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Learn7402Paper 3 · Paper 3: Practical skills and essayPractical skills and essayPractical techniques in biology
Practical skills and essay · Subtopic 1 of 2

Practical Techniques in Biology

Serial dilution: dilute by the same factor each time (e.g. 1 cm³ + 9 cm³ water = ×10). Colorimetry: measure absorbance or transmission; use a calibration curve of known concentrations to read unknowns. Osmosis practical: measure % change in mass of tissue in different solutions; where the line crosses zero, the solution's ψ equals the tissue's. Enzyme rates: rate = 1/t or gradient of the initial slope; control temperature with a water bath and pH with buffers. Biological drawings: clear lines, no shading, proportions correct, labels with ruled lines. Data processing: means (exclude anomalies), % change, standard deviation (spread around the mean), error bars; overlapping SD bars suggest the difference may not be significant. Aseptic technique for microbes: flame loops, work near a Bunsen, tape lids, incubate at 25 °C.

Cambridge 9700 Paper 3Edexcel YBI11 WBI13AQA 7402 Paper 3≈ 50 minRevision 1 · updated 2026-11-02
Section 1 of 14

Learning objectives

  • Carry out serial dilutions and colorimetry.
  • Calculate % change, means and standard deviation.
  • Draw accurate biological drawings.
  • Describe core practicals and aseptic technique.
Pause and think

Why do you calculate percentage change in mass, not just the change, in osmosis experiments?

Section 2 of 14

Core idea

Start hereGood data first

A brilliant hypothesis is worthless if the measurements are sloppy.

The big idea

Careful technique, fair comparisons (like % change) and clear presentation make results trustworthy.

Finding tissue water potential
  1. %Δm=mfinal−minitialminitial×100\%\Delta m=\frac{m_\text{final}-m_\text{initial}}{m_\text{initial}}\times100
    Fair comparison.
  2. plot %Δm against concentration\text{plot }\%\Delta m\text{ against concentration}
    Line of best fit.
  3. %Δm=0⇒ψtissue=ψsolution\%\Delta m=0\Rightarrow\psi_\text{tissue}=\psi_\text{solution}
    No net osmosis at the x-intercept.
Worked exampleMaking a dilution

Make 50 cm³ of 0.40 mol dm⁻³ from 2.0 mol dm⁻³.

Goal
Volumes.
Strategy
C₁V₁ = C₂V₂.
  1. V1=0.40×502.0=10 cm3V_1=\frac{0.40\times50}{2.0}=10\ \text{cm}^3
    Stock needed.
  2. 50−10=40 cm3 water50-10=40\ \text{cm}^3\ \text{water}
    Make up to volume.
Answer10 cm³ stock + 40 cm³ water.
✓ CheckFive-fold dilution ✓.

Summary

  • Dilutions; colorimetry.
  • % change; SD.
  • Drawings; aseptic technique.
Key words
Calibration curve
Graph of known standards used to find unknowns.
Aseptic technique
Methods preventing contamination by microbes.
HistoryPetri's dish. Julius Petri, assistant to Robert Koch, designed the shallow lidded dish in 1887 that every microbiology lab still uses.
Fun factVitamin C titration. DCPIP can measure vitamin C in fruit juice: the more vitamin C, the less juice needed to decolourise it.
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