Core Practicals and Experimental Design
The Importance of Practical Skills
AQA A-Level Biology assesses practical skills through Required Practicals (12 specified investigations) and questions on experimental design. At least 15% of the total marks in the written exams are based on practical skills. You must understand not only how to carry out experiments but also the principles of good experimental design, including controls, variables, accuracy, and reliability.
Key Principles of Experimental Design
Variables
Every experiment investigates the relationship between variables:
| Variable type | Definition | Example |
|---|---|---|
| Independent variable (IV) | The variable you deliberately change or manipulate | Temperature in an enzyme experiment |
| Dependent variable (DV) | The variable you measure in response to changes in the IV | Rate of reaction (e.g. volume of O₂ produced per minute) |
| Control variables (controlled/confounding) | All other variables that must be kept constant to ensure a fair test | pH, enzyme concentration, substrate concentration |
If control variables are not kept constant, they become confounding variables — factors other than the IV that could affect the DV, making it impossible to draw valid conclusions.
Controls
A control experiment provides a baseline for comparison:
- Negative control — treated identically to the experimental group but WITHOUT the variable being tested. Demonstrates that any observed effect is due to the IV, not some other factor. Example: a test tube with boiled (denatured) enzyme alongside active enzyme tubes.
- Positive control — uses a condition known to produce a specific result. Confirms that the experimental setup is working correctly.
Accuracy, Precision, and Reliability
| Term | Meaning |
|---|---|
| Accurate | The measured value is close to the true value |
| Precise | Repeat measurements are close to each other (low spread/small range) |
| Reliable | Consistent results are obtained when the experiment is repeated (by the same or different investigators) |
| Valid | The experiment measures what it claims to measure; the IV is the only factor causing changes in the DV |
| Reproducible | Other scientists can replicate the experiment and obtain similar results |
| Repeatable | The same scientist gets similar results when repeating the investigation |
Sample Size
Larger sample sizes:
- Reduce the effect of anomalies (outliers) on the mean
- Give a more representative estimate of the true population value
- Increase statistical power — the ability to detect a real effect if one exists
- Allow meaningful statistical tests to be performed
Random Sampling
To avoid sampling bias, samples should be selected randomly:
- Use random number generators or random number tables to determine where to place quadrats or which organisms to measure
- This ensures every individual or location has an equal chance of being selected
AQA Required Practicals
RP1: Effect of a Named Variable on the Rate of an Enzyme-Controlled Reaction
Example: Effect of temperature on the rate of breakdown of hydrogen peroxide by catalase.
- IV: Temperature (e.g. 10, 20, 30, 40, 50, 60°C using water baths)
- DV: Rate of reaction — measured as volume of O₂ gas collected per unit time (using a gas syringe or inverted measuring cylinder over water)
- Control variables: pH (buffer solution), enzyme concentration, substrate concentration, volume of substrate
- Method: Equilibrate enzyme and substrate to the desired temperature separately, then mix. Measure gas volume at timed intervals.
- Expected result: Rate increases with temperature up to the optimum (~40°C for catalase), then decreases sharply as the enzyme denatures
RP2: Preparation of Stained Squashes of Cells from Plant Root Tips
- Observe mitosis in rapidly dividing cells at the root tip (meristem)
- Cut ~1 cm of root tip, fix in ethano-acetic acid, hydrolyse in warm HCl (softens cell walls), stain with aceto-orcein or toluidine blue
- Squash under a coverslip to spread cells into a single layer
- Observe under a microscope — identify stages of mitosis (prophase, metaphase, anaphase, telophase)
- Mitotic index = number of cells in mitosis / total number of cells observed
RP3: Production of a Dilution Series of a Solute and Calibration Curve
- Make a serial dilution of a substance (e.g. glucose) — each step dilutes by a fixed factor (e.g. ×½)
- Measure absorbance (using a colorimeter with appropriate filter) after adding a reagent (e.g. Benedict's test for glucose, biuret for protein)
- Plot a calibration curve (absorbance vs. known concentration)
- Use the curve to determine the concentration of an unknown sample from its absorbance reading
RP4: Investigation into the Effect of a Named Variable on the Permeability of Cell Membranes
- Example: Effect of temperature or alcohol concentration on beetroot cell membrane permeability
- Cut uniform beetroot cylinders, rinse to remove surface pigment
- Place in water baths at different temperatures (or in alcohol solutions of different concentrations)
- After a set time, remove the beetroot and measure the absorbance (or transmission) of the surrounding solution using a colorimeter with a green/blue filter
- More pigment (betalain/anthocyanin) in the solution = more membrane damage = higher absorbance
RP5: Dissection of Animal or Plant Gas Exchange or Mass Transport System
- Dissect a heart, lung, or leaf to observe structures related to gas exchange or transport
- Heart: Identify chambers, valves, coronary arteries, differences in wall thickness between left and right ventricles, tendons holding AV valves
- Leaf: Observe stomata, spongy mesophyll, palisade mesophyll, vascular bundles using microscopy
RP6: Use of Aseptic Technique to Investigate the Effect of Antimicrobial Substances
- Inoculate nutrient agar plates with bacteria using aseptic technique (flame loops/bottle necks, work near Bunsen, tape plates — do not seal completely, incubate at ≤25°C in schools)
- Place filter paper discs soaked in different antimicrobials (e.g. antibiotics, disinfectants, plant extracts)
- Measure the diameter of inhibition zones around each disc after incubation
- Calculate the area of the inhibition zone (πr²) for quantitative comparison
RP7: Use of Chromatography to Investigate Pigments
- Extract photosynthetic pigments from leaves using acetone or ethanol
- Separate pigments using thin-layer chromatography (TLC) — apply extract as a spot near the base of a TLC plate, place in a solvent (e.g. propanone/petroleum ether mixture)
- Identify pigments by their Rf values (Rf = distance moved by pigment / distance moved by solvent front)
- Expected pigments: carotene (highest Rf, most soluble), xanthophyll, chlorophyll a, chlorophyll b (lowest Rf)
RP8-12 (Brief Summary)
| RP | Investigation |
|---|---|
| RP8 | Rates of respiration using a respirometer (measuring O₂ uptake or CO₂ release) |
| RP9 | Investigating the effect of an environmental variable on animal distribution using quadrats and transects |
| RP10 | Investigating gene expression — effect of an environmental variable on phenotype |
| RP11 | Gel electrophoresis to separate DNA fragments |
| RP12 | Investigating population size using a mark-release-recapture method and the Lincoln index |
Evaluating Experiments
When evaluating, consider:
- Systematic errors — consistent errors that affect all measurements in the same way (e.g. a miscalibrated instrument). Reduce accuracy but not precision.
- Random errors — unpredictable variations between measurements. Reduce precision. Can be minimised by taking more repeats and calculating a mean.
- Anomalies — results that do not fit the pattern. Identify them, exclude from the mean if justified, and suggest possible causes.
- Improvements — suggest changes to reduce errors, increase sample size, or control additional variables.
Presenting Data
- Tables: IV in the left column; DV and processed data in subsequent columns; units in headers, not in cells; appropriate decimal places
- Graphs: IV on x-axis; DV on y-axis; labelled axes with units; appropriate scale; line of best fit (curve or straight line as appropriate) or bar chart for categorical data; error bars where possible (using SD, SE, or range)
- Calculations of rate: Often calculated as 1/time (e.g. 1/t in seconds) to convert time measurements into rate. Alternatively, initial rate from the tangent to a curve at time zero.
Exam Tips
- AQA requires you to describe the method for each Required Practical — not just the result, but WHY each step is performed
- Always identify IV, DV, and at least two control variables when designing or evaluating an experiment
- For any practical question, mention how you would make results reliable (repeats + mean) and valid (control variables + appropriate controls)
- When asked to improve an experiment, suggest specific changes — more repeats, larger sample size, more precise measuring instruments, better temperature control, additional control variables
- Safety considerations should be mentioned where relevant: goggles for chemicals, aseptic technique with microorganisms, careful handling of sharp instruments during dissection