Decompose proteins to ammonium ions
Decomposers break down plant and animal proteins in dead or waste material, releasing ammonium ions into the soil cycle.
Exam scope and assessment notes
Extended only (Supplement) · Biology 0970, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked; human subject-teacher review, supervised practical competence and cumulative assessment remain pending. Cambridge states that IGCSE 0970 has the same subject content as IGCSE 0610, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Use the food-web pathway and distinguish matter cycling from energy flow.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is produced when decomposers break down plant and animal protein in this cycle?
What you will learn
- Trace nitrogen from proteins into ammonium ions
- Identify decomposition as a microorganism role
Organic nitrogen returns to the soil
Proteins in dead plants, dead animals and waste contain nitrogen. Decomposer microorganisms break these proteins down, transferring nitrogen into ammonium ions in the soil. This decomposition step reconnects nitrogen in organisms with soil nitrogen compounds.
The syllabus asks for decomposition of plant and animal protein to ammonium ions. Keep the substrate and product clear: protein is organic material; ammonium is an ion. Do not substitute nitrate for ammonium at this step.
Microorganisms have a defined role
Microorganisms carry out decomposition in the nitrogen cycle. They also have separate roles in nitrogen fixation, nitrification and denitrification. Not every microorganism carries out all four processes, and individual bacterial names are not required.
Dead material may contain carbon and nitrogen at the same time. Decomposition therefore participates in both cycles, but the carbon and nitrogen products should be labelled separately.
Pause and explain
Which microorganism role is used when protein becomes ammonium ions?
Worked example
Dead animal protein is decomposed in soil. State the nitrogen-containing product required in the cycle.
Show the worked solution
- The substrate is protein from dead animal material.
- Decomposition produces ammonium ions.
Answer Decomposition transfers nitrogen from the protein into ammonium ions in the soil.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Trace nitrogen from a dead leaf’s protein into the soil.
Give me a hint
Keep the protein substrate and ammonium product.
Compare my reasoning
- Plant protein contains nitrogen.
- Decomposer microorganisms break down the protein.
- Ammonium ions enter the soil.
Nitrogen in dead plant protein → ammonium ions in soil by decomposition by microorganisms.
Look for these in your work
- Substrate stated
- Correct product stated
A diagram shows animal protein → nitrate ions with no intermediate. What syllabus step is missing?
Give me a hint
Use the specified decomposition product before nitrification.
Compare my reasoning
- Protein is decomposed to ammonium ions.
- Nitrification then changes ammonium compounds towards nitrate.
Show decomposition of animal protein to ammonium ions first; nitrification is a separate subsequent process that produces nitrate ions.
Look for these in your work
- Ammonium intermediate restored
- Processes separated
A student labels decomposition and denitrification as the same bacterial process. Explain the difference.
Give me a hint
Compare their starting and ending nitrogen compounds.
Compare my reasoning
- Decomposition: protein → ammonium ions.
- Denitrification: nitrate → atmospheric nitrogen gas.
Decomposition releases ammonium ions from plant or animal protein, while denitrification changes nitrate compounds into atmospheric nitrogen gas.
Look for these in your work
- Starting materials contrasted
- Products contrasted
Common mistakes
- Skipping ammonium and drawing protein directly to nitrate
- Calling ammonium nitrogen gas
- Saying one microorganism performs every cycle role
What can you explain now?
What is produced when decomposers break down plant and animal protein in this cycle?
Compare with the explanation
Ammonium ions
This is the product specified in the syllabus.
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Make it stick
Describe how nitrogen in plant or animal protein becomes ammonium ions and identify the microorganism role.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.