In Photosynthesis and Respiration Together you saw that photosynthesis and respiration run in opposite directions. Follow one carbon atom through an ecosystem and you will see those two processes driving a loop.
Key idea
Carbon moves from the air (as carbon dioxide) into plants by photosynthesis, into animals by eating, and back to the air by respiration, decomposition and burning. Some carbon is stored for a long time in oceans, rock and fossil fuels.
Why it works
Atoms are never used up. When you digest a sandwich, its carbon atoms do not vanish: your cells break the sugar apart in respiration and you breathe the carbon out as carbon dioxide. A plant can take that same carbon dioxide in and build it into a new leaf. Matter cycles; only the energy is spent.
Worked example: Follow an atom
A carbon atom is in the air. Name a path that brings it back to the air through a deer.
- Photosynthesis: a clover plant takes in the carbon dioxide and builds sugar.
- Eating: a deer eats the clover.
- Respiration: the deer's cells break down the sugar and release carbon dioxide, which the deer breathes out.
Answer: Air → clover (photosynthesis) → deer (eating) → air (respiration).
Worked example: The slow route
How can carbon stay out of the air for millions of years?
- Dead plants and animals can be buried before they fully decompose.
- Over millions of years, heat and pressure turn some of them into coal, oil or natural gas (fossil fuels).
- Burning those fuels returns the carbon to the air quickly.
Answer: Burial as fossil fuels (and as rock and in the deep ocean) stores carbon for a very long time.
Common mistake
Students think the carbon in food disappears when it is used, or that respiration makes energy out of nothing. The atoms remain; respiration rearranges them into carbon dioxide and water and releases the energy that was stored in the bonds.