Biology 9.12 - 9.13 - Material cycling and carbon
Follow conserved matter between living organisms and non-living stores, with carbon cycling through photosynthesis, feeding, respiration, decomposition and combustion.
Materials through ecosystems
An ecosystem contains biotic components, such as plants, animals and microorganisms, and abiotic components, such as air, water and soil. Materials move between these components. The atoms are reused, although they may become part of different substances on the way.
For example, a carbon atom can be part of carbon dioxide in the air, then part of glucose in a leaf, then part of an animal and later carbon dioxide again. It has crossed from an abiotic component into living biomass and back again.
| Material | Examples in abiotic components | Examples in biotic components |
|---|---|---|
| carbon | carbon dioxide in air; carbon compounds in soil or fuels | carbohydrates, fats and proteins in organisms |
| water | oceans, rivers, soil water and water vapour | water inside cells and body fluids |
| nitrogen | nitrogen gas in air; nitrate and ammonium ions in soil | amino acids, proteins and other nitrogen-containing molecules |
Four kinds of movement recur:
- Uptake moves material from an abiotic store into an organism. Roots take up water and nitrate ions; plants take in carbon dioxide for photosynthesis.
- Feeding transfers matter from one organism to another.
- Waste and death move organic material to decomposers and the surroundings.
- Biological or physical processes return material to abiotic stores. Respiration releases carbon dioxide, while evaporation moves liquid water into the atmosphere.
Materials cycle because matter is conserved. Ecosystems do not receive a new supply of carbon or nitrogen each time an organism grows. Recycling makes existing atoms available again. Energy is different: it enters ecosystems, is transferred and eventually dissipates, so it does not follow the same closed material cycle.
When reading a cycle, name the material and store at both ends of an arrow, then identify the process. An arrow labelled respiration shows carbon moving from biomass to carbon dioxide; “respiration” is not a substance flowing around the cycle. Individual ecosystems exchange matter with their surroundings, even though the atoms are conserved.
Carbon and decomposers
Carbon is needed to build the molecules of living organisms. The carbon cycle moves it among atmospheric carbon dioxide, living biomass, dead organic material and longer-term stores such as fossil fuels.
From the atmosphere into food chains
Green plants take in carbon dioxide during photosynthesis. Carbon from carbon dioxide becomes part of glucose and then other carbon-containing compounds in plant biomass. When an animal eats the plant, feeding transfers those compounds to the animal. Further feeding can move the carbon along a food chain.
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