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Ammonia in soil is oxidised to nitrites and nitrates by species of nitrifying bacteria - AQA - A-Level Biology - Question 6 - 2018 - Paper 3
Question 6
Ammonia in soil is oxidised to nitrites and nitrates by species of nitrifying bacteria. Scientists investigated whether two soils with a different pH contained diff... show full transcript
Worked Solution & Example Answer:Ammonia in soil is oxidised to nitrites and nitrates by species of nitrifying bacteria - AQA - A-Level Biology - Question 6 - 2018 - Paper 3
Step 1
Suggest why, in this investigation, the scientists used these units.
Answer
The scientists used the units µg g⁻¹ to provide a precise measurement of the concentration of ammonia in the soil samples. This unit allows for standardization and comparison across different samples by measuring the amount of ammonia in micrograms per gram of soil, making it easier to quantify and assess variations in ammonia concentration.
Step 2
Calculate the difference in the rate of breakdown of ammonia per day between day 0 and day 2 in soil A and soil B.
Answer
For soil A:
- Day 0: Concentration = 15 µg g⁻¹
- Day 2: Concentration = 10 µg g⁻¹
- Difference = 15 - 10 = 5 µg g⁻¹
- Rate = 5 µg g⁻¹ / 2 days = 2.5 µg g⁻¹ per day
For soil B:
- Day 0: Concentration = 12 µg g⁻¹
- Day 2: Concentration = 8 µg g⁻¹
- Difference = 12 - 8 = 4 µg g⁻¹
- Rate = 4 µg g⁻¹ / 2 days = 2 µg g⁻¹ per day
Thus, the difference in rates between soils A and B is 2.5 - 2 = 0.5 µg g⁻¹ per day.
Step 3
Give reasons for your answer.
Answer
The results show that soil A has a higher concentration of ammonia at day 0 and a slower rate of breakdown compared to soil B. The pH difference between the two soils is likely influencing the activity of the nitrifying bacteria, affecting their rates of ammonia oxidation. Specifically, the optimal pH for these bacteria is closer to that of soil A (pH 6.9) compared to soil B (pH 4.3), leading to more efficient bacteria activity in soil A.
Step 4
Use a calculation to support your answer.
Answer
From the data, we can see that species S in acidic soil shows a change from 1,000 copies of mRNA to 990 copies, a decrease of 1%; whereas species T in neutral pH shows an increase from 900 copies to 990 copies, which is an increase of approximately 10%. The significant percentage change indicates species T is more affected by the pH changes, suggesting its growth and function are more influenced by pH conditions.
Step 5
Explain why.
Answer
This method estimates the expression of the amoA gene because the amount of mRNA reflects gene activity. However, it does not account for total bacterial cell numbers, as variations in mRNA levels can occur despite consistent or differing bacterial populations. Thus, while we can infer the expression of the amoA gene, we cannot determine how many bacteria are present or their overall health.
Step 6
Suggest one suitable method for sterilising the bottles and explain why it was necessary to sterilise them.
Answer
One suitable method for sterilising the bottles is using an autoclave, which employs steam under pressure to kill bacteria and spores. Sterilisation is necessary to prevent contamination from external microbial communities that could interfere with the results of the experiment by introducing variables outside the controlled conditions of the study.