Carbon, nitrogen and decay cycles

Living things are made of materials that cycle through the living world and the non-living world. Carbon, nitrogen and water are used by organisms, passed along food chains and returned to the air and soil, so they can be used again. Decomposers such as bacteria and fungi are what keep the cycles going.

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.
  • Decay is fastest when it is warm, moist and there is plenty of oxygen. Gardeners make compost this way; anaerobic decay makes methane, which can be used as biogas.
  • Nitrogen gas makes up most of the air, but plants cannot use it directly. Nitrogen-fixing bacteria (some in the root nodules of legumes such as peas and clover) and lightning turn it into nitrogen compounds.
  • Decomposers turn the proteins and urea in dead matter and waste into ammonium compounds. Nitrifying bacteria turn ammonium compounds into nitrites and then nitrates. Plants take in nitrates through their roots and use them to make proteins.
  • Denitrifying bacteria turn nitrates back into nitrogen gas. They live in waterlogged soils with little oxygen.

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

Describe how the nitrogen in a dead leaf can end up in a protein in a new plant.

  1. Decomposers break down the proteins in the leaf into ammonium compounds.
  2. Nitrifying bacteria turn ammonium compounds into nitrites, then nitrates.
  3. Root hair cells take in the nitrates by active transport.
  4. The plant uses the nitrogen to make amino acids and proteins.

Answer: decomposers make ammonium compounds; nitrifying bacteria make nitrates; plant roots take in the nitrates and make proteins

Example 3

Milk with lipase took 250 s to change the colour of an indicator at 20 °C. Find the rate as 1000 ÷ time.

  1. Lipase breaks down fat in the milk into fatty acids, so the pH falls and the indicator changes colour.
  2. Divide 1000 by the time. 1000 ÷ 250 = 4
  3. A shorter time would mean a faster rate of decay.

Answer: 4 s⁻¹