — pure water or dilute solution | Water enters by osmosis → cell swells → eventually bursts (haemolysis) because no cell wall | Water enters by osmosis → vacuole expands → pressure builds against cell wall → cell becomes turgid; wall prevents bursting |
| Equal Ψ (isotonic) | No net movement — cell stable | No net movement — cell flaccid; no turgor pressure |
| Lower Ψ than cell (hypertonic) — concentrated solution | Water leaves by osmosis → cell shrivels → crenation | Water leaves vacuole → protoplast shrinks and pulls away from cell wall → plasmolysis |
Method: Cut potato cylinders of equal length and mass. Place each in a different concentration of sucrose solution (e.g. 0.0, 0.2, 0.4, 0.6, 0.8, 1.0 mol dm−3). After about 30 minutes, blot dry and re-measure mass.
Calculate: percentage change in mass = (final mass − initial mass) / initial mass × 100. Plot percentage change against sucrose concentration.
Find the isotonic point: the concentration at which percentage change = 0 indicates the sucrose solution with the same water potential as the potato cells. Read this from the graph (where the line crosses zero).
Controls: use the same potato; identical-sized cylinders; same temperature; same time submerged; blot consistently before re-weighing.
Active transport
Active transport is the movement of specific substances across a membrane against their concentration gradient (from low concentration to high concentration). Energy from ATP (produced in respiration) is required.
- The substrate molecule binds to a specific binding site on the carrier protein on the side of low concentration
- ATP is hydrolysed to ADP + Pi; the phosphate becomes attached to the carrier protein
- The phosphorylation drives a conformational change in the carrier protein
- The substrate is now exposed to the side of high concentration and is released
- The phosphate group leaves the carrier protein, which returns to its original shape, ready to bind another substrate
Found in almost every animal cell. Pumps 3 Na+ out and 2 K+ in per ATP molecule. Maintains the resting potential of nerves and muscles (Topic 15) and creates gradients used to drive secondary active transport.
Plants take up nitrate, phosphate, and other mineral ions from soil that has lower ion concentrations than inside the root cells. Carrier proteins in the root hair membrane actively pump ions in — explaining why root hair cells have many mitochondria.
In the small intestine, glucose is absorbed against its gradient by sodium-glucose co-transporters. The gradient driving glucose uptake is the Na+ gradient maintained by the Na+/K+ pump — an example of secondary active transport (mechanism beyond the syllabus, but the example is on it).
Endocytosis and exocytosis
Some substances are too large to pass through any protein channel or carrier — for example, large polypeptides, polysaccharides, lipid droplets, whole cells. These are moved across the membrane in bulk, packaged inside membrane-bound vesicles. Both directions require ATP.
- The cell surface membrane folds inwards, surrounding the material
- The membrane pinches off, forming an endocytic vesicle that enters the cytoplasm
Two types in syllabus terminology:
- Phagocytosis (“cell eating”) — uptake of solid material such as bacteria. Used by phagocytes in the immune system (Topic 11)
- Pinocytosis (“cell drinking”) — uptake of fluid containing dissolved substances; smaller vesicles, more general process
- A vesicle inside the cell (often from the Golgi apparatus) moves to the cell surface membrane
- The vesicle membrane fuses with the cell surface membrane
- The vesicle contents are released into the extracellular space
Examples:
- Secretion of digestive enzymes (e.g. amylase) from gland cells
- Release of neurotransmitters at synapses
- Secretion of insulin by β-cells of pancreas
Which transport process(es) require ATP from respiration?
- A. Simple diffusion only
- B. Osmosis and facilitated diffusion
- C. Active transport, endocytosis, and exocytosis
- D. All transport processes require ATP
A potato cylinder of initial mass 5.00 g is placed in a 0.4 mol dm−3 sucrose solution for 30 minutes. After blotting, its mass is 4.65 g.
(a) Calculate the percentage change in mass. Show your working. [2]
(b) Explain, using the term water potential, why the mass changed in this way. [3]
(c) Suggest two changes the student could make to improve the reliability of the experiment. [2]
(a) Percentage change [2 marks]
Change in mass = 4.65 − 5.00 = −0.35 g [1]
Percentage change = (−0.35 / 5.00) × 100 = −7.0% [1]
(b) Explanation [3 marks]
- The 0.4 mol dm−3 sucrose solution had a lower (more negative) water potential than the potato cells [1]
- Water moved out of the potato cells by osmosis, from higher water potential (inside cells) to lower water potential (sucrose solution), through partially permeable cell membranes [1]
- Loss of water reduced cell volume and overall mass of the cylinder — the cells became less turgid / began to plasmolyse [1]
(c) Reliability improvements [2 marks; any 2 from list]
- Use replicates (e.g. 3 cylinders per concentration) and calculate the mean
- Standardise initial dimensions and mass more precisely (e.g. all to within 0.01 g)
- Standardise blotting (same number of pats with paper towel) before each weighing
- Maintain constant temperature throughout (water bath)
- Standardise the time submerged exactly
- Cut all cylinders from the same potato to control variation between potatoes
Mark scheme guidance: Part (b) is a classic phrasing trap — using “water concentration” instead of “water potential” loses the explanation marks. The membrane must also be described as partially permeable.
Topic 4 Practice — Comprehensive
Mixed practice covering both sub-sections in 9700 P1/P2 style. Try each before revealing the answer.
Which of the following correctly describes cholesterol in the cell surface membrane?
- A. It is a phospholipid forming part of the bilayer.
- B. It spans the entire membrane and acts as a transport protein.
- C. It is a steroid lipid that sits between phospholipids and modulates membrane fluidity.
- D. It is found only on the outer membrane surface and acts as a cell recognition marker.
A cell uses a sodium-potassium pump to maintain ion gradients. Which statement about this transport is correct?
- A. Na+ and K+ move down their concentration gradients without ATP.
- B. The pump uses kinetic energy of the ions to maintain the gradient.
- C. ATP hydrolysis powers a conformational change in the carrier protein, moving ions against their gradients.
- D. The pump is a channel protein that passively allows ion movement.
A red blood cell is placed in pure water. What happens, and why?
- A. The cell shrinks because water moves out by osmosis.
- B. The cell swells and bursts because pure water has higher water potential than the cell contents.
- C. The cell becomes turgid as the wall resists pressure from incoming water.
- D. No change, because animal cells maintain constant volume by active transport.
Steroid hormones such as oestrogen are non-polar molecules. They cross cell membranes very differently from peptide hormones such as insulin.
(a) Explain how oestrogen crosses the cell surface membrane to reach receptors inside the cell. [3]
(b) Insulin cannot cross the cell surface membrane and binds to receptors on the outer membrane surface. Explain why, and outline what happens after insulin binds. [5]
(a) Oestrogen crosses by simple diffusion [3 marks]
- Oestrogen is small and non-polar / hydrophobic / lipid-soluble [1]
- It can dissolve in / pass directly through the hydrophobic core of the phospholipid bilayer [1]
- Movement is by simple diffusion, down its concentration gradient, no carrier protein or ATP required [1]
(b) Insulin signalling [5 marks]
- Insulin is a polypeptide / protein, so it is large and polar [1]
- It cannot dissolve in / pass through the hydrophobic core of the bilayer [1]
- Insulin binds to a complementary insulin receptor (a glycoprotein) on the outer surface of the target cell membrane [1]
- Receptor binding triggers signal transduction — a cascade of events inside the cell (often via second messengers) [1]
- This produces the cellular response (e.g. insertion of glucose transporters into the membrane → increased glucose uptake) [1]
Synoptic note: This question links Topic 2.2 (lipids) with Topic 4 (membrane structure and signalling). The same hormone reaches every cell in the body, but only cells with the correct receptor respond — an important specificity principle for endocrinology (Topic 14).
Root hair cells take up mineral ions from soil even when the ion concentration in the soil is much lower than inside the cells. Root hair cells contain many mitochondria.
Explain how the structure and contents of root hair cells are adapted to this function. [6]
Six creditable points (any six):
- Mineral ions enter the root hair cell by active transport because the ion concentration in soil is lower than inside the cell (movement against the concentration gradient) [1]
- Active transport requires energy in the form of ATP [1]
- ATP is produced by aerobic respiration, which takes place in mitochondria [1]
- Root hair cells have many mitochondria to supply the high ATP demand for active transport [1]
- The cell surface membrane contains specific carrier proteins (pumps) for each type of ion (e.g. nitrate, phosphate) [1]
- The root hair is a long, thin extension of the cell, increasing the surface area in contact with soil — this increases the rate of ion uptake [1]
- Larger surface area also increases water uptake by osmosis (water moves into the cell after ion concentration is raised, lowering Ψ inside) [1]
Topic 4 — Common Mistakes
- 💧Saying “water concentration” instead of “water potential”The 9700 examiner reports specifically penalise this. Water moves from higher water potential to lower water potential — not from higher water concentration to lower water concentration. Get the terminology right; it is heavily marked.
- 🧳“Water moves from a high water potential gradient to a low water potential gradient”Also wrong. Water moves down a water potential gradient, or from a high water potential to a low water potential. There is no “high gradient” or “low gradient” — gradients are themselves directional.
- 🔒Saying osmosis happens through any membraneOsmosis specifically requires a partially permeable membrane (allows water but restricts solutes). Forgetting to specify this loses easy marks.
- ♻Confusing facilitated diffusion with active transportBoth involve membrane proteins. The difference is direction and energy: facilitated diffusion is passive, down the gradient, no ATP. Active transport is against the gradient and requires ATP.
- ⚡Saying simple diffusion needs proteinsSimple diffusion goes directly through the phospholipid bilayer — no proteins required. This works only for small non-polar molecules (O2, CO2, steroids). Polar molecules and ions need facilitated diffusion or active transport.
- 🌻Confusing plasmolysis with crenationBoth occur in hypertonic solutions, but plasmolysis is in plant cells — the protoplast pulls away from the cell wall. Crenation is in animal cells — the whole cell shrivels because there is no wall to peel from.
- 🧹Saying turgid cells “burst”Plant cells become turgid (rigid, full) but the strong cell wall prevents bursting. Only animal cells lacking a wall lyse / burst when placed in hypotonic conditions.
- 🍺Cholesterol is a phospholipidWrong. Cholesterol is a steroid lipid — a different chemical family from phospholipids. It sits between phospholipids in the bilayer but is not a phospholipid itself.
- 🎯Glycoproteins are inside the cellThe carbohydrate chains of glycoproteins (and glycolipids) always face the extracellular side of the membrane — they need to be exposed to the outside world to function in cell recognition and signalling.
- ⚔Saying endocytosis and exocytosis don’t need ATPBoth processes require ATP — for cytoskeletal rearrangement that bends the membrane, and for vesicle transport. Bulk transport is always active.
- 🧬Treating osmosis as completely separate from diffusionOsmosis IS diffusion — specifically the diffusion of water across a partially permeable membrane. Saying “not diffusion” loses marks. The right description is “a special case of diffusion involving water”.
Topic 4 underpins Topic 7 (transport in plants), Topic 8 (transport in mammals), Topic 11 (immunology — cell surface antigens), Topic 14 (homeostasis), and Topic 15 (control and coordination — nerve impulses). Highest-yield items: fluid mosaic structure with all six components, the five transport routes with their energy requirements, water potential terminology (the most penalised topic in the syllabus), and the potato osmosis experiment with percentage change calculations.