A-Level · Biology · AQA · Mark scheme decoded
AQA A-Level Biology: Nutrient Cycling in Ecosystems — mark scheme explained
The short answer
Nutrient cycling is a fundamental process in ecosystems, ensuring the continuous availability of essential elements for living organisms. This section focuses on the nitrogen and phosphorus cycles, highlighting the vital roles played by microorganisms such as saprobionts, mycorrhizae, and bacteria.
The question
Describe the role of saprobionts in nutrient cycling. [Paraphrased for study — not reproduced from any exam paper.]
Mark scheme, decoded
What each mark is really for — in plain English — and the wording trap that loses it.
- S1
Step 1: Identify what saprobionts are.
- S2
Saprobionts are decomposers that break down dead organic matter.
- S3
Step 2: Explain their role in the nitrogen cycle.
- S4
In the nitrogen cycle, saprobionts decompose dead plant and animal matter, converting organic nitrogen compounds into ammonia (NH 3 ). This process is known as ammonification.
- S5
Step 3: Explain their role in the phosphorus cycle.
- S6
In the phosphorus cycle, saprobionts break down dead organic matter, returning phosphates to the soil. This makes phosphorus available for plant uptake.
Model answer
Worked through, with each step tagged to the mark it earns.
- S1
Step 1: Identify what saprobionts are.
- S2
Saprobionts are decomposers that break down dead organic matter.
- S3
Step 2: Explain their role in the nitrogen cycle.
- S4
In the nitrogen cycle, saprobionts decompose dead plant and animal matter, converting organic nitrogen compounds into ammonia (NH 3 ). This process is known as ammonification.
- S5
Step 3: Explain their role in the phosphorus cycle.
- S6
In the phosphorus cycle, saprobionts break down dead organic matter, returning phosphates to the soil. This makes phosphorus available for plant uptake.
Final answer: Saprobionts are decomposers that break down dead organic matter. In the nitrogen cycle, they convert organic nitrogen compounds into ammonia (NH 3 ) through ammonification. In the phosphorus cycle, they return phosphates to the soil, making them available for plant uptake.
Common mistakes
- Confusing nitrogen fixation with ammonification. — Remember that nitrogen fixation is performed by nitrogen-fixing bacteria, while ammonification is carried out by saprobionts.
- Forgetting the role of mycorrhizae in nutrient uptake. — Always mention that mycorrhizae form symbiotic associations with plant roots, increasing the surface area for nutrient uptake and benefiting both the plant and the fungus.
- Confusing nitrifying bacteria with denitrifying bacteria. — Remember that nitrifying bacteria (e.g., Nitrosomonas and Nitrobacter) convert ammonia to nitrite and then to nitrate, while denitrifying bacteria reduce nitrates back to nitrogen gas (N 2 ).
- Failing to link fertiliser use with environmental issues. — Always include a discussion of leaching and eutrophication when explaining the use of natural and artificial fertilisers.
- Incorrectly describing the process of nitrification. — Clearly state that nitrification involves ammonia oxidation to nitrite by Nitrosomonas and nitrite oxidation to nitrate by Nitrobacter.
- Failing to explain the role of saprobionts in both nitrogen and phosphorus cycles. — Always mention that saprobionts play a crucial role in both cycles by breaking down dead organic matter and returning nutrients to the soil.
Where the marks go
- Full worked solution (all marking points)4 marks