Laboratory Practical Work and Reporting
What this domain tests
This section examines the mandatory activities and how you record them. You must be able to describe a procedure, identify variables and controls, present results in tables and graphs, and distinguish a source of error from a mistake.
Reporting structure
- Purpose or hypothesis, then a method written in the past tense and in enough detail for another student to repeat it.
- Results: a table with headings and units, replicates and a mean. Graphs need labelled axes with units, a sensible scale, plotted points and a best-fit line - never dot-to-dot.
- Conclusion: state the trend and whether the hypothesis was supported, then link it to the biological principle.
Core practical techniques
- Food tests: iodine for starch (blue-black), Benedict's for reducing sugar (brick-red precipitate on heating), Biuret for protein (purple), Sudan III or brown paper for fat.
- Enzyme work: control pH and temperature; use a water bath rather than a flame for a temperature series; include a boiled-enzyme control.
- Osmosis: Visking tubing or potato cylinders in sucrose solutions; calculate percentage change in mass.
- Microbiology: work near a flame, flame the loop, lift the lid only slightly, subculture aseptically.
- Ecology: quadrat for frequency or percentage cover, transect for distribution, pooter and pitfall trap for invertebrates.
Worked example
Potato cylinders in 0.0, 0.2, 0.4, 0.6 and 0.8 mol/L sucrose changed mass by +8%, +4%, 0%, −5% and −9%. Reasoning: at 0.4 mol/L there is no net water movement, so the tissue is isotonic to that solution; below it the tissue gains mass by osmosis; above it water leaves and the cells become plasmolysed.
Traps
Writing 'human error' as a source of error; describing colour changes without naming the reagent; forgetting to decolourise a leaf in hot ethanol before the starch test; heating alcohol over a flame; omitting units from graph axes.