What is Unit 3?
Unit 3 is the practical-skills examination for the Edexcel International AS Biology course. It assesses practical knowledge and experimental skills developed while studying Units 1 and 2.
What can the paper test?
- Experimental design
- Independent, dependent and controlled variables
- Apparatus and measurement
- Microscopy and biological drawings
- Sampling techniques
- Enzyme investigations
- Osmosis investigations
- Food tests and biological molecules
- Cell and tissue investigations
- Plant practicals
- Data processing
- Graph plotting and interpretation
- Uncertainty and experimental error
- Evaluation and improvements
- Mathematical calculations
Planning an Investigation
A strong practical investigation begins with a clear question or hypothesis and a method designed to produce reliable evidence.
A good experimental plan should include
- Independent variable
- Dependent variable
- Controlled variables
- Suitable apparatus
- Measurement method
- Repeated measurements
- Safety precautions
- Data-processing method
- Method of reducing uncertainty
Hypothesis → Variables → Apparatus → Method → Repeats → Data → Graph → Analysis → Conclusion → Evaluation
Hypothesis
A hypothesis should predict a relationship between variables that can be tested experimentally.
Investigate the effect of temperature on the rate of an enzyme-controlled reaction.
Independent, Dependent and Controlled Variables
Independent variable
The variable deliberately changed by the investigator.
Dependent variable
The variable measured in response to the independent variable.
Controlled variable
A factor kept constant so that it does not affect the dependent variable.
Confounding variable
An uncontrolled factor that could influence the result and weaken the conclusion.
Example: enzyme experiment
| Variable | Example |
|---|---|
| Independent | Temperature |
| Dependent | Initial reaction rate |
| Controlled | pH, substrate concentration, enzyme concentration |
Measurements, Accuracy and Reliability
Biological investigations involve measurement of quantities such as length, mass, volume, temperature, time, pH and biological counts.
Accuracy vs precision
Accuracy
How close a measurement is to the true value.
Precision
How closely repeated measurements agree with one another.
Repeat readings
Repeating measurements helps identify anomalous values and allows a mean to be calculated.
sum of all measurements ÷ number of measurements
Significant figures
Results should normally be reported with an appropriate number of significant figures consistent with the precision of the measurements.
Microscopy and Biological Drawings
Microscopy is important throughout Biology Unit 3. You may need to interpret images, calculate magnification, identify structures and evaluate microscope techniques.
Unit conversion
- 1 mm = 1000 μm
- 1 μm = 1000 nm
- 1 mm = 1,000,000 nm
An image of a cell is 30 mm long. Its actual length is 60 μm. Calculate the magnification.
30 mm = 30,000 μm
Magnification = 30,000 ÷ 60
Magnification = ×500
Biological drawings
- Use clear single lines.
- Do not shade structures unnecessarily.
- Draw only what is observed.
- Use labels with ruled lines.
- Include an appropriate scale or magnification where required.
- Make the drawing large enough to show relevant detail.
Resolution
Resolution is the ability to distinguish two objects that are close together as separate objects.
Investigating Enzyme Activity
Enzyme investigations can involve changing temperature, pH, enzyme concentration or substrate concentration and measuring the initial reaction rate.
Initial rate
The initial rate is useful because substrate concentration and product concentration can change during an experiment.
Temperature
Increasing temperature generally increases reaction rate initially because molecules have greater kinetic energy and collide more frequently.
Above the optimum temperature, bonds maintaining the enzyme's three-dimensional structure can be disrupted, changing the active site.
pH
Each enzyme has a range of pH values in which it works effectively. Extreme pH can alter interactions within the enzyme and affect the active site.
Important controls
- Enzyme concentration
- Substrate concentration
- Temperature
- pH
- Reaction volume
- Time intervals
Show answer
A water bath helps maintain a stable and controlled temperature throughout the reaction.
Investigating Osmosis
Osmosis is the net movement of water molecules through a partially permeable membrane from a region of higher water potential to a region of lower water potential.
Typical investigation
Plant tissue such as potato cylinders can be placed in solutions of different concentrations.
Variables
- Independent variable: concentration of external solution
- Dependent variable: percentage change in mass
- Controlled variables: tissue dimensions, temperature, volume of solution and time
A potato cylinder changes from 2.50 g to 2.75 g. Calculate the percentage change in mass.
Percentage change = (0.25 ÷ 2.50) × 100
= 10%
Interpreting a graph
The concentration at which percentage change in mass is approximately zero can be used to estimate the external solution concentration at which there is no net movement of water.
Plant Practical Skills
Unit 3 can require practical application of plant biology from Unit 2.
Plant tissue microscopy
- Roots
- Stems
- Leaves
- Xylem
- Phloem
- Sclerenchyma fibres
- Sieve tubes
Plan diagrams
A plan diagram should show the overall arrangement of tissues rather than individual cells in unnecessary detail.
Tensile strength
Plant fibres can be investigated by measuring the force required to break a fibre or by examining how fibres behave as increasing loads are applied.
Fibre length, fibre type, fibre width and environmental conditions should be controlled when appropriate.
Plant mineral deficiencies
Plant growth can be investigated by growing plants in solutions lacking specific mineral ions and comparing their development with a control.
Food Tests
| Food substance | Test | Positive result |
|---|---|---|
| Starch | Iodine solution | Blue-black |
| Reducing sugar | Benedict's reagent + heating | Green → yellow → orange → red precipitate |
| Protein | Biuret test | Lilac/purple |
| Lipid | Ethanol emulsion test | White/cloudy emulsion |
Microbial Investigations and Aseptic Technique
Practical investigations involving microorganisms require careful control of contamination and safe handling.
Aseptic technique
- Use sterile equipment.
- Minimise exposure of cultures to the environment.
- Disinfect appropriate surfaces.
- Use appropriate protective equipment.
- Dispose of biological material safely.
Antimicrobial investigations
The effectiveness of an antimicrobial substance can be investigated by comparing zones of inhibition around treated discs.
Diffusion through the agar, concentration and other experimental factors can affect zone size.
Processing Experimental Data
Mean
Range
Percentage change
Ratio
Ratios can be simplified by dividing each quantity by the same factor.
Mitotic index
number of cells undergoing mitosis ÷ total number of cells
24 cells in a root tip are undergoing mitosis out of 120 cells observed.
= 0.20
As a percentage: 20%
Plotting and Interpreting Graphs
Choosing axes
The independent variable normally goes on the x-axis and the dependent variable goes on the y-axis.
Good graph practice
- Label both axes.
- Include units.
- Use a sensible scale.
- Use most of the available graph area.
- Plot points accurately.
- Use a line or curve of best fit where appropriate.
Gradient
change in y ÷ change in x
When determining a gradient from a graph, use a large triangle on the line of best fit rather than two points that are unnecessarily close together.
Correlation
A positive relationship means that the dependent variable generally increases as the independent variable increases.
A negative relationship means that the dependent variable generally decreases as the independent variable increases.
Uncertainty and Error
Random error
Random errors cause measurements to vary unpredictably. Repeats and calculating a mean can reduce their effect.
Systematic error
A systematic error shifts measurements consistently in one direction and is not normally removed simply by taking more repeats.
Reducing uncertainty
- Use an instrument with a suitable resolution.
- Increase the size of the measured quantity where practical.
- Repeat measurements.
- Use appropriate measuring techniques.
- Keep relevant variables controlled.
(absolute uncertainty ÷ measured value) × 100
Evaluating an Experiment
Evaluation questions are among the most important skills in Unit 3.
Ask five questions
- Was the method valid?
- Were the variables controlled?
- Were enough repeats taken?
- Were measurements precise enough?
- How could the experiment be improved?
Weak evaluation
Strong evaluation
Improvement rule
Do not merely say "make it more accurate." State exactly what should be changed and explain why that change would improve the investigation.
Common Practical Contexts
Microscopy
Cell structure, magnification, scale, biological drawings.
Enzymes
Temperature, pH, substrate concentration, enzyme concentration and rate.
Osmosis
Plant tissue mass changes and solution concentration.
Cell division
Root tip squash, mitosis and mitotic index.
Plant fibres
Tensile strength and comparison of materials.
Antimicrobials
Aseptic technique and zones of inhibition.
Unit 3 Practice
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The pH of the reaction mixture.
Show answer
Repeats improve reliability, allow anomalous values to be identified and allow a mean to be calculated.
Show answer
40 mm = 40,000 μm.
Magnification = 40,000 ÷ 80
×500
Show answer
Repeat measurements and calculate a mean. Increasing the number of readings can also improve reliability.
Show answer
The independent variable is the variable deliberately changed by the investigator. Conventionally it is plotted on the x-axis.
Show answer
Random errors cause unpredictable variation, whereas systematic errors cause a consistent shift in measurements.
Show answer
Heterozygosity index = 15 ÷ 60
= 0.25
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Correctly labelled axes with units and an appropriate scale. Points should also be plotted accurately.
How to Approach a Unit 3 Question
Determine what biological process is being investigated.
Find the independent, dependent and controlled variables.
Check units, significant figures, repeats and possible anomalies.
Calculate means, percentages, rates, ratios or other required quantities.
Describe the pattern and use biological knowledge to explain it.
Discuss reliability, validity, uncertainty, errors and realistic improvements.