A student investigates the effect of a synthetic preservative (sodium benzoate) on the germination and development of fungal spores (Aspergillus niger).
She inoculates 40 spores on agar plates in each of five Petri dishes. Each dish contains a total volume of 20 cm3 of nutrient agar mixture, prepared with different proportions of distilled water and a concentrated sodium benzoate solution.
The table below shows her results after 4 days:
| Volume of distilled water in dish (cm3\text{cm}^3cm3) | Volume of sodium benzoate solution in dish (cm3\text{cm}^3cm3) | Number of spores germinated |
|---|---|---|
| 20 | 0 | 36 |
| 16 | 4 | 32 |
| 10 | 10 | 22 |
| 4 | 16 | 12 |
| 0 | 20 | 4 |
State the dependent variable in this investigation.
For the spores grown in 20 cm3 of sodium benzoate solution (with 0 cm3 of distilled water), the mean hyphal length was 3.5 μm3.5\ \mu\text{m}3.5 μm and the mean sporangiophore height was 1.2 μm1.2\ \mu\text{m}1.2 μm.
Calculate the Spore Development Index (SDI\text{SDI}SDI) for these spores.
Use the formula:
SDI=mean hyphal length (μm)×mean sporangiophore height (μm)×percentage of spores germinated \text{SDI} = \text{mean hyphal length }(\mu\text{m}) \times \text{mean sporangiophore height }(\mu\text{m}) \times \text{percentage of spores germinated} SDI=mean hyphal length (μm)×mean sporangiophore height (μm)×percentage of spores germinatedExplain why using the Spore Development Index (SDI\text{SDI}SDI) is a more informative way to measure the effect of the preservative on fungal development than simply using the number of spores germinated.
44 exam-style questions on OCR GCSE Biology Practical skills. Each one has a worked solution and a mark scheme showing where the marks go.