Practical Skills, Laboratory Techniques and Data Interpretation
Practical Skills, Laboratory Techniques and Data Interpretation tests whether you can use instruments correctly, choose reagents and techniques, and draw valid conclusions from measurements.
Instruments and techniques
- Total magnification = eyepiece magnification × objective magnification. Magnification is not the same as resolution.
- An ocular micrometer must be calibrated with a stage micrometer; each division then has a known value.
- Stains: acetocarmine for chromosomes, iodine for starch, methylene blue for animal cells, safranin and fast green for plant tissues, Leishman for blood smears.
- Mitosis is studied in onion root-tip squashes; meiosis in grasshopper testes.
- Chromatography separates pigments; Rf = distance travelled by solute ÷ distance travelled by solvent, and Rf cannot exceed 1.
- Colorimetry and centrifugation are used for quantitative estimation and separation.
Data handling
- Rate = change in value ÷ time. Dilution uses C1V1 = C2V2.
- Report the mean with standard deviation; use a bar chart for categories and a line graph for continuous change.
- Identify systematic errors (instrument bias) and random errors (fluctuation), and use repeats and controls.
Worked example
A 10× eyepiece and 40× objective give 400× magnification. If 1 mm on the stage micrometer equals 100 ocular divisions, each division is 10 µm. An object spanning 8 divisions is 80 µm long. The reasoning is to calibrate first, then multiply the division value by the count.
Traps
Do not confuse magnification with resolution; Rf above 1 is impossible; forgetting units invalidates a calculation; standard deviation measures spread, not error of the mean; always read the meniscus at eye level.