Carbon cycle

Carbon cycles between the air and living things. Photosynthesis takes carbon dioxide out of the air and plants use the carbon to make glucose. Animals get carbon by eating plants. Respiration by plants, animals and decomposers, and combustion of fuels, put carbon dioxide back into the air. Burning fossil fuels faster than they formed is raising the carbon dioxide in the air.

Key facts

  • Photosynthesis takes carbon dioxide out of the air. Plants use the carbon to make glucose and other carbon compounds.
  • Carbon passes to animals when they eat plants (feeding). Respiration by plants, animals and decomposers returns carbon dioxide to the air. Combustion of fossil fuels and wood releases carbon dioxide too.
  • When plants and animals die, decomposers break them down. They respire, releasing carbon dioxide, and return mineral ions to the soil.

Worked examples

Example 1

Name the process shown by each arrow: carbon dioxide → plants; plants → animals; animals → carbon dioxide; fossil fuels → carbon dioxide.

  1. Plants take carbon dioxide from the air in photosynthesis.
  2. Animals get carbon compounds by eating plants: feeding.
  3. Animals respire, releasing carbon dioxide.
  4. Burning fossil fuels, combustion, releases carbon dioxide.

Answer: photosynthesis; feeding; respiration; combustion

Example 2

What does photosynthesis do to the carbon dioxide in the air?

  1. Photosynthesis takes carbon dioxide out of the air.

Answer: takes carbon dioxide out of the air

Example 3

What does burning wood do to the carbon dioxide in the air?

  1. Burning wood puts carbon dioxide back into the air.

Answer: puts carbon dioxide back into the air

Example 4

Why are decomposers important in the carbon cycle?

  1. They release carbon dioxide back into the air by respiration.

Answer: they release carbon dioxide back into the air by respiration

Example 5

Give two processes that add carbon dioxide to the air and one that removes it. Why has the carbon dioxide in the air risen over the last 200 years?

  1. Respiration by living things, and decay by decomposers, release carbon dioxide. So does combustion (burning).
  2. Photosynthesis by plants and algae takes carbon dioxide out of the air.
  3. Since about 1800 people have burned huge amounts of fossil fuels and cleared forests, so more carbon dioxide is added than is removed.

Answer: added by respiration and combustion (and decay); removed by photosynthesis; it has risen because we burn more fossil fuels and cut down forests

Example 6

What does decay by microorganisms do to the carbon dioxide in the air?

  1. Decay by microorganisms puts carbon dioxide back into the air.

Answer: puts carbon dioxide back into the air