Cell Adaptations and Membrane Permeability Revision Notes | A Level AQA Biology

1. Why Membrane Permeability Defines Cell Function

Every cell in your body is separated from its environment by a membrane — but that membrane is not a passive wall. It is a dynamic, selective barrier that controls exactly what enters and leaves. Get the permeability wrong and the cell cannot do its job.

This lesson tackles two interconnected ideas: how specialised cells adapt their membranes to maximise transport rates, and how you can experimentally investigate what changes membrane permeability. Both ideas appear regularly in AQA exam questions, often together in a single extended-response question.

2. The Fluid Mosaic Model — The Foundation You Need

Picture the membrane as a sea of phospholipid molecules arranged in two layers — the bilayer — with their hydrophilic (water-attracting) heads facing outward and their hydrophobic (water-repelling) tails pointing inward. Proteins float within this bilayer like icebergs in a slow-moving ocean.

The bilayer's hydrophobic core is the key to selective permeability. Small, non-polar molecules such as oxygen and carbon dioxide dissolve directly into the lipid tails and diffuse across freely. Larger or charged molecules — glucose, ions, amino acids — cannot cross the hydrophobic core unaided and rely on transmembrane proteins to carry or channel them through.

• Small, non-polar molecules (O2, CO2, steroid hormones): cross freely by simple diffusion through the lipid bilayer
• Small, polar molecules (water): cross slowly by osmosis through the bilayer or rapidly via aquaporin channel proteins
• Large, polar molecules (glucose, amino acids): require specific carrier proteins for facilitated diffusion or active transport
• Ions (Na+, K+, Cl-): require specific ion channel proteins or carrier proteins; cannot cross the hydrophobic core unaided

3. Specialised Cell Adaptations for Membrane Transport

Now that you understand what the membrane is, the next question is: how do cells that need to transport enormous quantities of material manage to do so fast enough? The answer lies in structural adaptations that increase either the surface area of membrane available or the density of transport proteins within it.

The rate of transport across a membrane depends on the surface area available, the concentration gradient, and the number of functional transport proteins present. Specialised cells exploit all three of these variables through their adaptations.

3a. Epithelial Cells of the Small Intestine (Ileum)

The cells lining the small intestine must absorb the products of digestion — glucose, amino acids, fatty acids — at a rate that matches the body's metabolic demands. Their adaptations are a masterclass in maximising transport efficiency.

• Microvilli (brush border): finger-like projections of the cell-surface membrane on the apical (lumen-facing) side, dramatically increasing the surface area of the external membrane available for absorption
• High density of carrier proteins and co-transporter proteins: embedded in the microvilli membrane to facilitate rapid uptake of glucose and amino acids via facilitated diffusion and active transport
• Numerous mitochondria: positioned near the apical membrane to supply ATP for active transport processes, particularly the sodium-glucose co-transporter (SGLT1)
• Tight junctions between adjacent cells: prevent substances leaking between cells, ensuring all transport is transcellular and therefore controlled by the membrane

The microvilli increase the surface area of the apical membrane by approximately 20-fold compared with a flat surface. This directly increases the rate of facilitated diffusion and active transport because more carrier and channel proteins can be accommodated, and more membrane area is exposed to the concentration gradient in the lumen.

3b. Proximal Convoluted Tubule (PCT) Cells of the Kidney

PCT cells reabsorb glucose, amino acids, and ions from the glomerular filtrate back into the blood. Their adaptations mirror those of intestinal epithelial cells, which makes sense — both cell types face the same challenge of moving large quantities of solutes rapidly across a membrane.

• Microvilli on the apical surface (facing the tubule lumen): increase surface area for reabsorption
• Infoldings of the basal membrane (facing the blood capillary): increase the surface area of the internal membrane on the blood side, facilitating rapid exit of reabsorbed solutes into the capillary
• Very high mitochondrial density: PCT cells have among the highest mitochondrial counts of any cell type, reflecting the enormous ATP demand for active transport of glucose and ions against concentration gradients
• Sodium-potassium ATPase pumps in the basal membrane: maintain a low intracellular sodium concentration, driving sodium (and co-transported glucose) into the cell from the lumen

Notice the pattern: both the apical and basal membranes are adapted. The apical membrane maximises uptake from the filtrate; the basal membrane maximises exit into the blood. This two-sided adaptation is a hallmark of any transporting epithelium.

3c. Red Blood Cells (Erythrocytes)

Red blood cells present a different kind of adaptation. Their primary job is gas exchange — loading oxygen in the lungs and releasing it in respiring tissues — so their membrane must allow rapid diffusion of oxygen and carbon dioxide.

• Biconcave disc shape: maximises the surface area to volume ratio of the cell, reducing the diffusion distance from the membrane to the centre of the cell
• No nucleus or membrane-bound organelles: the entire cell interior is available for haemoglobin, and there are no internal membrane barriers to slow gas diffusion
• Aquaporin channels (AQP1) in the membrane: facilitate rapid water movement, allowing the cell to adjust its volume as it moves between environments of different osmolarity
• High membrane flexibility: the spectrin-actin cytoskeletal network beneath the membrane allows the cell to deform as it squeezes through narrow capillaries without rupturing

3d. Root Hair Cells

Moving to plants, root hair cells absorb mineral ions and water from the soil. Root hair extensions are elongations of the cell wall and plasma membrane, distinct from the microvilli seen in intestinal epithelial cells, but serving the same function of increasing surface area of the cell-surface membrane in contact with soil water.

• Long hair-like extensions of the cell wall and membrane: increase the surface area in contact with soil water, increasing the rate of osmosis and ion uptake
• High density of ion channel and carrier proteins: facilitate active transport of mineral ions (e.g. nitrate, phosphate) against concentration gradients from the dilute soil solution
• Large central vacuole with a low water potential: maintains a steep water potential gradient between the soil solution and the cell interior, driving osmosis inward
• Numerous mitochondria: provide ATP for the active transport of mineral ions

4. Factors Affecting Membrane Permeability

Understanding how specialised cells adapt their membranes leads naturally to a deeper question: what can change the permeability of a membrane in the first place? This is where the Required Practical comes in — and it is also a rich source of exam questions asking you to explain experimental results.

The permeability of a membrane depends on the integrity of the phospholipid bilayer and the proteins embedded within it. Any factor that disrupts the bilayer structure or denatures membrane proteins will increase permeability — often dramatically.

• Temperature: increasing temperature increases the kinetic energy of phospholipid molecules, increasing membrane fluidity; at high temperatures (above approximately 40–45°C for most biological membranes), the bilayer becomes too fluid and proteins begin to denature, causing a large increase in permeability
• Solvents (e.g. ethanol): dissolve the lipid bilayer by disrupting hydrophobic interactions between fatty acid tails, increasing permeability; higher solvent concentrations cause greater disruption
• pH extremes: very low or very high pH values denature membrane proteins (including channel and carrier proteins) and can disrupt the phospholipid arrangement, altering permeability
• Detergents: act as surfactants that disrupt the hydrophobic core of the bilayer, solubilising membrane lipids and proteins and causing a large, often irreversible increase in permeability

The classic experimental model for studying membrane permeability is beetroot. Beetroot cells contain a red pigment called betalain (often referred to as betacyanin) stored in the vacuole. The pigment is normally retained inside the cell by the tonoplast (vacuole membrane) and the cell-surface membrane. When these membranes are damaged, betalain leaks out into the surrounding solution, and the intensity of the red colour can be measured using a colorimeter.

5. Required Practical 4 — Investigating Membrane Permeability

The AQA specification requires you to carry out an investigation into the effect of a named variable on the permeability of cell-surface membranes. The most common version uses beetroot and temperature, but the same design principles apply if you investigate ethanol concentration or pH instead.

A key evaluation point that examiners frequently reward is the distinction between two mechanisms operating at different temperature ranges. Below approximately 40–45°C, increased fluidity is the dominant effect. Above this threshold, protein denaturation becomes the dominant cause of increased permeability — and this is an irreversible change, unlike the reversible fluidity increase at lower temperatures.

5a. Evaluating the Investigation

Evaluation questions are worth significant marks in AQA exams. You need to be able to identify sources of error, suggest improvements, and assess the reliability and validity of the data.

• Reliability: repeating each temperature at least three times and calculating a mean reduces the effect of anomalous results; using the same colorimeter throughout removes instrument variability
• Validity: rinsing cylinders before the experiment removes surface pigment released by cutting, ensuring any absorbance measured is due to membrane permeability changes, not pre-existing surface damage
• Validity: controlling cylinder dimensions ensures the same surface area is exposed to water at each temperature, making comparisons fair
• Limitation: water bath temperatures may fluctuate slightly; using a thermometer to monitor and record actual temperature improves accuracy
• Limitation: the subjective endpoint of 'water runs clear' during rinsing introduces variability; a standardised number of rinses would improve consistency
• Extension: using a spectrophotometer instead of a simple colorimeter allows a more precise wavelength to be selected, improving the accuracy of absorbance measurements

Looking at the data above, you can see a gradual increase in absorbance between 0°C and 40°C, followed by a steep rise between 40°C and 60°C, then a plateau. The gradual phase reflects increasing membrane fluidity; the steep phase reflects protein denaturation; the plateau suggests maximum permeability has been reached — the membranes are fully disrupted.

6. Applying the Concepts — Worked Exam Question

Exam questions on this topic often combine the practical with the underlying biology. Here is a typical AQA-style question that requires you to link experimental observations to membrane structure and cell adaptations.

Worked example: Worked Exam Question: Explaining Membrane Permeability Data

A student investigated the effect of ethanol concentration on the permeability of beetroot cell membranes. She placed beetroot cylinders in solutions of 0%, 10%, 20%, 30%, and 40% ethanol for 30 minutes, then measured the absorbance of each solution using a colorimeter. She found that absorbance increased as ethanol concentration increased. Explain these results in terms of membrane structure. [4 marks]

1. Higher absorbance = more betalain pigment in solution = more pigment has leaked out of the beetroot cells — Always translate the measurement into biological meaning first. This earns the first mark by linking the dependent variable to membrane permeability. [B1]
2. Ethanol is a solvent that dissolves lipids. It disrupts the hydrophobic interactions between the fatty acid tails of phospholipid molecules in the bilayer. — Name the specific structural component affected (the phospholipid bilayer / hydrophobic core). This is the mechanism mark. [B1]
3. Disruption of the bilayer increases membrane fluidity and creates gaps or discontinuities in the membrane structure, reducing its ability to act as a selective barrier. — Link the structural disruption to the functional consequence — loss of selective permeability. [B1]
4. Higher ethanol concentrations cause greater disruption to the bilayer because more lipid molecules are dissolved, so more betalain leaks out, giving a higher absorbance reading. — Address the trend (why it increases with concentration), not just the direction. This is the analysis mark. [B1]

Answer: Higher absorbance indicates more betalain has leaked out, meaning greater membrane permeability. Ethanol dissolves the lipid bilayer by disrupting hydrophobic interactions between phospholipid fatty acid tails, creating discontinuities in the membrane. This reduces the membrane's selective permeability, allowing betalain to leak out. Higher ethanol concentrations cause greater disruption, so more pigment leaks out and absorbance is higher.

7. Exam Technique and Mark-Winning Strategies

Questions on cell adaptations and membrane permeability test your ability to link structure to function — a core skill at A Level. The most common reason students lose marks is giving a vague structural description without explaining the functional consequence, or vice versa.