Interdependence of autotrophs and heterotrophs and recycling of elementsSpec C4.2.21, C4.2.22
In short
Aerobic respiration depends on atmospheric oxygen produced by photosynthesis, and photosynthesis depends on atmospheric carbon dioxide produced by respiration. These huge annual fluxes make this a major interaction between autotrophs and heterotrophs. All chemical elements required by living organisms, not just carbon, are recycled in ecosystems, with decomposers playing a key role.
Photosynthesis and aerobic respiration are linked through the atmosphere. Photosynthesis takes in CO₂ and releases O₂; aerobic respiration takes in O₂ and releases CO₂. Aerobic respiration depends on oxygen produced by photosynthesis, and photosynthesis depends on carbon dioxide produced by respiration. The fluxes involved each year are huge, so this is a major interaction between autotrophs and heterotrophs.
Recycling of all elements
Living organisms need many elements besides carbon: nitrogen, phosphorus, sulfur, potassium, calcium, magnesium, iron and others. The supply of each element on Earth is finite, so all of them are recycled. Producers absorb them as inorganic ions or molecules; they pass to consumers by feeding; and decomposers break down dead organic matter, releasing the elements again as inorganic forms that producers can reuse.
This is why matter can be recycled in ecosystems but energy cannot: atoms are conserved and reused, while energy ends up as heat that organisms cannot use.
Details of the nitrogen cycle and other nutrient cycles are not required for C4.2. Focus on the general principle and the role of decomposers.
Quick check
What can cross the boundary of a closed system?
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Only energy; matter cannot enter or leave.
What is the energy source of iron-oxidising bacteria?
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Energy released by oxidising iron(II) ions to iron(III) ions, an oxidation reaction.
What are the usual units of primary production?
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Mass of carbon per unit area per unit time, usually g m⁻² yr⁻¹.
Which way do arrows point in a food chain?
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From the organism eaten to the organism that eats it, showing the direction of transfer of energy and biomass.
Why does atmospheric CO₂ fall each northern summer?
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Photosynthesis exceeds respiration on the large northern land masses, so there is a net uptake of CO₂.
Written and checked against the IB Biology HL specification · Updated October 2026