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Techniques for the A2 Core Practicals

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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 ───────────────────────────────────────────────────────────────── 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.

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

Practice Questions

Practice Question 1

Explain the purpose of the soda lime in a respirometer and predict what would happen without it.

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More Biology 6 (IAL) topics

Respirometry & Respiratory Quotient
Designing a Whole Investigation
Aseptic Technique & Culturing Microorganisms
Planning & Analysing Whole Investigations
Uncertainty, Statistics & Evaluation
Chromatography & Photosynthetic Pigments
Quadrats, Transects & Field Sampling
Measuring Rates: Enzymes, Growth & Photosynthesis
Standard Deviation, Standard Error & Error Bars
The t-test: Comparing Two Means