A marsh plant produces an antibacterial chemical in its leaves. A scientist investigated the effect of the chemical on two species of bacteria that can infect the plant.
The scientist used leaf tissue from plants that were unable to produce the chemical. None of the tissue samples contained the chemical at the start. In samples marked ‘Yes’, the scientist added enough chemical to give the concentration normally present in the leaves of plants that produce it.
The scientist added the same starting number of bacteria to each sample. The table shows the mean number of bacteria present after 24 hours.
| Bacterial species | Chemical added | Mean number of bacteria after 24 hours in millions |
|---|---|---|
| P | No | 9.2 |
| P | Yes | 1.1 |
| Q | No | 8.0 |
| Q | Yes | 7.7 |
A scientist concluded that the chemical gives the plant the same protection against both bacterial species.
Explain why the results do not support this conclusion. Include in your answer how the risk of disease caused by species P would change if the plant stopped producing the chemical.
133 exam-style questions on AQA GCSE Biology 3.3 Plant disease, covering 3.3.1 Detection and identification of plant diseases and 3.3.2 Plant defence responses. Each one has a worked solution and a mark scheme showing where the marks go.