QUESTION 129 (6 marks)
A harmless nitrogen-isotope tracer is initially supplied as nitrate to plants. Later it is detected successively in plant proteins, grazer proteins, dead organic matter, ammonium and nitrate. The isotope identifies the nitrogen atoms; it does not alter their chemical behaviour.
a) Describe how the labelled nitrogen reaches grazer proteins from the initial nitrate. [2 marks]
b) Identify the processes producing ammonium and then nitrate from the dead organic matter. [2 marks]
c) Explain why return of the labelled nitrogen to nitrate does not show that energy cycles in the same way. [2 marks]
Practice marking scheme
Answer
Assimilation places nitrate nitrogen into plant organic molecules; feeding transfers it to grazers. Decomposition/ammonification produces ammonium, followed by nitrification to nitrate. Matter can return to a reservoir while energy is progressively dissipated.
Working
The tracer follows atoms between inorganic and organic pools. Plants take up inorganic nitrogen and incorporate it into organic molecules such as amino acids and proteins. Grazers obtain that nitrogen by eating plant material. Decomposers mineralise organic nitrogen to ammonium; nitrifying microorganisms oxidise ammonium through nitrite to nitrate. Reappearance of labelled nitrate demonstrates recycling of matter, rather than recycling of the original usable energy.
Marking criteria
- Describes uptake and incorporation into plant organic compounds. [1 mark]
- Describes feeding and incorporation into grazer proteins. [1 mark]
- Identifies ammonification/decomposition to ammonium. [1 mark]
- Identifies nitrification to nitrate. [1 mark]
- Distinguishes recycled nitrogen atoms from energy. [1 mark]
- Explains heat dissipation and need for renewed energy input. [1 mark]
Practice question aligned to the current QCAA syllabus; review the worked solution and marking criteria.
View the QCAA syllabusCompare your working with the guide above.