IB Biology HLEnergy & Matter in EcosystemsPaper 1 & 2~9 min read
Nutrient Cycling
The atoms in your body have been used before. Every element a living thing needs has already been through countless other organisms, and it only keeps working because two groups of organisms hand it back and forth: producers who lock it up, and decomposers who set it free.
📚 What you need to know
Photosynthesis takes in carbon dioxide and produces carbon compounds and oxygen. It is the source of atmospheric oxygen on Earth.
Aerobic respiration uses that oxygen and produces carbon dioxide, which can be used in photosynthesis. Both autotrophs and heterotrophs respire.
Combined photosynthesis and combined respiration move huge volumes of both gases. These transfers are fluxes.
In a functioning ecosystem the elements organisms need are constantly recycled.
Producers take up inorganic nutrients from the abiotic environment; consumers gain them by ingestion; decomposers release them again.
As well as carbon, ecosystems cycle nitrogen, calcium, phosphorus, sulfur and potassium.
Autotrophs and heterotrophs feed each other’s gases
The process of photosynthesis, carried out by most autotrophs, takes in carbon dioxide from the atmosphere and converts it into carbon compounds and oxygen. This process is the source of atmospheric oxygen on Earth – every oxygen molecule you breathe was released by a photosynthetic organism.
Aerobic respiration depends on oxygen, so organisms that respire aerobically can make use of the oxygen produced during photosynthesis. In turn they produce carbon dioxide as a waste product, which can be used in photosynthesis. Note that both autotrophs and heterotrophs make use of oxygen in respiration.
Neither box can run for long without the other. Take one away and the whole system stalls within a few generations.
Scale matters here. The combined photosynthesis of all photosynthetic organisms on Earth removes a huge volume of carbon dioxide and releases a huge volume of oxygen, while the combined respiration of all aerobically respiring organisms does the exact reverse. Processes that transfer carbon like this are called fluxes.
Recycling the elements
In a functioning ecosystem the elements that living organisms need are constantly recycled. Three groups keep that going, and each one has a specific job.
Group
What they do with nutrients
Producers
Access inorganic nutrients from the abiotic environment and convert them into carbon compounds
Consumers
Gain organic nutrients by ingesting the tissues of producers and other consumers
Decomposers
Break down the organic molecules in dead tissues and waste matter, making them available again to producers
Compare this with the energy diagrams earlier in the unit. Those had an entrance and an exit. This one has neither.
It is not only carbon
This cycling applies to carbon, which is taken in from the atmosphere by producers and returned to the atmosphere by decomposers. But many other mineral elements are cycled through ecosystems in the same way:
Nitrogen – for amino acids, proteins and nucleic acids
Calcium – for bones, shells and cell walls
Phosphorus – for DNA, ATP and phospholipids
Sulfur – for certain amino acids
Potassium – for nerve impulses and guard cell function
These elements are incorporated into biological molecules within the tissues of living organisms, and then released back into the environment when decomposers break down those tissues after death.
You do not need the detail of the nitrogen or phosphorus cycles here – just the pattern. Whatever the element, the shape is identical: taken up by producers, passed along by feeding, released by decomposers.
Back to where the unit started. Ecosystems are open, so nutrients can drift in and out. But the Earth as a whole is closed to matter, which is exactly why the recycling has to work. There is no delivery of fresh phosphorus from space.
Worked examples
WE 1
Explain the gas interaction
Outline the interaction between autotrophs and heterotrophs in terms of the gases they exchange. (3 marks)
Point 1: photosynthesis
Autotrophs take in carbon dioxide and produce carbon compounds and oxygen, which is the source of atmospheric oxygen.
Point 2: respiration
Aerobically respiring organisms use that oxygen and release carbon dioxide as a waste product, which can be used in photosynthesis.
Point 3: the qualifier
Both autotrophs and heterotrophs respire, so autotrophs use oxygen as well as producing it.
Each process supplies the raw material for the otherthe third point is the one most students miss – plants are on both sides of this exchange
WE 2
Explain nutrient recycling
Explain how the chemical elements needed by living organisms are recycled in an ecosystem. (4 marks)
Point 1: producers
Producers access inorganic nutrients from the abiotic environment and convert them into carbon compounds.
Point 2: consumers
Consumers gain those nutrients as organic molecules by ingesting the tissues of producers and other consumers.
Point 3: decomposers
Decomposers break down the organic molecules in dead tissue and waste, releasing inorganic nutrients again.
Point 4: the return
Those nutrients become available to producers once more, so the same atoms are used repeatedly.
Absorbed → eaten → decomposed → absorbed againname all three groups – answers that only mention decomposers cap out at two marks
WE 3
Energy versus matter
Explain why matter is recycled in an ecosystem but energy is not. (3 marks)
Point 1: what happens to matter
Atoms are rearranged into different molecules but not lost, so the same elements are used again and again by producers, consumers and decomposers.
Point 2: what happens to energy
Energy is lost from every trophic level as heat, during respiration and when ATP is used, and radiates into the environment.
Point 3: why it cannot come back
Organisms cannot capture that heat and use it to build tissue, so energy must be constantly resupplied by sunlight.
Matter cycles, energy flows through and leavesthis single distinction underpins the whole unit – learn the two verbs
💡 Exam tips
Say photosynthesis is the source of atmospheric oxygen – it is a stated point in the syllabus.
Always add that autotrophs respire too when writing about gas exchange.
Use the term flux for a transfer between stores.
For recycling questions, structure your answer as producers, consumers, decomposers.
Be able to name five elements other than carbon that are cycled.
End any matter-versus-energy question with matter cycles, energy flows.
⚠ Common mistakes
Saying plants only photosynthesise and animals only respire. Plants do both, all the time.
Writing that energy is recycled. It is not – only matter is.
Naming only carbon. Nitrogen, calcium, phosphorus, sulfur and potassium are cycled too.
Saying decomposers “eat” nutrients and keep them. They absorb some, but the rest is released into the soil.
Confusing organic and inorganic nutrients. Producers take up inorganic ones; consumers take in organic ones.
Assuming recycling is automatic. Remove the decomposers and the cycle stops.
That completes Energy & Matter in Ecosystems. The nine pages tell one story: sunlight enters, gets locked into carbon compounds, passes along a chain while leaking heat at every step, and finally leaves as heat – while the atoms it borrowed stay behind and go round again.
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