Fluid-Mosaic Model and Membrane Structure — AQA A-level Biology

SPEC 3.2.3 - CELLS — MEMBRANES & TRANSPORT

Cell Membranes: Fluid Mosaic, Diffusion, Osmosis & Active Transport

Fluid mosaic model · Five transport mechanisms · Required Practicals 3 & 4 · Co-transport in the ileum

The Fluid Mosaic Model

All cell membranes share the same basic structure, described by the fluid mosaic model (Singer & Nicolson, 1972).

Definition

Fluid mosaic model: The model of membrane structure in which phospholipids form a fluid bilayer in which proteins are embedded and free to move laterally.

Why "Fluid"? Why "Mosaic"?

Fluid — phospholipids and proteins can move laterally within the membrane (not fixed in place)

Mosaic — proteins are distributed unevenly across the surface, like tiles in a mosaic pattern

Synoptic Link

The fluid mosaic model applies to ALL membranes in cells — not just the cell-surface membrane. The inner mitochondrial membrane, chloroplast thylakoid membrane and nuclear envelope all share this structure. Exam questions may test this in the context of respiration or photosynthesis.

Membrane Components & Their Roles

ComponentLocationFunction

Phospholipid bilayerForms the bilayer itselfHydrophobic core excludes ions and polar molecules; selectively permeable barrier

CholesterolEmbedded between phospholipidsRestricts phospholipid movement; reduces fluidity at high temperatures; prevents solidification at low temperatures

GlycoproteinsOuter surface only (asymmetric)Cell signalling; cell recognition; act as antigens

GlycolipidsOuter surface only (asymmetric)Cell recognition; cell adhesion; stability

Channel proteinsSpan bilayer (intrinsic)Allow specific ions / polar molecules to cross by facilitated diffusion

Carrier proteinsSpan bilayer (intrinsic)Bind specific molecules, change shape; used in facilitated diffusion AND active transport

Receptor proteinsInner or outer surfaceBind hormones and neurotransmitters; involved in signal transduction

Phospholipid Bilayer — Labelled Diagram

The hydrophilic heads face the aqueous environments inside and outside the cell. The hydrophobic tails face inward, creating a barrier to polar molecules and ions.

Cholesterol & Membrane Fluidity

Cholesterol molecules are inserted between phospholipids in the bilayer. They play a critical role in regulating membrane fluidity across temperature ranges.

High Temperature

Phospholipids have more kinetic energy → membrane becomes too fluid → risk of losing integrity. Cholesterol restricts phospholipid movement, preventing excessive fluidity.

Low Temperature

Phospholipids lose kinetic energy → risk of solidifying. Cholesterol prevents tight packing of phospholipid tails, maintaining fluidity at low temperatures.

Key Fact

Cholesterol is found in animal cell membranes and is absent (or minimal) in most bacterial cell membranes. This is why membrane fluidity is particularly important in animal physiology — it affects hormone receptor function, neurotransmitter release and phagocytosis.

Synoptic Link — Required Practical 4

When beetroot is heated, increasing temperature increases membrane fluidity and disrupts the lipid bilayer, allowing betalain pigment to leak. This is why absorbance increases with temperature in RP4.

Five Transport Mechanisms

MechanismEnergy?DirectionProteins?Examples

Simple diffusionNoneDown conc. gradientNoO₂, CO₂, lipid-soluble molecules, ethanol

Facilitated diffusionNoneDown conc. gradientYes (channel or carrier)Glucose (into RBC), ions (Na⁺, K⁺)

OsmosisNoneDown water potential gradientNo — through partially permeable membraneWater into root hair cells; water across gut wall

Active transportATP (direct)Against conc. gradientYes (carrier + ATPase)Na⁺/K⁺ pump; mineral ions into root hair cells

Co-transportNone directly; ATP used indirectly by Na⁺/K⁺ pumpNa⁺ down; other molecule againstYes (co-transporter)Glucose + Na⁺ absorption in ileum

Examiner Alert ⚠️

Facilitated diffusion uses no ATP — it is a passive process. Active transport uses carrier proteins, NOT channel proteins. Every year, students incorrectly state that active transport uses channel proteins — this costs marks. Channel proteins are exclusively for facilitated diffusion.

Simple Diffusion & Facilitated Diffusion

Definition — Diffusion

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, by random movement.

Simple Diffusion

Molecules pass directly through the phospholipid bilayer

Only works for small, non-polar molecules: O₂, CO₂, ethanol, glycerol

Lipid-soluble molecules dissolve in the hydrophobic core

Rate ∝ concentration gradient, surface area, temperature

Facilitated Diffusion

Uses channel proteins (for ions) or carrier proteins (for larger molecules)

Still passive — moves DOWN the concentration gradient

Rate limited by number of channel/carrier proteins available

Glucose enters red blood cells via GLUT carrier proteins (facilitated diffusion)

Exam Tip

Ions (Na⁺, K⁺, Cl⁻) are charged — they cannot cross the hydrophobic core of the bilayer by simple diffusion. They require channel proteins for facilitated diffusion. Always state that ions move via ion channels.

Osmosis & Water Potential

Definition — Osmosis

Osmosis is the movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane.

Definition — Water Potential (Ψ)

Water potential (symbol: Ψ) is a measure of the tendency of water to move from one region to another, measured in kilopascals (kPa). Pure water has a water potential of zero. Adding solute makes water potential more negative.

Key Water Potential Rules

Pure water: Ψ = 0 kPa (maximum water potential)

Solutions always have negative Ψ values (e.g. −800 kPa)

More solute → more negative Ψ → lower water potential

Water moves from higher Ψ (less negative) to lower Ψ (more negative)

In plant cells: Ψ = solute potential (Ψ_s) + pressure potential (Ψ_p)

Examiner Alert ⚠️

Never write: "water moves from high to low solute concentration" or "water moves from dilute to concentrated solution." Always write: "water moves from higher water potential to lower water potential through a partially permeable membrane." AQA mark schemes require both the term water potential AND the phrase partially permeable membrane.

Active Transport

Definition — Active Transport

Active transport is the movement of molecules or ions against their concentration gradient across a membrane, using energy from ATP hydrolysis and carrier proteins.

The mechanism involves ATP hydrolysis releasing energy to change the shape of a carrier protein:

1

The molecule or ion binds to the complementary carrier protein on one side of the membrane

2

ATP is hydrolysed to ADP + Pᵢ by the enzyme ATPase; the released energy causes the carrier protein to change shape

3

The shape change releases the molecule on the other side of the membrane, against the concentration gradient; the carrier protein then returns to its original shape, ready for the next cycle

Classic Example: Na⁺/K⁺ ATPase Pump

Pumps 3 Na⁺ out of the cell and 2 K⁺ in — against both gradients. This is essential for nerve impulse transmission and maintaining cell volume.

Co-transport: Glucose Absorption in the Ileum

Co-transport couples the movement of one molecule down its gradient to drive another molecule against its gradient. The glucose and Na⁺ absorption mechanism is the AQA required example.

1

Na⁺/K⁺ ATPase actively pumps Na⁺ out of the ileum epithelial cell (into blood) using ATP → creates very low [Na⁺] inside the cell

2

Na⁺ moves from the intestinal lumen into the epithelial cell down its concentration gradient via the sodium-glucose co-transporter protein

3

Glucose is co-transported simultaneously against its own concentration gradient, piggybacking on the Na⁺ movement — this is secondary active transport

4

Glucose exits the epithelial cell into the blood via facilitated diffusion through carrier proteins on the basolateral surface (blood side)

Exam Tip — Secondary Active Transport

ATP is not used directly to move glucose, but without the Na⁺/K⁺ ATPase pump creating the sodium gradient, co-transport cannot occur. This is called secondary active transport — ultimately dependent on ATP.

Co-transport — Ileum Diagram

Factors Affecting Rate of Transport

AQA requires you to explain how specific factors affect the rate of movement across membranes. These apply differently to different mechanisms.

FactorEffect on RateMechanism

↑ Concentration gradient↑ Rate of diffusion (simple and facilitated)Greater driving force for net movement

↑ Temperature↑ Rate (all passive mechanisms)Greater kinetic energy → more random movement; also increases membrane fluidity

↑ Surface area of membrane↑ Rate (all mechanisms)More membrane = more space for diffusion and more transport proteins

↑ Number of channel/carrier proteins↑ Rate (facilitated diffusion & active transport)More proteins = more simultaneous transport events; limits rate at saturation

↑ Difference in water potential (Ψ)↑ Rate of osmosisSteeper water potential gradient = greater driving force

↑ ATP availability↑ Rate of active transportMore ATP → more carrier proteins can operate simultaneously

Specialised Cells & Adaptations

Cells adapted for rapid transport have: increased surface area of membranes (e.g. microvilli in ileum epithelium) AND more channel/carrier proteins in their membranes. Both features together maximise transport rate.

Required Practical 3 — Water Potential of Plant Tissue

Required Practical 3

Aim: Produce a calibration curve to determine the water potential of plant tissue (e.g. potato).

VariableDetail

Independent variableConcentration of sucrose solution (e.g. 0, 0.1, 0.2, 0.3, 0.4, 0.5 mol dm⁻³)

Dependent variable% change in mass of potato cylinder (weigh before and after immersion)

Control variablesSame mass/length of potato; same volume of solution; same temperature; same time immersed; all cut from same potato

Method:

1

Prepare a dilution series of sucrose solutions

2

Blot, weigh and immerse potato cylinders in each concentration

3

After a set time, remove, blot dry and reweigh

4

Calculate % change in mass: [(final − initial) / initial] × 100

5

Plot % change in mass vs. solute concentration (or water potential)

6

The x-intercept (0% mass change) = water potential of the potato tissue

Interpretation: Gain in mass → solution has higher Ψ than tissue → water entered by osmosis. Loss in mass → solution has lower Ψ than tissue → water left the tissue.

Required Practical 4 — Membrane Permeability

Required Practical 4

Aim: Investigate the effect of a named variable on the permeability of cell-surface membranes (using beetroot).

VariableDetail

Independent variableTemperature (°C) OR ethanol concentration (%)

Dependent variableAbsorbance of leaked betalain pigment — measured with a colorimeter (using an appropriate filter)

Control variablesSame size/mass beetroot cylinders; same volume of water; same time incubated; same colorimeter filter wavelength

Why beetroot? Beetroot vacuoles contain betalain (red/purple pigment) held in by the tonoplast and cell-surface membrane. Damage to these membranes allows pigment to leak out.

Expected results (temperature):

Low temperatures (0–20°C): minimal leakage — membrane intact; low kinetic energy limits phospholipid movement

~30–50°C: gradually increasing absorbance — increasing membrane fluidity allows greater permeability

High temperatures (60°C+): sharp increase in absorbance — membrane proteins begin to denature ; bilayer structure severely disrupted

Examiner Alert ⚠️

Do NOT say the pigment is released because "cells are killed." The correct explanation is that increased temperature increases membrane fluidity and causes denaturation of membrane proteins, disrupting the bilayer structure and increasing permeability.

DEEP Chain — Osmosis (Extended Answer)

Exam questions worth 4–6 marks on osmosis require the DEEP structure. Here is a model answer for: "Explain why a potato cylinder placed in a concentrated sucrose solution loses mass."

DEEP Chain

DDefine: The concentrated sucrose solution has a lower water potential (more negative Ψ) than the cytoplasm of the potato cells.

EExplain mechanism: Water moves by osmosis — down the water potential gradient, from the potato cells (higher Ψ) through the partially permeable membrane into the sucrose solution (lower Ψ).

EEvidence/named detail: The potato cells lose water from their vacuoles and cytoplasm; the cell contents shrink away from the cell wall (plasmolysis if severe), reducing the mass of the cylinder.

PPractical application: The % mass change is calculated as [(final − initial)/initial] × 100. At the concentration where % mass change = 0, the water potential of the solution equals the water potential of the potato tissue.

Practice Question 1

A student investigates osmosis using potato cylinders. She places cylinders of equal mass into sucrose solutions of different concentrations and measures the % change in mass after 30 minutes.

In 0.3 mol dm⁻³ sucrose solution, the cylinder gains 8% in mass. In 0.5 mol dm⁻³ sucrose, the cylinder loses 5% in mass.

(a) Explain why the potato cylinder gains mass in 0.3 mol dm⁻³ sucrose solution. [3 marks]

(b) Describe how the student could determine the water potential of the potato tissue from these results. [3 marks]

D:

The potato tissue has a lower water potential (more negative Ψ) than the 0.3 mol dm⁻³ sucrose solution — i.e. the tissue has a higher solute concentration.

E:

Water moves by osmosis from the sucrose solution (higher Ψ) through the partially permeable membrane into the potato cells (lower Ψ), down the water potential gradient.

E:

Water entering the cells causes the vacuoles and cytoplasm to expand (cells become turgid), increasing the overall mass of the cylinder. [3 marks]

P(b):

Plot % change in mass (y-axis) against sucrose concentration (x-axis). Draw a line of best fit. The x-intercept — where % mass change = 0 — corresponds to the sucrose concentration that is isotonic with the potato tissue. This sucrose concentration has the same water potential as the tissue. Use a water potential table or formula to convert concentration to Ψ in kPa. [3 marks]

Practice Question 2

The ileum epithelium absorbs glucose from digested food. Glucose is present in the intestinal lumen at a concentration lower than inside the epithelial cells after digestion begins.

(a) Explain how glucose is absorbed from the lumen into the epithelial cell despite its concentration being lower in the lumen. [4 marks]

(b) A student suggests that if ATP synthesis in the epithelial cells was blocked, glucose absorption would stop completely. Evaluate this suggestion. [3 marks]

D:

Co-transport (secondary active transport) is used. Na⁺/K⁺ ATPase pumps sodium ions out of the epithelial cell using ATP, creating a very low [Na⁺] inside the cell.

E:

Sodium ions move from the lumen into the cell down their concentration gradient via the sodium-glucose co-transporter protein. Glucose is simultaneously co-transported into the cell against its own concentration gradient.

E:

Glucose then moves from the epithelial cell into the blood via facilitated diffusion through carrier proteins on the basolateral membrane. [4 marks]

P(b):

The student's suggestion is correct. Without ATP, Na⁺/K⁺ ATPase cannot function → [Na⁺] inside the cell would rise → Na⁺ gradient (driving co-transport) would collapse → glucose can no longer be co-transported into the cell. Although glucose could still enter by facilitated diffusion if its concentration were higher in the lumen, the co-transport mechanism — which allows absorption against a gradient — would cease. [3 marks]

Summary

Key Points — Spec 3.2.3

TopicWhat to Know

Fluid mosaic modelPhospholipid bilayer + embedded proteins; fluid (lateral movement); mosaic (uneven protein distribution); cholesterol regulates fluidity

Simple diffusionNo protein, no energy; small non-polar molecules only (O₂, CO₂); down concentration gradient

Facilitated diffusionChannel proteins (ions) or carrier proteins (glucose); passive; down gradient; rate limited by number of proteins

OsmosisWater moves from higher Ψ to lower Ψ; always use water potential terminology; Ψ always ≤ 0

Active transportCarrier protein + ATPase; against gradient; ATP hydrolysed; carrier proteins, NOT channel proteins

Co-transportNa⁺/K⁺ pump creates Na⁺ gradient → co-transporter brings Na⁺ + glucose in → glucose exits by facilitated diffusion

RP3 (water potential)% mass change of potato vs. sucrose concentration; x-intercept = water potential of tissue

RP4 (permeability)Beetroot + temperature/ethanol; measure absorbance of betalain; higher abs = more permeable

Number to Know

Pure water Ψ = 0 kPa. All solutions have negative Ψ values. Water always moves toward MORE NEGATIVE Ψ.

Protein Types

Channel proteins: ions (facilitated diffusion only). Carrier proteins: larger molecules; facilitated diffusion AND active transport.

Top Exam Tip

The three most common errors in this topic: (1) Using "concentration" instead of "water potential" for osmosis. (2) Saying active transport uses channel proteins — it uses carrier proteins. (3) Forgetting that co-transport is ultimately ATP-dependent (secondary active transport). Fix all three and you secure most of the marks for this section.