Transport Across Membranes

Spec mapping: AQA 7402 Section 3.2.3 — transport across cell membranes (refer to the official AQA specification document for exact wording). Required Practical 3 (effect of an environmental variable on membrane permeability — the beetroot practical) and Required Practical 4 (water potential of plant tissue) are anchored in this lesson.

The cell surface membrane is a selective barrier, and the rules governing what crosses it — and how fast — determine virtually every physiological process. Gas exchange at alveoli, glucose absorption in the gut, ion movements that generate the resting potential of a neurone, water uptake by root hair cells, the osmotic regulation of blood plasma, the swelling of red blood cells in hypotonic solutions: all are applications of the principles in this lesson. AQA examines these mechanisms in both qualitative ("describe and explain") and quantitative ("calculate water potential", "interpret rate-of-uptake data") forms, and Paper 3 often combines them with a Required Practical context. Mastery of the vocabulary — diffusion, facilitated diffusion, osmosis, active transport, co-transport — and the precise differences between them is essential.

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Diffusion

Key Definition: 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. It is a passive process — no metabolic energy (ATP) is required.

Principles

• Diffusion occurs because molecules are in constant random motion (Brownian motion). There is no directional force; the net movement arises statistically because more molecules move out of a concentrated region than into it.
• Diffusion continues until dynamic equilibrium is reached — molecules still move in both directions, but there is no net movement.
• Only small, non-polar molecules (e.g., O₂, CO₂, N₂) and some very small polar molecules (e.g., water, ethanol) can diffuse directly through the phospholipid bilayer.

Factors Affecting the Rate of Diffusion

The rate of diffusion is described by Fick's law:

Rate of diffusion ∝ (surface area × concentration difference) / thickness of membrane (diffusion distance)

Factor	Effect on Rate
Concentration gradient	Steeper gradient → faster diffusion
Surface area	Greater surface area → faster diffusion (e.g., microvilli in the small intestine)
Diffusion distance (thickness)	Shorter distance → faster diffusion (e.g., thin alveolar walls)
Temperature	Higher temperature → faster diffusion (molecules have more kinetic energy)
Size of molecule	Smaller molecules diffuse faster
Polarity	Non-polar molecules cross lipid bilayers more easily

Exam Tip: When asked to explain how a biological surface is adapted for rapid diffusion, always link back to Fick's law. For example, the alveoli of the lungs have a large total surface area (~70 m²), thin walls (one cell thick, ~0.5 µm), a rich blood supply maintaining the concentration gradient, and ventilation continuously replacing air — all features that maximise the rate of gas exchange.

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Facilitated Diffusion

Key Definition: Facilitated diffusion is the passive movement of molecules or ions across a membrane through specific transport proteins (channel proteins or carrier proteins), down a concentration gradient. No ATP is required.

Channel Proteins

• Provide a hydrophilic pore through the membrane for ions or small polar molecules.
• Are specific — each channel typically allows only one type of ion or molecule through.
• Many are gated: they can open or close in response to specific stimuli (e.g., voltage-gated Na⁺ channels in neurones open in response to depolarisation; ligand-gated channels open when a specific molecule binds).
• When open, ions flow through very rapidly — up to 10⁸ ions per second per channel.

Carrier Proteins

• Bind to a specific molecule on one side of the membrane.
• Undergo a conformational change (change in shape) that moves the molecule to the other side of the membrane and releases it.
• Are slower than channel proteins because each transport event requires a shape change.
• Examples: glucose transporter (GLUT) proteins in cell membranes facilitate the uptake of glucose into cells down its concentration gradient.

Key Differences from Simple Diffusion

• Facilitated diffusion requires specific proteins and is therefore saturable — the rate reaches a maximum (V_max) when all transport proteins are occupied. Simple diffusion is not saturable.
• Facilitated diffusion shows specificity — only certain molecules are transported.
• Both are passive processes driven by the concentration gradient.

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Osmosis

Key Definition: Osmosis is the net movement of water molecules across a partially permeable membrane from a region of higher water potential (Ψ) to a region of lower (more negative) water potential.

Water Potential (Ψ)

• Water potential (Ψ, measured in kilopascals, kPa) is a measure of the tendency of water molecules to move from one place to another. It is the 'free energy' of water per unit volume.
• Pure water has a water potential of 0 kPa. Dissolving solutes lowers the water potential, making it negative.
• Water always moves from a higher (less negative) water potential to a lower (more negative) water potential.

For plant cells:

Ψ (cell) = Ψ_s + Ψ_p

Where:

• Ψ_s = solute potential (always negative or zero) — the effect of dissolved solutes lowering water potential.
• Ψ_p = pressure potential (usually positive in plant cells due to turgor pressure, can be zero or negative in xylem).

Osmosis in Animal Cells

Animal cells have no cell wall, so they are sensitive to changes in osmotic conditions:

Solution	Water Potential	Effect on Animal Cell
Hypotonic (lower solute concentration than cell)	Higher Ψ outside	Water enters by osmosis → cell swells → may lyse (burst) — haemolysis in red blood cells
Isotonic (same solute concentration)	Equal Ψ	No net water movement → cell remains normal
Hypertonic (higher solute concentration than cell)	Lower Ψ outside	Water leaves by osmosis → cell shrinks → crenation in red blood cells

Osmosis in Plant Cells

Plant cells have a rigid cellulose cell wall that prevents them from bursting:

Solution	Effect on Plant Cell
Hypotonic	Water enters by osmosis → vacuole expands → cytoplasm pushes against cell wall → cell becomes turgid (Ψ_p increases until Ψ_cell = Ψ_solution, reaching equilibrium)
Isotonic	No net water movement → cell is flaccid (Ψ_p = 0)
Hypertonic	Water leaves by osmosis → vacuole shrinks → cytoplasm pulls away from cell wall → plasmolysis (Ψ_p = 0 and Ψ_cell = Ψ_s)

Exam Tip: Use the correct terminology: plants become turgid or plasmolysed; animal cells undergo lysis or crenation. Do not mix these terms.

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Active Transport

Key Definition: Active transport is the movement of molecules or ions across a membrane against their concentration gradient (from low concentration to high concentration), using energy from the hydrolysis of ATP. It requires specific carrier proteins (pumps).

The Sodium–Potassium Pump (Na⁺/K⁺-ATPase)

This is the best-studied active transport pump and is present in virtually all animal cells:

1. Three Na⁺ ions from the cytoplasm bind to specific sites on the carrier protein on the intracellular side.
2. ATP is hydrolysed to ADP + P_i. The phosphate group attaches to the protein (phosphorylation), causing a conformational change.
3. The protein opens to the extracellular side, and the three Na⁺ ions are released outside the cell.
4. Two K⁺ ions from the extracellular fluid bind to the protein on the extracellular side.
5. The phosphate group is released (dephosphorylation), causing the protein to return to its original shape.
6. The two K⁺ ions are released into the cytoplasm.

Net result: 3 Na⁺ out, 2 K⁺ in, per cycle. This creates an electrochemical gradient — the inside of the cell is more negative relative to the outside (approximately −70 mV in neurones), which is essential for nerve impulse transmission and muscle contraction.

Evidence for Active Transport

• The rate of active transport is reduced by metabolic inhibitors (e.g., cyanide, which inhibits the electron transport chain) because less ATP is produced.
• The rate is reduced at low temperatures because enzyme activity (including the ATPase activity of the pump) decreases.
• The rate is reduced by a lack of oxygen in aerobic cells because oxidative phosphorylation is the main source of ATP.
• Active transport shows specificity — each pump transports only certain ions or molecules.

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Co-transport (Secondary Active Transport)

Co-transport uses the concentration gradient established by active transport to drive the movement of another substance. It does not directly use ATP, but it depends on the gradient created by an ATP-driven pump.

Absorption of Glucose in the Small Intestine (Ileum)

This is a key example for AQA:

1. The Na⁺/K⁺-ATPase on the basolateral membrane (the membrane facing the blood) of epithelial cells pumps Na⁺ out of the cell and K⁺ into the cell, maintaining a low Na⁺ concentration inside the cell.
2. This creates a steep concentration gradient for Na⁺ across the apical membrane (the membrane facing the intestinal lumen, with microvilli).
3. Na⁺ moves down its concentration gradient from the lumen into the epithelial cell through a sodium-glucose co-transporter (SGLT1) on the apical membrane. As Na⁺ moves in, it carries a glucose molecule with it — this is co-transport (symport).
4. Glucose accumulates inside the cell to a higher concentration than in the blood.
5. Glucose then leaves the cell by facilitated diffusion through GLUT2 carrier proteins on the basolateral membrane into the blood capillary.

Exam Tip: The absorption of amino acids in the small intestine follows a similar co-transport mechanism involving Na⁺ co-transporters. Many exam questions ask you to describe and explain the role of the Na⁺/K⁺ pump in maintaining the conditions for co-transport — make sure you explain the link between the two processes.

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Comparing Transport Mechanisms

Feature	Simple Diffusion	Facilitated Diffusion	Osmosis	Active Transport
Direction	Down gradient	Down gradient	Down water potential gradient	Against gradient
Energy (ATP)	No	No	No	Yes
Proteins needed	No	Yes (channel or carrier)	Aquaporins may be involved	Yes (carrier/pump)
Specificity	Low	High	Water only	High
Saturable	No	Yes	No	Yes
Affected by metabolic inhibitors	No	No	No	Yes

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Required Practical 3 — Membrane Permeability (Beetroot)

This investigates how a chosen variable (most commonly temperature, but ethanol concentration is an equally valid alternative) affects membrane permeability, measured by the leakage of the red vacuolar pigment betacyanin.

Method

1. Cut equal-sized cylinders of beetroot using a cork borer. Trim them to the same length (e.g., 1.0 cm).
2. Wash thoroughly in distilled water until the rinse runs clear. This removes pigment released from the cells that were physically damaged by cutting — an essential control step.
3. Set up water baths at a range of temperatures (e.g., 0, 20, 30, 40, 50, 60, 70 °C). Pre-warm tubes of distilled water at each temperature.
4. Place one beetroot cylinder in each tube. Leave for a standardised time (e.g., 30 minutes).
5. Remove the beetroot. Transfer the remaining solution to a cuvette.
6. Measure absorbance at ~530 nm (blue-green filter) using a colorimeter, having calibrated against distilled water.

Variables

• Independent variable: temperature.
• Dependent variable: absorbance (proxy for betacyanin concentration, proxy for membrane damage).
• Controlled variables: size and surface area of cylinders, volume of water, immersion time, beetroot variety, freshness, light exposure, colorimeter filter wavelength.

Expected results

• Little leakage below ~40 °C.
• Sharp increase above ~40 °C as proteins denature and the bilayer becomes increasingly fluid and disorganised.
• Maximum absorbance at the highest temperatures, where the membrane has effectively lost integrity.

Risk assessment

• Hot water baths >50 °C — burn risk; use heat-proof tongs.
• Sharp cork borers — cut away from the body, use a tile.
• Beetroot stains — wear lab coat; juice stains clothing and skin.

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Required Practical 4 — Water Potential of Plant Tissue

This determines the water potential of plant tissue (typically potato) by finding the external solute concentration that produces no mass change.

Method

1. Prepare a serial dilution of sucrose solutions (e.g., 0, 0.2, 0.4, 0.6, 0.8, 1.0 mol dm⁻³).
2. Cut potato cylinders of equal size (cork borer + scalpel). Blot, then weigh each cylinder to record initial mass.
3. Place one cylinder in each sucrose solution; leave 30–60 minutes at constant temperature.
4. Remove cylinders, blot and reweigh. Calculate percentage mass change = ((final mass − initial mass) / initial mass) × 100.
5. Plot percentage mass change against external sucrose concentration. The x-intercept (where mass change = 0) gives the external concentration at which Ψ_solution = Ψ_tissue.
6. Use a calibration table relating sucrose molarity to water potential (provided in the data sheet) to convert the intercept concentration into Ψ_tissue in kPa.

Why mass change works