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
How the examiner actually awards the marks on this topic.
Gradora's own decode of the marking approach — not the exam board's published mark scheme.
How marks are awarded
For descriptive questions, marks are typically awarded for providing accurate and detailed information about the process or phenomenon. For explanatory questions, marks are given for clear reasoning and understanding of underlying principles. For identification questions, marks are awarded for correctly naming specific elements or processes. For discussion questions, marks are given for considering multiple aspects and providing a balanced view.
What the command words demand
- Describe
- Provide a detailed account of a process or phenomenon, including key steps and components.
- Explain
- Give reasons for why something happens, including underlying principles and relationships.
- Identify
- Recognize and name specific elements or processes.
- Discuss
- Consider multiple aspects of a topic, including causes, effects, and implications.
Model answer
A full-mark response to the question above, worked through step by step.
Timing: Allocate approximately 5-7 minutes per question to ensure you have enough time to carefully read the problem, provide detailed answers, and check your work.
- Identify what saprobionts are.0 marksSaprobionts are decomposers that break down dead organic matter.
- Explain their role in the nitrogen cycle.2 marksIn the nitrogen cycle, saprobionts decompose dead plant and animal matter, converting organic nitrogen compounds into ammonia (NH 3 ). This process is known as ammonification.
- Explain their role in the phosphorus cycle.2 marksIn 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.
Work through every step correctly and you earn all 4 marks.
Another worked example
Explain the process of nitrification and identify the bacteria involved.
- Define nitrification.2 marksNitrification is a two-step process where ammonia (NH 3 ) is oxidised to nitrite (NO 2 - ) and then to nitrate (NO 3 - ).
- Identify the bacteria involved in each step.2 marksThe first step, ammonia oxidation to nitrite, is carried out by nitrifying bacteria such as Nitrosomonas. The second step, nitrite oxidation to nitrate, is performed by other nitrifying bacteria like Nitrobacter.
Final answer: Nitrification is a two-step process where ammonia (NH 3 ) is oxidised to nitrite (NO 2 - ) and then to nitrate (NO 3 - ). The first step, ammonia oxidation to nitrite, is carried out by nitrifying bacteria such as Nitrosomonas. The second step, nitrite oxidation to nitrate, is performed by other nitrifying bacteria like Nitrobacter.
Work through every step correctly and you earn all 4 marks.
Common mistakes
Confusing nitrogen fixation with ammonification.
Why it happens: Students may mix up the processes of converting atmospheric nitrogen (N 2 ) into ammonia (NH 3 ) and breaking down organic nitrogen compounds into ammonia.
Fix: 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.
Why it happens: Students might overlook the importance of mycorrhizae in enhancing plant nutrient absorption.
Fix: 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.
Why it happens: Students may mix up the roles of different types of bacteria in the nitrogen cycle.
Fix: 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.
Why it happens: Students might describe the use of fertilisers without discussing their environmental impacts.
Fix: Always include a discussion of leaching and eutrophication when explaining the use of natural and artificial fertilisers.
Incorrectly describing the process of nitrification.
Why it happens: Students may forget that nitrification is a two-step process involving different types of bacteria.
Fix: 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.
Why it happens: Students might focus on one cycle and neglect the other.
Fix: 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
The question types you’ll meet on this topic and the marks each one carries — so you know what to expect and where to focus.
| Question type | What you’re asked to do | Marks |
|---|---|---|
| Describe Nutrient Cycling | Describe how saprobionts decompose dead matter and release nutrients back into ecosystems. | 4 |
| Explain And Identify | Describe the two oxidation steps of nitrification and name the bacteria performing each. | 4 |
| Describe Biological Role | Describe how mycorrhizae increase surface area and help plants absorb water and mineral nutrients. | 3 |
| Fertilisers And Eutrophication | Describe how fertilisers replace lost nutrients and explain the environmental problems their use causes. | 5 |
| Total across these question types | 16 | |
Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.