IB Biology SLTopic 3 — Energy & MatterPaper 1 & 2Core idea~9 min read
Nutrient Cycling
The supply of chemical elements on Earth is fixed. No new nitrogen is arriving. So every ecosystem depends on the same atoms being released and reused, again and again — and the organisms doing the releasing are the ones nobody thinks about.
📘 What you need to know
The supply of chemical elements is finite, so elements must be recycled within ecosystems.
Producers take up inorganic nutrients from the abiotic environment and build them into carbon compounds.
Consumers obtain nutrients by ingesting other organisms.
Decomposers break down dead matter and waste, releasing inorganic nutrients for producers to use again.
Nitrogen, phosphorus, calcium, sulfur and potassium all cycle in the same general way as carbon.
Photosynthesis and respiration form a large-scale interaction between autotrophs and heterotrophs, exchanging oxygen and carbon dioxide.
Why recycling is not optional
Energy keeps arriving from the sun, so an ecosystem can afford to lose it as heat. Matter is different. The nitrogen atoms in a leaf were in something else last year and will be in something else next year. If nutrients stayed locked inside dead bodies, producers would run out and the ecosystem would collapse.
That is why decomposition matters so much. It is not tidying up. It is the step that keeps the whole system running.
Break the decomposition arrow and the whole loop stops.
Who does what
Group
Where its nutrients come from
What it contributes to the cycle
Producers
Inorganic nutrients absorbed from soil, water or air
Converts them into carbon compounds available to everything else
Consumers
Organic nutrients from ingesting other organisms
Passes nutrients along, and returns some as faeces and urine
Detritivores
Dead material they ingest
Breaks tissue into smaller pieces, hugely increasing surface area
Saprotrophs
Dead material digested externally
Releases inorganic nutrients back into the soil or water
Detritivores and saprotrophs usually work as a team. Earthworms and woodlice shred a fallen leaf into fragments; fungi and bacteria then secrete enzymes onto those fragments and absorb the products. Whatever the saprotrophs do not use themselves stays in the soil, where roots can take it up.
A neat exam line: decomposers “make nutrients available again to producers”. That single phrase links the start and the end of the cycle and is often worth the mark on its own.
Which elements cycle
Carbon gets all the attention, but the same principle applies to every element organisms need.
Nitrogen — needed for amino acids, proteins and nucleic acids.
Phosphorus — needed for DNA, ATP and phospholipids.
Calcium — needed for bones, shells and cell walls.
Sulfur — needed for some amino acids and the proteins built from them.
Potassium — needed for nerve impulses and for plant water balance.
Each is absorbed as an inorganic ion by producers, built into biological molecules, passed along food chains, and released again by decomposers when organisms die.
Why farmers add fertiliser: harvesting a crop physically removes matter from the ecosystem, so those nutrients never get recycled back into that field. Fertiliser replaces what the harvest took away.
The oxygen and carbon dioxide exchange
Photosynthesis and aerobic respiration are close to being mirror images of each other, and together they form the biggest interaction between autotrophs and heterotrophs on the planet.
The waste product of one process is the raw material of the other.
Two details are worth pinning down:
Photosynthesis is the origin of essentially all the oxygen in Earth’s atmosphere.
Autotrophs respire as well. They do not photosynthesise and leave it at that; they need ATP just like everything else.
Added up across every organism on the planet, these are enormous carbon fluxes. Photosynthesis removes billions of tonnes of carbon dioxide each year, and respiration and decomposition put a comparable amount back.
WORKED EXAMPLE
A woodland is treated with a fungicide that kills most of its soil fungi. Suggest and explain the effects on the woodland over the following years. [4]
Point 1: identify the role removedmost soil fungi are saprotrophs, which digest dead material externally and release inorganic nutrientsPoint 2: immediate effectrate of decomposition falls, so dead leaves and wood build up on the forest floorPoint 3: effect on the nutrient supplyfewer inorganic nutrients such as nitrate are released into the soilPoint 4: effect on producersproducers cannot absorb enough nutrients, so growth and primary production falla good structure for any “suggest the effects” question: role removed, first effect, knock-on, final consequence
Pulling the topic together
If you remember nothing else from this unit, remember the shape of it:
The one-line summary
Energy flows through an ecosystem and is lost as heat. Matter cycles within it and is used again.
Every page in this topic is a detail hanging off that sentence. Sunlight enters and heat leaves, which is the flow. Carbon and nitrogen go round through producers, consumers and decomposers, which is the cycle. Food chains are short because the flow leaks; ecosystems keep going because the cycle does not.
💡 Exam tip
Say that decomposers release inorganic nutrients and make them available to producers again.
Name a specific nutrient when you can, e.g. nitrate for nitrogen, rather than saying “nutrients”.
Remember autotrophs respire as well as photosynthesise. It is checked constantly.
When explaining a disruption to the cycle, work through the chain step by step rather than jumping to the conclusion.
Use finite supply of elements as the reason recycling is necessary.
Photosynthesis is the source of atmospheric oxygen — a one-mark fact worth having ready.
⚠ Common mix-up
Nutrients are not energy. Nutrients are recycled; energy is not.
Decomposers do not “produce” nutrients. They release ones that were already there, locked in dead tissue.
Detritivore is not the same as saprotroph. Internal versus external digestion.
Plants do not absorb nitrogen gas from the air. They take up nitrogen as ions such as nitrate from the soil.
Respiration is not only done by animals. Every living organism respires.
Fertiliser is not the same as recycling. It is an input added because harvesting removed matter from the system.
Up next: Climate Change — you now know how carbon moves between pools. The next topic looks at what happens when we shift the balance between those pools faster than the cycle can absorb.
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