Revise Techniques for the A2 Core Practicals for Biology 6 (IAL) WBI16 (A2 Level) — revision notes and instant AI marking. Free to start.
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Edexcel IAL Biology • Unit 6: Practical Skills in Biology II
Techniques for the A2 Core Practicals
🧫 Big idea: Unit 6 assesses the same skills as Unit 3, applied to harder experiments. The techniques in Core Practicals 10–18 — respirometers, aseptic work, chromatography, field sampling — each have a handful of steps that must be right for the reading to mean anything, and those steps are what the paper asks about.
Summary — What This Topic Covers
What changes between Unit 3 and Unit 6, and what does not
The respirometer and where its errors come from
Aseptic technique and why each step exists
Field sampling, chromatography and rate measurement — signposted to their own topics
The techniques most likely to appear in a Unit 6 question
1. What Changes at A2
SAME AS UNIT 3 NEW AT A2
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variables and controls statistical testing of results
tables, graphs, units standard deviation and error bars
random vs systematic error judging significance, not just trend
reliability and validity designing a WHOLE investigation
percentage uncertainty sampling strategy in the field
The shift
Unit 3 asks whether your method was sound. Unit 6 asks whether your conclusion is justified — which usually means whether a difference is large enough to be more than chance.
Worth remembering
Every Unit 3 skill is still examinable at A2. The statistics are added on top, not instead.
2. The Respirometer
Used in Core Practicals 15 and 16 to measure oxygen uptake by respiring organisms or germinating seeds.
HOW IT WORKS
organisms respire → O₂ consumed, CO₂ released
soda lime / KOH absorbs CO₂ → only the O₂ change remains
gas volume falls → manometer fluid moves toward the tube
distance moved × tube area → volume of O₂ taken up
The control tube is identical but contains glass beads of the same mass instead of organisms — it corrects for changes in temperature and atmospheric pressure
The whole apparatus sits in a water bath, because gas volume changes with temperature and would swamp the respiration signal
A screw clip or tap is opened to equalise pressure before each run and closed to start it
Leave to equilibrate before taking the first reading, or you measure the apparatus settling rather than respiration
The one they ask
Without soda lime, the CO₂ released would replace the O₂ consumed and the volume would barely change. The absorbent is what makes the measurement possible, not an optional extra.
3. Aseptic Technique
Key Term
Aseptic technique is the set of precautions that prevents unwanted microorganisms entering a culture, and prevents the cultured organism escaping.
STEP WHY
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work near a Bunsen flame convection carries airborne spores
upwards, away from the plate
flame the loop before AND after kills organisms on the wire; cool
before touching the culture or you
kill the sample
flame the bottle neck destroys organisms at the opening
lift the lid at an angle, minimises the opening through which
only as far as needed contaminants can fall in
tape at 2 or 4 points holds the lid but admits air, so
anaerobes are not selected for
incubate at 25 °C or below avoids favouring human pathogens
invert the plate condensation collects in the lid,
not on the agar surface
What mark schemes look for
Each step paired with its reason. Listing the steps earns far less than explaining what each one prevents.
4. Where the Other A2 Techniques Are Covered
Chromatography & Rf values → Chromatography & Photosynthetic Pigments
Quadrats, transects, sampling → Quadrats, Transects & Field Sampling
Rate measurement and 1/t → Measuring Rates: Enzymes, Growth &
Photosynthesis
Spread, error bars, t-test, → the four statistics topics in this unit
chi-squared, Spearman
Easy marks
Whatever the technique, the same three questions recur: what is the control, what variable must be held constant, and where does the uncertainty come from. Prepare those three for each practical and you can answer most of what is asked.
Practice Questions
Practice Question 1
Explain the purpose of the soda lime in a respirometer and predict what would happen without it.
Practice Question 2
Describe the control used in a respirometer experiment and explain what it corrects for.
Practice Question 3
Explain why a Petri dish is taped at two or four points and incubated at 25 °C rather than 37 °C.
Practice Question 4
A student flames the inoculating loop and immediately touches it to the bacterial culture. Explain the problem.
What to Memorise
Unit 6 = Unit 3 skills + statisticsSoda lime absorbs CO₂Control tube = glass beads, same massWater bath controls gas expansionFlame the loop before and after — then coolLid at an angle, brieflyTape 2 or 4 points, never all roundIncubate ≤ 25 °CInvert the plate
Concepts Checklist
Exam Tips
What mark schemes look for
Technique steps paired with reasons. "Flame the loop" is a step; "flame the loop to kill organisms on the wire, then cool it so the sample is not killed" is the answer.
The trap
Forgetting that the respirometer control corrects for physical change, not for respiration. It contains beads precisely because it must respire nothing.
Easy marks
For any practical: name the control, name a controlled variable, name a source of uncertainty. Three answers that fit almost any technique question.
Worth remembering
Every Unit 3 skill is still examined here. The statistics are additional, not a replacement.