Chapter 5 of 8

All 8 chapters
  1. 1Natural selection, readunderstood
  2. 2Evidence for evolution, readunderstood
  3. 3New species and extinction, readunderstood
  4. 4Classification, readunderstood
  5. 5The carbon and nitrogen cycles, readunderstood
  6. 6Measuring populations, readunderstood
  7. 7Interdependence, readunderstood
  8. 8Biodiversity and people, readunderstood

S4.3.5 Chapter 5

The carbon and nitrogen cycles

The carbon in your body was once in the air. How did it get from there into you, and where will it go next?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Draw the carbon cycle with photosynthesis, respiration, decay and burning
  • Explain how bacteria in the nitrogen cycle help plants grow

The atoms in a living thing are not made new, and they are not used up. They are used again and again. The path of an element round the living and non living world is a cycle. This chapter follows two elements, carbon and nitrogen.

First, carbon. The air holds a very small part of carbon dioxide, about 0.04%. Plants and algae take carbon dioxide in by photosynthesis. They use the carbon to make glucose and, from it, the other substances in their bodies. Carbon moves from the air into living things.

Animals get carbon by eating plants, or by eating animals that ate plants. So carbon passes along a food chain. Every living thing also respires. Plants, animals and decomposers all give out carbon dioxide by respiration. Decomposers are the bacteria and fungi that break down dead things and waste.

Draw it first

Follow the carbon

  1. Boxes joined by arrows, from top to bottom. The strip grows by one box in each picture. The boxes read: carbon dioxide in the air; carbon in plants; carbon in animals; carbon in dead things and waste; carbon dioxide back in the air. The words on the arrows read: photosynthesis; eating; death and waste; decay.carbon dioxide in the airphotosynthesiscarbon in plants

    1 Plants take carbon dioxide from the air by photosynthesis. Now the carbon is in the plants.

  2. Boxes joined by arrows, from top to bottom. The strip grows by one box in each picture. The boxes read: carbon dioxide in the air; carbon in plants; carbon in animals; carbon in dead things and waste; carbon dioxide back in the air. The words on the arrows read: photosynthesis; eating; death and waste; decay.carbon dioxide in the airphotosynthesiscarbon in plantseatingcarbon in animals

    2 An animal eats the plants. Now the carbon is in the animal.

  3. Boxes joined by arrows, from top to bottom. The strip grows by one box in each picture. The boxes read: carbon dioxide in the air; carbon in plants; carbon in animals; carbon in dead things and waste; carbon dioxide back in the air. The words on the arrows read: photosynthesis; eating; death and waste; decay.carbon dioxide in the airphotosynthesiscarbon in plantseatingcarbon in animalsdeath and wastecarbon in dead things andwaste

    3 The animal dies and leaves waste. Now the carbon is in dead things and waste.

  4. Boxes joined by arrows, from top to bottom. The strip grows by one box in each picture. The boxes read: carbon dioxide in the air; carbon in plants; carbon in animals; carbon in dead things and waste; carbon dioxide back in the air. The words on the arrows read: photosynthesis; eating; death and waste; decay.carbon dioxide in the airphotosynthesiscarbon in plantseatingcarbon in animalsdeath and wastecarbon in dead things andwastedecaycarbon dioxide back in theair

    4 Decomposers break them down and respire. They give carbon dioxide back to the air.

Four boxes joined by arrows in a ring. The boxes read: carbon dioxide in air; carbon in plants; carbon in animals; carbon in dead animals. The words on the arrows, going round, are: photosynthesis; eating; dying; decay.carbondioxidein aircarboninplantscarboninanimalscarbonin deadanimalsphotosynthesiseatingdyingdecay
The main path of carbon through living things. Dead plants and animal waste decay in the same way. Respiration is drawn in the next ring.

Respiration is a second way back to the air. An animal gives out carbon dioxide when it respires. Plants and decomposers respire too, in the same way.

Three boxes joined by arrows in a ring. The boxes read: carbon in plants; carbon in animals; carbon dioxide in air. The words on the arrows, going round, are: eating; respiration; photosynthesis.carboninplantscarboninanimalscarbondioxidein aireatingrespirationphotosynthesis
Carbon dioxide goes into plants by photosynthesis. Respiration by animals gives it back to the air.

Carbon dioxide also goes back to the air when things burn. Wood burns, and so do fuels. A fuel such as coal, oil or gas is a fossil fuel. Fossil fuels formed from the remains of dead plants and tiny sea living things called plankton. These were buried. Over millions of years, heat and pressure changed them. So fossil fuels are stored carbon.

Four boxes joined by arrows, from top to bottom. The boxes read: Dead plants and tiny sea living things are buried; Heat and pressure over millions of years change them; They become coal, oil and gas; Burning the fuel puts the carbon back in the air.Dead plants and tiny sealiving things are buriedHeat and pressure overmillions of years change themThey become coal, oil and gasBurning the fuel puts thecarbon back in the air
Fossil fuels took millions of years to form. Burning them returns their carbon to the air much faster.
Four boxes joined by arrows in a ring. The boxes read: carbon in plants; carbon in dead plants; carbon in fossil fuels; carbon dioxide in air. The words on the arrows, going round, are: death; buried; burning; photosynthesis.carboninplantscarbonin deadplantscarboninfossilfuelscarbondioxidein airdeathburiedburningphotosynthesis
Burning fossil fuels adds carbon dioxide to the air. The carbon had been stored for millions of years.
A table with the columns Process and CO₂, which stands for carbon dioxide. photosynthesis: taken in. respiration: given out. decay: given out. burning: given out.ProcessCO₂photosynthesistaken inrespirationgiven outdecaygiven outburninggiven out
What each process does to the carbon dioxide (CO₂) in the air.

Carbon also moves through the sea and into rocks. This chapter follows the main path, through the air and living things. When the amount taken in equals the amount given out, the amount in the air stays steady.

A bar chart of carbon in one year in a small wood, in units. The axis is labelled units of carbon, with a scale from 0 to 120 in steps of 20. The bar for taken in reaches 120. The bar for given out reaches 85.020406080100120taken ingiven outunits of carbon
Example numbers, in made up units, for one year in a small wood.

Worked example

In one year a small wood takes in 120 units of carbon by photosynthesis. It gives out 85 units by respiration. How many units of carbon are added to the wood?

  1. The wood takes in 120 units and gives out 85 units.
  2. What stays in the wood is what is taken in, minus what is given out.
  3. 120 − 85 = 35 units.

Answer: 35 units

Worked example

A forest takes in 120 units of carbon in a year. Plant respiration gives out 50 units, decay gives out 40 units and a fire gives out 10 units. How many units of carbon are added to the forest?

  1. Add up what is given out: 50 + 40 + 10 = 100 units.
  2. The forest takes in 120 units.
  3. 120 − 100 = 20 units are added to the forest.

Answer: 20 units

Now nitrogen. Plants need nitrogen to make proteins. About 78% of the air is nitrogen gas. But plants cannot use nitrogen gas. Their roots take in nitrates, which are salts that dissolve in soil water. Animals get nitrogen by eating proteins in plants and in other animals.

Some bacteria can change nitrogen gas into compounds that plants can use. These are nitrogen fixing bacteria. Some live free in the soil. Others live in lumps called root nodules on the roots of plants such as beans and peas. Lightning also fixes a little nitrogen.

Draw it first

Nitrogen from the air to an animal

  1. Boxes joined by arrows, from top to bottom. The strip grows by one box in each picture. The boxes read: nitrogen gas in the air; nitrogen compounds in the soil; proteins in plants; proteins in animals. The words on the arrows read: fixing; roots take in; eating.nitrogen gas in the airfixingnitrogen compounds in thesoil

    1 Fixing bacteria in the soil and in root nodules turn nitrogen gas into nitrogen compounds such as ammonium. Lightning makes a little nitrate.

  2. Boxes joined by arrows, from top to bottom. The strip grows by one box in each picture. The boxes read: nitrogen gas in the air; nitrogen compounds in the soil; proteins in plants; proteins in animals. The words on the arrows read: fixing; roots take in; eating.nitrogen gas in the airfixingnitrogen compounds in thesoilroots take inproteins in plants

    2 The roots of a plant take in nitrogen compounds, mostly nitrates. The plant uses them to make proteins.

  3. Boxes joined by arrows, from top to bottom. The strip grows by one box in each picture. The boxes read: nitrogen gas in the air; nitrogen compounds in the soil; proteins in plants; proteins in animals. The words on the arrows read: fixing; roots take in; eating.nitrogen gas in the airfixingnitrogen compounds in thesoilroots take inproteins in plantseatingproteins in animals

    3 An animal eats the plant and takes in the proteins.

When living things die, and when animals pass waste, the nitrogen in their proteins goes back to the soil. Decomposers break down the proteins and make ammonium. Then nitrifying bacteria turn the ammonium into nitrates. The nitrates can be taken in by roots again.

Four boxes joined by arrows in a ring. The boxes read: nitrates in soil; protein in living things; dead things; ammonium. The words on the arrows, going round, are: taken in; dying; decomposers; nitrifying.nitratesin soilproteininlivingthingsdeadthingsammoniumtaken indyingdecomposersnitrifying
The nitrogen cycle through the soil and living things. Roots take in the nitrates. Animals get protein by eating. Animal waste also puts nitrogen back in the soil. The arrow marked nitrifying is the work of nitrifying bacteria.

One more group closes the cycle. In waterlogged soil there is little oxygen. There, denitrifying bacteria turn nitrates into nitrogen gas, which goes back to the air. So nitrogen leaves the soil as well as arriving.

Two boxes joined by an arrow, from top to bottom. The top box reads nitrates in waterlogged soil. The bottom box reads nitrogen gas back in the air. The word on the arrow is denitrifying bacteria.nitrates inwaterlogged soildenitrifying bacterianitrogen gas backin the air
Denitrifying bacteria return nitrogen to the air.

Farmers use this knowledge. When a crop is taken away, nitrogen goes with it. A farmer can add fertiliser that holds nitrates. Or the farmer can grow beans or peas in a field. Their root nodules fix nitrogen, so the soil gains nitrogen compounds for the next crop.

Try it

Follow one carbon atom

You need: a sheet of paper, a pencil.

  1. Start with one carbon atom in carbon dioxide in the air.
  2. Write a short story. The atom is taken in by a leaf, and then the leaf is eaten by a goat.
  3. Say what happens when the goat dies, and where the atom goes at the end.
  4. Draw your story as a ring of boxes, with an arrow from each box to the next.

What you will see: The atom ends up as carbon dioxide in the air again, ready to start another journey. The ring has no end. Try a different route. The atom could also leave the goat as carbon dioxide each time the goat breathes out.

If you cannot try it, this is what happens.
Four boxes joined by arrows in a ring. The boxes read: carbon dioxide in air; carbon in a leaf; carbon in a goat; carbon in a dead goat. The words on the arrows, going round, are: photosynthesis; eating; dying; decay.carbondioxidein aircarbon ina leafcarbon ina goatcarbon ina deadgoatphotosynthesiseatingdyingdecay

A wrong idea many people have

Plants take in nitrogen from the air, because the air is 78% nitrogen.

Why it feels right: Plants take in carbon dioxide from the air, so it seems that they would take in nitrogen too.

The test that breaks it: Farmers add nitrates to their fields to help crops grow. If plants could use the nitrogen gas in the air, there would be no need.

What is true: Plants cannot use nitrogen gas themselves. Their roots take in nitrates from the soil. Only some bacteria can change nitrogen gas into forms that plants can use.

Key words

cycle
the path of an element round living things and the non living world, so that it is used again and again.
decomposers
bacteria and fungi that break down dead things and waste.
fossil fuel
coal, oil or gas, formed over millions of years from the buried remains of living things.
nitrates
salts of nitrogen that dissolve in soil water. Plant roots take them in.
nitrogen fixing bacteria
bacteria that change nitrogen gas into compounds that plants can use.
nitrifying bacteria
bacteria that turn ammonium into nitrates.
denitrifying bacteria
bacteria that turn nitrates into nitrogen gas, in waterlogged soil.

Back to your guess

The answer is C: Plants take carbon dioxide from the air to make food, and you get carbon by eating plants or animals.

Plants take carbon from carbon dioxide in the air and build it into food. You get carbon by eating. When you breathe out, you give carbon dioxide back to the air.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. Which process takes carbon dioxide out of the air?

    Tap your answer.

    1. Not this one. Respiration gives out carbon dioxide. It is the opposite of photosynthesis. Try another.

    2. Not this one. Burning gives out carbon dioxide. It does not take it in. Try another.

    3. Yes, that is right. Plants and algae take in carbon dioxide by photosynthesis and use its carbon to make glucose.

    See the answer

    Photosynthesis

    Plants and algae take in carbon dioxide by photosynthesis and use its carbon to make glucose.

    • If you chose A: Respiration gives out carbon dioxide. It is the opposite of photosynthesis.
    • If you chose B: Burning gives out carbon dioxide. It does not take it in.

  2. Which list has only processes that put carbon dioxide into the air?

    Tap your answer.

    1. Not this one. Photosynthesis takes carbon dioxide in. Decay and burning give it out. Try another.

    2. Yes, that is right. Living things respire and give out carbon dioxide. Decomposers give it out as they break down dead things. Burning gives it out too.

    3. Not this one. Photosynthesis takes carbon dioxide in. Respiration and decay give it out. Try another.

    See the answer

    Respiration, decay and burning

    Living things respire and give out carbon dioxide. Decomposers give it out as they break down dead things. Burning gives it out too.

    • If you chose A: Photosynthesis takes carbon dioxide in. Decay and burning give it out.
    • If you chose C: Photosynthesis takes carbon dioxide in. Respiration and decay give it out.

  3. Which statement about plants is true?

    Tap your answer.

    1. Not this one. Plants respire all the time, day and night. Photosynthesis needs light, so it stops in the dark. Try another.

    2. Not this one. Plants take in carbon dioxide by photosynthesis. But they also respire, and respiration gives out carbon dioxide. Try another.

    3. Yes, that is right. Every living cell respires. A plant photosynthesises only when there is enough light.

    See the answer

    Plants respire all the time, and photosynthesise when there is light.

    Every living cell respires. A plant photosynthesises only when there is enough light.

    • If you chose A: Plants respire all the time, day and night. Photosynthesis needs light, so it stops in the dark.
    • If you chose B: Plants take in carbon dioxide by photosynthesis. But they also respire, and respiration gives out carbon dioxide.

  4. Why does burning fossil fuels change the carbon dioxide in the air so quickly?

    Tap your answer.

    1. Yes, that is right. Fossil fuels hold carbon from living things of long ago. Burning them returns that carbon to the air as carbon dioxide.

    2. Not this one. Atoms are not made when something burns. The carbon was already in the fuel, and burning joins it to oxygen. Try another.

    3. Not this one. Burning a fuel also gives out carbon dioxide. It takes in oxygen and gives out carbon dioxide. Try another.

    See the answer

    The carbon was stored for millions of years, and burning puts it into the air in a short time.

    Fossil fuels hold carbon from living things of long ago. Burning them returns that carbon to the air as carbon dioxide.

    • If you chose B: Atoms are not made when something burns. The carbon was already in the fuel, and burning joins it to oxygen.
    • If you chose C: Burning a fuel also gives out carbon dioxide. It takes in oxygen and gives out carbon dioxide.

  5. How do most plants get the nitrogen they need to make proteins?

    Tap your answer.

    1. Not this one. Plants cannot use nitrogen gas themselves. Only some bacteria can change it into a form that plants use. Try another.

    2. Yes, that is right. Nitrates dissolve in water in the soil. Roots take them in, and the plant builds the nitrogen into proteins.

    3. Not this one. An element cannot be made from other elements in this way. Plants must take in nitrogen compounds. Try another.

    See the answer

    Their roots take in nitrates that dissolve in soil water.

    Nitrates dissolve in water in the soil. Roots take them in, and the plant builds the nitrogen into proteins.

    • If you chose A: Plants cannot use nitrogen gas themselves. Only some bacteria can change it into a form that plants use.
    • If you chose C: An element cannot be made from other elements in this way. Plants must take in nitrogen compounds.

  6. Which bacteria turn ammonium in the soil into nitrates?

    Tap your answer.

    1. Yes, that is right. Decomposers make ammonium from dead things and waste. Nitrifying bacteria then turn it into nitrates, which roots can take in.

    2. Not this one. Denitrifying bacteria do the opposite kind of job. They turn nitrates into nitrogen gas. Try another.

    3. Not this one. Fixing bacteria change nitrogen gas into compounds. They do not make nitrates from ammonium. Try another.

    See the answer

    Nitrifying bacteria

    Decomposers make ammonium from dead things and waste. Nitrifying bacteria then turn it into nitrates, which roots can take in.

    • If you chose B: Denitrifying bacteria do the opposite kind of job. They turn nitrates into nitrogen gas.
    • If you chose C: Fixing bacteria change nitrogen gas into compounds. They do not make nitrates from ammonium.

  7. From an earlier chapter

    Resistant bacteria survive an antibiotic and divide. Which step of natural selection is the survival of the resistant bacteria?

    Tap your answer.

    1. Yes, that is right. The antibiotic is the pressure. The resistant bacteria survive and divide, so they pass on their alleles.

    2. Not this one. If every one survived, nothing would be selected. The antibiotic kills most of the bacteria. Go to S4.3.1. Try another.

    3. Not this one. Bacteria do not change on purpose. The resistant ones already had the allele. Go to S4.3.1. Try another.

    See the answer

    Those best suited to the conditions survive and breed more.

    The antibiotic is the pressure. The resistant bacteria survive and divide, so they pass on their alleles.

    • If you chose B: If every one survived, nothing would be selected. The antibiotic kills most of the bacteria. Go to S4.3.1.
    • If you chose C: Bacteria do not change on purpose. The resistant ones already had the allele. Go to S4.3.1.

  8. Air is about 78% nitrogen gas by volume. How many litres of nitrogen gas are in 200 litres of air?

    See the answer

    156 litres

    78% of 200 is 0.78 × 200 = 156 litres.

  9. In one year a wood takes in 140 units of carbon by photosynthesis and gives out 95 units. How many units of carbon are added to the wood?

    See the answer

    45 units

    What stays is what is taken in minus what is given out. 140 − 95 = 45 units.

Keep it

Copy this into your exercise book.

Photosynthesis takes carbon dioxide in. Respiration, decay and burning give it out. Roots take in nitrates. Fixing bacteria add nitrogen compounds, nitrifying bacteria make nitrates, and denitrifying bacteria return nitrogen gas to the air.

Draw: A ring of boxes for the carbon cycle, with arrows for photosynthesis, respiration, decay and burning.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

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