# Cycles within ecosystems

> Edexcel International GCSE Biology · 4BI1 2017 Spec
> Source: https://www.owlsprep.com/study/edexcel-igcse-biology-s4-cycles-within-ecosystems/

This guide covers the two key nutrient cycles required for Edexcel IGCSE Biology Section 4: the core carbon cycle for all candidates, and the Biology-only nitrogen cycle for Paper 2, including all mark scheme required processes.

**Prerequisites:** [Ecosystems and food chains](https://www.owlsprep.com/study/edexcel-igcse-biology-s4-ecosystems-food-chains/); [Photosynthesis and respiration basics](https://www.owlsprep.com/study/edexcel-igcse-biology-s2-photosynthesis-respiration/)

## Learning objectives

- Describe all stages of the carbon cycle including photosynthesis, respiration, decomposition and combustion
- (Biology only) Describe stages of the nitrogen cycle including roles of key functional bacterial groups
- Explain how nutrients are recycled between biotic and abiotic components of ecosystems

## Overview of Nutrient Cycles in Ecosystems

Nutrients such as carbon and nitrogen are finite resources. Ecosystems rely on continuous cycling of these elements between living organisms (the biotic community) and non-living parts of the environment (atmosphere, soil, water) to support new growth and sustain life.

**Nutrient cycle** — A natural process that constantly recycles essential elements between the biotic and abiotic components of an ecosystem, so elements can be reused repeatedly.

**Check your understanding**

1. Why are nutrient cycles essential for ecosystems?

   - They create new elements to support growth
   - They recycle finite elements for repeated use
   - They remove toxic waste from the atmosphere
   - They increase the total mass of elements available

   *Answer:* They recycle finite elements for repeated use

   *Why:* Correct! Elements like carbon and nitrogen are not created out of nothing, so cycles allow them to be reused by new generations of organisms.

*Calculator:* allowed

## The Carbon Cycle (All Candidates, 4.10)

The carbon cycle describes the continuous movement of carbon atoms between the atmosphere (as $CO_2$), living biomass, fossil fuels, and oceans. All mark scheme answers must include the four required processes listed in the specification.

**Worked example:** Outline the main stages of the carbon cycle as required for Edexcel IGCSE Biology exam answers.

1. 1. Atmospheric $CO_2$ is the main abiotic reservoir of carbon.
2. 2. Photosynthesis: Plants and algae absorb $CO_2$ from the air to make glucose for growth, locking carbon into plant biomass. Carbon passes along food chains when animals eat plants or other animals.
3. 3. Respiration: All living organisms (plants, animals, decomposers) carry out respiration to release energy, which returns $CO_2$ to the atmosphere.
4. 4. Decomposition: Decomposers (fungi, bacteria) break down dead organisms and waste products, releasing $CO_2$ back to the air via their own respiration.
5. 5. Combustion: Burning of fossil fuels (formed from dead organisms compressed over millions of years) or wood releases stored carbon as $CO_2$ into the atmosphere.

> **Exam Tip**
>
> Always explicitly name all four processes (photosynthesis, respiration, decomposition, combustion) in 3+ mark carbon cycle questions to get full marks, even if you include a clear diagram.

**Summary**

- Photosynthesis removes $CO_2$ from the atmosphere; respiration, decomposition, and combustion return $CO_2$ to the atmosphere
- Carbon is passed between trophic levels via feeding

*Calculator:* allowed

## The Nitrogen Cycle (Biology Only, Paper 2, 4.11B)

> **Biology Only Content**
>
> The nitrogen cycle is only tested on Paper 2 for Edexcel IGCSE Biology (4BI1) single award candidates. Double Award (4SD0) candidates do not need to learn this content.

Nitrogen is a key component of amino acids (the building blocks of proteins) and DNA, so it is essential for growth and reproduction in all living organisms. 78% of the atmosphere is $N_2$ gas, but it is unreactive and cannot be used directly by most plants or animals.

**Worked example:** Describe the role of bacteria in the nitrogen cycle, as required by the Edexcel specification.

1. 1. Nitrogen-fixing bacteria: Convert unreactive atmospheric $N_2$ into usable nitrogen compounds (ammonium or nitrates) that plants can absorb. Some live in root nodules of legume plants (e.g., peas, beans), others are free-living in soil. Lightning also fixes small amounts of atmospheric $N_2$ into usable forms.
2. 2. Decomposers: Break down proteins in dead organisms and urea in animal waste into ammonium compounds in the soil.
3. 3. Nitrifying bacteria: Convert ammonium compounds in soil into nitrates, the form of nitrogen most easily absorbed by plant roots.
4. 4. Denitrifying bacteria: Found in waterlogged, low-oxygen soil, they convert excess soil nitrates back into $N_2$ gas, returning it to the atmosphere to complete the cycle.
5. 5. Additional stage: Plants absorb nitrates to make their own proteins; animals get the nitrogen they need by eating plants or other animals and digesting their proteins.

> **Scope Warning**
>
> You do NOT need to memorise specific genus names of bacteria, e.g., *Rhizobium* or *Nitrosomonas*. Only refer to their functional roles as listed above to get full marks.

> **mnemonic**
>
> Remember: Nitrifying bacteria make *Nitrates* (both start with 'Nitr'), *De*nitrifying bacteria *De*lete nitrates from soil.

*Calculator:* allowed

## Common pitfalls

- **Wrong:** Forgetting to include decomposition as a process that returns $CO_2$ to the atmosphere in the carbon cycle.
  - Why it fails: Many students only list respiration and combustion, missing that decomposers respire as they break down dead matter, which is a required mark point.
  - Correct: Always explicitly name photosynthesis, respiration, decomposition, and combustion as the four key carbon cycle processes.
- **Wrong:** Stating that plants can absorb atmospheric nitrogen gas directly.
  - Why it fails: Nitrogen gas is unreactive, so plants cannot use it without first being converted into nitrates or ammonium by bacteria.
  - Correct: Specify that plants only absorb nitrogen in the form of nitrates from soil via their roots.
- **Wrong:** Naming specific bacteria species (e.g., *Rhizobium*) in nitrogen cycle answers.
  - Why it fails: The Edexcel specification explicitly states specific bacterial names are not required, and including them wastes time and risks misspelling and losing marks.
  - Correct: Refer only to the functional groups: nitrogen-fixing, nitrifying, denitrifying, and decomposer bacteria.
- **Wrong:** Confusing the roles of nitrifying and denitrifying bacteria.
  - Why it fails: Students often mix up which bacteria convert ammonium to nitrates and which convert nitrates back to nitrogen gas.
  - Correct: Use the mnemonic: nitrifying bacteria make nitrates, denitrifying bacteria remove nitrates from soil.
- **Wrong:** Omitting feeding as a way carbon and nitrogen move along food chains.
  - Why it fails: Many students focus only on abiotic transfers, forgetting that both elements are passed between trophic levels when organisms eat other organisms.
  - Correct: Add a line stating carbon/nitrogen is passed up food chains via feeding in longer answer questions.

## Cheatsheet

| Cycle | Key Processes / Roles | Inputs to atmosphere | Outputs from atmosphere |
| --- | --- | --- | --- |
| Carbon Cycle (All candidates) | Photosynthesis, respiration, decomposition, combustion, feeding | Respiration, decomposition, combustion | Photosynthesis ($CO_2$ uptake) |
| Nitrogen Cycle (Biology only) | Nitrogen fixation, decomposition, nitrification, denitrification, feeding | Denitrification ($N_2$ release) | Nitrogen fixation (bacteria + lightning) |

## What's next

Now that you have mastered the core nutrient cycles in ecosystems, you can move on to learning about human impacts on these cycles, including how excess $CO_2$ from combustion drives global warming and how excess nitrogen fertilisers cause eutrophication, which are covered in the next ecology sub-topic. You should also practice past paper questions on both cycles to familiarise yourself with mark scheme wording, and make sure you can label blank cycle diagrams from memory for 4-6 mark structured questions. Double Award candidates can skip nitrogen cycle revision and move directly to human impacts content.

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