Transfer nitrogen through feeding and digestion
Feeding transfers plant and animal proteins between organisms; digestion breaks proteins into amino acids that can be absorbed.
Section 19.3 · Nutrient cycles
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 does digestion do to dietary protein in this cycle?
What you will learn
- Trace protein nitrogen along a food chain
- Explain digestion as protein breakdown before absorption
Feeding moves protein nitrogen
Plants use nitrate to make amino acids and proteins. When an animal eats plant material, nitrogen in the protein enters its digestive system. When a carnivore eats another animal, protein nitrogen can transfer again. Feeding therefore moves nitrogen through food chains.
The nitrogen atom remains matter while it is rearranged into different molecules. It is not converted into energy. Food chains describe the transfer between organisms; digestion describes breakdown of proteins inside the consumer.
Digestion supplies absorbable amino acids
Digestive enzymes break proteins into amino acids. The amino acids are absorbed and can be used to make the consumer’s proteins. Some amino acids may be in excess and undergo deamination, which is covered next.
Avoid saying intact dietary proteins are simply absorbed into the blood as whole proteins. Use the syllabus sequence of feeding and digestion of proteins, then connect absorbed amino acids to protein production.
Pause and explain
How can nitrogen in a plant protein reach an animal?
Worked example
A rabbit eats plant protein. State what digestion produces and how the nitrogen can be used.
Show the worked solution
- The rabbit ingests protein by feeding.
- Digestion breaks protein into amino acids.
- Absorbed amino acids can be used to make rabbit proteins.
Answer Digestion breaks the plant protein into amino acids; absorbed amino acids can be used to make rabbit proteins containing that nitrogen.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
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 leaf protein into a herbivore amino acid.
Give me a hint
Include feeding, digestion and absorption.
Compare my reasoning
- The herbivore eats the leaf protein.
- Digestion breaks protein into amino acids.
- Amino acids are absorbed.
Leaf protein nitrogen → herbivore by feeding → amino acids by digestion → absorbed amino acids.
Look for these in your work
- Feeding link
- Digestion products
A wolf eats a rabbit. Explain how nitrogen in rabbit muscle protein can enter the wolf’s amino-acid pool.
Give me a hint
Follow protein ingestion and digestion.
Compare my reasoning
- The wolf ingests rabbit protein.
- Digestive enzymes break it into amino acids.
- Amino acids are absorbed.
The wolf eats rabbit muscle protein; digestion breaks the protein into amino acids, which are absorbed into the wolf.
Look for these in your work
- Protein source
- Amino acids and absorption
A learner says “nitrogen moves from rabbit to wolf by denitrification.” Correct the process.
Give me a hint
Choose between food transfer and soil-to-atmosphere change.
Compare my reasoning
- The wolf eats the rabbit.
- Feeding transfers protein nitrogen.
Nitrogen moves from rabbit to wolf by feeding. Denitrification instead changes soil nitrate into atmospheric nitrogen gas.
Look for these in your work
- Feeding named
- Denitrification distinguished
Common mistakes
- Saying digestion changes nitrogen into energy
- Skipping amino acids
- Reversing the feeding direction
What can you explain now?
What does digestion do to dietary protein in this cycle?
Compare with the explanation
Breaks protein into amino acids
Amino acids are the products that can be absorbed.
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Make it stick
Explain how protein nitrogen moves from one animal to another and what digestion does to the protein.
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.