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 B18.3 · Carbon cycle
Exam scope and assessment notes
Mapped to the named 0973 section using teaching from the corresponding single-science course. A 0973-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade. This qualification reports two grades on the 9–1 scale; confirm the entry route for your zone and exam series.
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.
Sources for this lesson
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.
After trying it yourself, choose your next review. This is your self-assessment.
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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.