Explain how phosphorus from the bodies of dead organisms is returned to the soil and then absorbed by living plants.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Phosphorus is an essential element for living organisms. It is found in biological molecules such as DNA and cell membranes. Unlike carbon, phosphorus does not have a gaseous phase in its cycle.
Model answer (5 marks)
1. Decomposers such as bacteria and fungi break down the bodies of dead organisms.
2. During decomposition, phosphorus is released into the soil as phosphate ions or mineralised phosphate.
3. Plants absorb these dissolved phosphate ions through their root cells.
4. The uptake of phosphate ions is carried out by active transport, moving the ions against the concentration gradient.
5. Once inside the plant, phosphorus is incorporated into biological molecules such as DNA, RNA and phospholipids of cell membranes.
2. During decomposition, phosphorus is released into the soil as phosphate ions or mineralised phosphate.
3. Plants absorb these dissolved phosphate ions through their root cells.
4. The uptake of phosphate ions is carried out by active transport, moving the ions against the concentration gradient.
5. Once inside the plant, phosphorus is incorporated into biological molecules such as DNA, RNA and phospholipids of cell membranes.
Examiner tips
- Use the exact terms – decomposers, phosphate ions, active transport, DNA, phospholipids.
- Show the sequence from decomposition to absorption to utilisation.
- Mention that uptake is against a concentration gradient to justify active transport.
Mark scheme (5 marks)
- Decomposers (bacteria / fungi) break down the bodies of dead organisms
- Decomposers release phosphorus (as phosphate ions / minerals) into the soil
- Plants absorb (dissolved) mineral ions / phosphate ions through their roots
- Active transport is used to absorb mineral ions against the concentration gradient
- Phosphorus is used to make biological molecules / DNA / phospholipids (cell membranes)
Related
- All AQA A-Level Biology (7402) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →