Phosphorus cycle
Laura Armstrong & Joe Wolfensohn
Teachers
Recall Questions
This topic requires prior knowledge of biological molecules. You can test your knowledge on these below.
Why do organisms need phosphorus?
To make ATP, DNA, RNA, and phospholipids.
Where is most phosphorus found?
Most phosphorus on Earth is found in sedimentary rocks and ocean sediments, primarily as phosphate ions
Is phosphorus required in respiration or photosynthesis?
Yes – it's needed to make ATP, which stores and transfers energy in both processes.
Its also found in NADP and RuBP needed in photosynthesis!
Topic Explainer Video
Check out this @JoeDoesBiology video that explains phosphorus cycle or read the full notes below. Once you've gone through the whole note, try out the practice questions!
Practice question videos
The Phosphorus Cycle
Unlike the nitrogen cycle, the phosphorus cycle does not involve a gaseous phase – phosphorus is mainly found in sedimentary rocks in the form of phosphate ions.
Overview of the Cycle
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Erosion and weathering of rocks
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Absorption by plants
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Feeding and Digestion
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Excretion and decomposition
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Deposition and sedimentation
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Erosion and Weathering of Rocks
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Phosphate ions (PO₄³⁻) are released from phosphate-containing rocks by erosion and weathering.
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These phosphate ions dissolve in soil and water.
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Phosphate ions from fertilisers also leach into bodies of water.
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Absorption by Plants
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Plants absorb phosphate ions from the soil through active transport.
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They incorporate these into organic compounds like DNA, RNA, ATP, and phospholipids.
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Feeding & Digestion
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Animals obtain phosphorus by feeding on plants (or other animals).
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They incorporate these into their organic compounds like DNA, RNA, ATP, and phospholipids.
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Excretion & Decomposition
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Excretion (e.g. urine, faeces) and death return organic phosphate compounds to the soil.
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Saprobionts decompose organic material, releasing phosphate ions back into the soil.
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Deposition and Sedimentation
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In aquatic systems, phosphate ions can be deposited in sediments and form rocks again over time.
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This process is very slow.
Mycorrhizae
Mycorrhizae play an important part in the uptake of ions such as phosphate and nitrate.
Mycorrhizae are mutualistic associations between certain types of fungi and the roots of plants.
Key Points
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The fungus grows around and into the plant’s root cells.
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The fungal hyphae (fine filaments) increase the surface area of the plant’s roots.
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This helps the plant absorb water and mineral ions (especially phosphate ions) more efficiently.
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In return, the plant provides the fungus with carbohydrates (e.g. sugars from photosynthesis).
Key Terms
- Phosphate ions (PO₄³⁻): Inorganic form of phosphorus used by organisms.
- Weathering: The breakdown of rocks releasing phosphate into soil and water.
- Saprobionts: Microorganisms that decompose dead matter, releasing nutrients like phosphate ions.
- Sedimentation: Formation of solid phosphate deposits from dissolved phosphate ions.
- Assimilation: The uptake and incorporation of nutrients into organic compounds in living organisms.
No answer provided.
Exam Tip
Phosphorus doesn’t have a gaseous phase, so don’t mention atmospheric phosphorus! This is a common exam mistake.
Make sure you can explain how a poor supply of phosphate ions can reduce plant growth.
No answer provided.
Suggest and explain two reasons why a poor supply of phosphate ions results in poor growth of plants. (2 marks)
Required to make ATP so less energy for growth
Required to make nucleotides, so less DNA / mRNA / tRNA for cell division / production of protein for growth
Required to make RuBP / NADP, so less CO2 fixed / less GP reduced into triose phosphate
Required to make phospholipids for membranes
Practice Question
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!