What is Unit 6?
Unit 6 is the practical-skills examination for the Edexcel International A Level Biology qualification. Unlike Units 4 and 5, this unit is not primarily about learning another block of biological theory.
Instead, you are expected to apply practical knowledge from Units 1, 2, 4 and 5 to unfamiliar experimental situations.
Exam format
Written examination, 50 marks, 1 hour 20 minutes. Candidates answer all questions.
What is assessed?
Planning, practical methods, variables, data processing, graphing, uncertainty, evaluation and scientific reasoning.
Synoptic nature
Questions may draw on practical work across Units 1, 2, 4 and 5.
Mathematics
The paper contains at least five marks targeting Level 2 or above mathematical skills.
How to Plan a Biological Investigation
Planning questions are among the most important skills in Unit 6. A strong plan should allow another scientist to reproduce the investigation and should produce data that can answer the hypothesis.
1. Start with the hypothesis
A hypothesis predicts a relationship between two variables. It should be testable using measurable data.
Increasing temperature will increase the rate of enzyme activity up to an optimum temperature, after which the rate will decrease.
2. Formulate a null hypothesis
A null hypothesis predicts that there is no significant relationship or difference between the variables being investigated.
3. Select appropriate apparatus
State the apparatus precisely and consider its range and resolution. The apparatus must be suitable for the quantity being measured.
- Use a balance for mass.
- Use a stopwatch for time.
- Use a thermometer or temperature probe for temperature.
- Use a colorimeter when measuring changes in absorbance.
- Use a gas syringe or suitable sensor when measuring gas volume.
- Use a microscope and graticule when measuring microscopic structures.
4. Decide how the data will be collected
State how measurements will be taken, how frequently they will be taken and how many repeats will be performed.
5. Consider safety and ethics
Identify hazards associated with organisms, chemicals, heat, electrical equipment, glassware or biological samples.
- Hypothesis / null hypothesis
- Independent variable
- Dependent variable
- Control variables
- Apparatus and resolution
- Measurement method
- Range of values
- Repeats
- Safety
- Ethical considerations
- Data analysis method
Independent, Dependent and Control Variables
Independent variable
The variable deliberately changed by the investigator.
Dependent variable
The variable measured as the response to the independent variable.
Control variable
A factor kept constant so that it does not influence the result.
Standardised variable
A factor controlled consistently between experimental conditions.
Independent: temperature.
Dependent: rate of respiration.
Controls: organism type, organism mass, volume of solution, concentration of substrate, duration and apparatus.
Fair testing
A fair test changes one independent variable while keeping other relevant variables constant. This allows changes in the dependent variable to be attributed to the independent variable with greater confidence.
Measurements, Resolution and Calibration
Resolution
Resolution is the smallest change that an instrument can reliably distinguish.
Calibration
Instruments should be checked before use. For example, a balance should read zero before measuring mass.
Range
The selected range of values should be large enough to reveal a meaningful relationship but practical and safe for the investigation.
Repeats
Repeating measurements helps identify anomalous results and improves reliability by allowing a mean to be calculated.
Mean = Σx / n
If one measurement is very different from the others, investigate whether an experimental error occurred before automatically removing it.
Processing and Analysing Data
Tables
Tables should have clear headings, units and consistent decimal places or significant figures.
| Temperature / °C | Time / s | Rate / s⁻¹ |
|---|---|---|
| 20 | 52.0 | 0.0192 |
| 30 | 36.0 | 0.0278 |
| 40 | 24.0 | 0.0417 |
Percentages
Percentage change = ((new − original) / original) × 100
Rate calculations
Rate = change in quantity / time taken
Magnification
Magnification = image size / actual size
Ratio
Ratios are commonly used when comparing biological quantities, concentrations, proportions or experimental outcomes.
Constructing and Interpreting Graphs
Choosing axes
- Independent variable → x-axis.
- Dependent variable → y-axis.
- Include units.
- Use an appropriate scale.
- Use most of the available graph area.
Line graphs
Continuous independent variables are normally represented using a line graph or suitable curve.
Bar charts
Bar charts are suitable for discrete categories or independent variables that are not continuous.
Histograms
Histograms represent continuous data grouped into class intervals. The bars touch because the intervals are continuous.
Gradient
Gradient = Δy / Δx
When determining a gradient, use a large triangle along the line of best fit rather than using two points that are very close together.
Correlation
A positive correlation means that the dependent variable generally increases as the independent variable increases.
A negative correlation means that the dependent variable generally decreases as the independent variable increases.
Uncertainty, Errors and Anomalies
Random error
Random errors cause measurements to vary unpredictably around the true value.
Repeats and calculating a mean can reduce the effect of random error.
Systematic error
A systematic error shifts measurements consistently in one direction. Repeating the experiment does not remove this type of error.
Accuracy vs precision
Accuracy
How close a measurement is to the true or accepted value.
Precision
How closely repeated measurements agree with each other.
Percentage uncertainty
% uncertainty = (absolute uncertainty / measured value) × 100
Reducing uncertainty
- Use equipment with greater resolution.
- Increase the size of the measured quantity where appropriate.
- Take repeated measurements.
- Use a larger sample size where practical.
- Use appropriate calibration.
Errors in practical investigations
| Problem | Possible improvement |
|---|---|
| Temperature changes during experiment | Use a controlled water bath or temperature probe. |
| Small volume measurements | Use apparatus with greater resolution. |
| Large variation between repeats | Increase repeats and investigate experimental conditions. |
| Instrument does not read zero | Calibrate or zero the instrument before measurement. |
| Subjective colour judgement | Use a colorimeter where appropriate. |
Evaluating an Experiment
Evaluation questions require more than saying that an experiment was "good" or "bad". Identify a specific limitation, explain its effect and propose a realistic improvement.
Limitation → effect → improvement.
Example: The temperature was not maintained consistently, so changes in reaction rate may have been caused partly by temperature variation. A thermostatically controlled water bath could maintain a constant temperature.
Reliability
Reliability concerns the consistency of results. Repeats, larger sample sizes and consistent procedures can improve reliability.
Validity
Validity concerns whether the investigation actually tests the intended hypothesis.
Conclusion
A conclusion should refer directly to the data and should avoid making claims stronger than the evidence allows.
Core Practicals Relevant to Unit 6
Unit 6 is synoptic. You should be prepared to apply practical techniques from Units 1, 2, 4 and 5.
CP1
Investigate the effect of temperature or pH on enzyme activity.
CP2
Investigate the effect of membrane structure/permeability using suitable biological material.
CP3
Investigate biological molecules using food-test procedures.
CP4
Investigate transport or biological processes using appropriate measurements and controls.
CP5–9
Cell microscopy, mitosis, plant tissues, plant fibres and antimicrobial investigations.
CP10–14
Photosynthesis, ecological sampling, microbiology, microorganisms and antibiotics.
CP15
Use a redox indicator to investigate respiration in yeast.
CP16
Use a simple respirometer to investigate respiration rate and respiratory quotient.
CP17
Investigate the effects of exercise on tidal volume, breathing rate, respiratory minute ventilation and oxygen consumption.
CP18
Investigate amylase production in germinating cereal grains.
You do not need to memorise every practical as a script. You need to understand the scientific principle, variables, apparatus, measurements, controls, data analysis, safety, limitations and possible improvements.
Respiration Example
A respirometer can be used to investigate respiration in germinating seeds or small invertebrates.
Basic principle
Respiring organisms consume oxygen. Changes in gas volume or pressure can therefore be used to estimate respiration rate.
Rate of respiration = change in gas volume / time
Respiratory quotient
RQ = CO₂ produced / O₂ consumed
RQ can provide information about the respiratory substrate being used.
Variables
- Independent: experimental condition being changed.
- Dependent: change in gas volume per unit time.
- Controls: organism mass, temperature, apparatus and duration.
Exercise and Ventilation Data
Tidal volume
Tidal volume is the volume of air moved into or out of the lungs during a normal breath.
Breathing rate
The number of breaths taken per unit time.
Respiratory minute ventilation
Minute ventilation = tidal volume × breathing rate
A person has a tidal volume of 0.50 dm³ and breathes 18 times per minute.
Minute ventilation = 0.50 × 18
= 9.0 dm³ min⁻¹
Homeostasis and Experimental Data
Homeostasis maintains internal conditions within narrow limits despite changes in the external or internal environment.
Example: thermoregulation
The hypothalamus detects changes in body temperature and coordinates responses that help restore temperature toward its normal range.
Example: osmoregulation
Osmoreceptors in the hypothalamus monitor plasma concentration. ADH affects water reabsorption in the kidneys.
More ADH → greater water reabsorption → smaller volume of concentrated urine
Sampling and Biodiversity
Unit 6 can use ecological contexts from Unit 4. You should be able to select an appropriate sampling method and analyse the resulting data.
Quadrats
Quadrats are used to sample organisms that are stationary or move slowly, such as plants.
Transects
Transects can investigate how the distribution or abundance of organisms changes across an environmental gradient.
Important controls
- Quadrat size
- Sampling locations
- Number of samples
- Environmental conditions
- Sampling method
Population estimate = mean number per quadrat × total habitat area / quadrat area
Microorganisms and Aseptic Technique
Microbiological investigations require careful control of contamination and safe handling of microorganisms.
Important principles
- Use sterile apparatus where appropriate.
- Minimise exposure of cultures.
- Use appropriate aseptic technique.
- Dispose of biological material safely.
- Follow laboratory safety procedures.
Growth measurements
Microbial growth can be investigated using cell counts, dilution plating, mass measurements or optical methods such as turbidity.
Dilution factor = final volume / volume of sample transferred
PCR, Gel Electrophoresis and DNA Analysis
PCR
Polymerase chain reaction is used to amplify a selected DNA sequence, producing many copies from a small starting amount.
Gel electrophoresis
DNA fragments can be separated according to length. Smaller fragments generally migrate further through the gel than larger fragments under the same conditions.
DNA profiling
DNA profiling compares patterns of DNA fragments and can be used to investigate genetic relationships or identify biological samples.
Unit 6 question approach
If a question gives you unfamiliar molecular data, first identify what has been measured, what the axes represent and what biological process the experiment is testing. Then analyse the evidence.
Unit 6 Practice Questions
Show answer
Show answer
Show answer
Show answer
Show answer
Show answer
How to Attack a Unit 6 Question
Problem → Why it matters → Improvement.
Avoid vague statements such as "make it more accurate." State exactly what should be changed and why it would improve the investigation.