A student investigated the effect of carbohydrate concentration in an agar medium on the growth of one species of bacterium.
The agar media contained the same concentrations of sources of nitrogen and minerals. Before the investigation, the Petri dishes and agar media were sterilised.
The student spread bacterial suspension onto three plates at each carbohydrate concentration. She incubated the plates at 25 °C for 48 hours.
The student measured the mean diameter of well-separated colonies on each plate.
| Carbohydrate concentration / g dm⁻³ | Mean colony diameter on plate 1 / mm | Mean colony diameter on plate 2 / mm | Mean colony diameter on plate 3 / mm |
|---|---|---|---|
| 5 | 1.8 | 2.0 | 1.9 |
| 10 | 3.8 | 4.0 | 4.2 |
| 15 | 4.6 | 4.8 | 7.2 |
| 20 | 5.0 | 5.1 | 5.2 |
Give one reason why the Petri dishes and agar media were sterilised before use.
Use the table. Explain the effect of carbohydrate concentration on bacterial growth. You should also comment on any result that may be anomalous.
Use the results at a carbohydrate concentration of 10 g dm⁻³.
Calculate the cross-sectional area of a colony with the mean diameter for the three plates.
Use π = 3.14.
Give your answer in mm² to 3 significant figures.
The student prepares three more plates containing 20 g dm⁻³ carbohydrate but no sources of nitrogen or minerals.
Suggest how the mean colony diameter on these plates would differ from the result at 20 g dm⁻³ in the table. Explain your answer.
282 exam-style questions on AQA GCSE Biology 1.1 Cell structure, covering 1.1.1 Eukaryotes and prokaryotes, 1.1.2 Animal and plant cells, 1.1.3 Cell specialisation, 1.1.4 Cell differentiation, 1.1.5 Microscopy, and 1.1.6 Culturing microorganisms. Each one has a worked solution and a mark scheme showing where the marks go.