Explain the role of decomposers in the cycling of phosphorus within an ecosystem.

IB DP Biology Standard Level (2023 syllabus) — C4.2 Transfers of energy and matter · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Decomposers such as bacteria and fungi break down dead organic matter that contains phosphorus compounds (e.g. nucleic acids, phospholipids, ATP). In the process of mineralisation they release inorganic phosphate ions (PO₄³⁻) into the soil or water. These phosphate ions are then absorbed by producers – plants and algae – through their roots or via active transport. Thus decomposers return phosphorus to the biotic component of the ecosystem, preventing its permanent loss and allowing it to be reused by living organisms.

Examiner tips

Use the term ‘mineralisation’ to show understanding of the process.
Show the flow: dead matter → decomposers → PO₄³⁻ → uptake by producers.
Mention both soil and water as potential reservoirs.
Keep the answer concise – 4 marks only.

Common mistakes

Confusing decomposers with producers; not mentioning mineralisation.
Using vague terms like ‘break down’ without specifying inorganic phosphate release.
Failing to state that phosphorus is returned to living organisms, implying permanent loss.

Mark scheme (4 marks)

Decomposers (bacteria and fungi) break down dead organic matter / detritus containing phosphorus compounds (e.g. nucleic acids, phospholipids, ATP)
Decomposers release inorganic phosphate ions (PO₄³⁻) into the soil / water through the process of mineralisation
Inorganic phosphate ions are absorbed / taken up by producers (plants / algae) via roots / active transport
This cycling ensures phosphorus is returned to the biotic components of the ecosystem / prevents permanent loss of phosphorus from the ecosystem / allows phosphorus to be reused by living organisms

Key terms in this question

decomposers · cycling