Chapter 8 of 8

All 8 chapters
  1. 1Cell structure in detail, readunderstood
  2. 2Microscopes and size, readunderstood
  3. 3Diffusion, osmosis and active transport, readunderstood
  4. 4Cell division, readunderstood
  5. 5Enzymes, readunderstood
  6. 6Photosynthesis, readunderstood
  7. 7Testing photosynthesis, readunderstood
  8. 8Respiration, readunderstood

S4.1.8 Chapter 8

Respiration

After I run, I keep breathing hard even though I have stopped. Why?

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

  • Compare aerobic and anaerobic respiration in animals, plants and yeast
  • Explain why we breathe hard for a while after exercise

Breathing and respiration are not the same. Breathing moves air in and out of your lungs. Respiration is a reaction inside cells. It releases energy from glucose. Breathing brings the oxygen that most respiration needs.

Every living cell respires, all the time, whether you are running or asleep. A plant cell respires too. A plant cell has chloroplasts for photosynthesis and mitochondria for respiration.

A plant cell with the labels cell wall, chloroplasts and mitochondria, and a key that says the black dots are ribosomes.mitochondriachloroplastscell wallribosome
Chloroplasts make glucose. Mitochondria release the energy from glucose. A leaf cell does both.

Cells use the energy to move, to keep warm, to build large molecules and for active transport. The energy is not made. It is released from the chemical energy stored in glucose.

When there is plenty of oxygen, a cell uses aerobic respiration. Glucose reacts with oxygen. The products are carbon dioxide and water. Most of the energy in glucose is released. This happens mostly in the mitochondria.

The word equation for aerobic respiration. A box labelled glucose + oxygen, then an arrow labelled energy released, then a box labelled carbon dioxide + water.glucose + oxygenenergy releasedcarbon dioxide + water
Photosynthesis goes the other way. It takes in carbon dioxide and water and makes glucose and oxygen.
The symbol equation for aerobic respiration. A box labelled C₆H₁₂O₆ + 6O₂, then an arrow labelled energy released, then a box labelled 6CO₂ + 6H₂O.C₆H₁₂O₆ + 6O₂energy released6CO₂ + 6H₂O
The sides are balanced. Each has 6 carbon, 12 hydrogen and 18 oxygen atoms.

Worked example

How many molecules of oxygen are needed to respire 4 molecules of glucose?

  1. In the equation, 1 molecule of glucose reacts with 6 molecules of oxygen.
  2. 4 × 6 = 24.

Answer: 24 molecules

What if the cell runs short of oxygen? It can still release some energy, with no oxygen. This is anaerobic respiration. The glucose is only partly broken down, so much less energy is released.

In your muscles, anaerobic respiration makes lactic acid. During hard exercise the blood cannot bring oxygen as fast as the muscles use it. The muscles then use both kinds of respiration. Lactic acid builds up, and the muscles tire.

The word equation for anaerobic respiration in muscle. A box labelled glucose, then an arrow labelled no oxygen, then a box labelled lactic acid.glucoseno oxygenlactic acid
Anaerobic respiration in muscles makes lactic acid and releases little energy. No carbon dioxide is made.

Yeast and plant cells do it differently. Without oxygen they make ethanol and carbon dioxide. This is fermentation. A plant root in waterlogged soil, where there is little oxygen, may do this.

The word equation for anaerobic respiration in yeast and plants. A box labelled glucose, then an arrow labelled no oxygen, then a box labelled ethanol + carbon dioxide.glucoseno oxygenethanol + carbon dioxide
Yeast uses this reaction to make bread dough rise. The carbon dioxide forms bubbles in the dough. The ethanol goes off as the bread bakes. The same reaction makes the ethanol in some drinks.
A table with the columns Kind, Oxygen and Energy. Aerobic: yes, much. Anaerobic: no, little.KindOxygenEnergyaerobicyesmuchanaerobicnolittle
Much and little describe the energy released from each glucose. Aerobic respiration releases far more.
A table with the columns Where and Makes. Muscle: lactic acid. Yeast: ethanol and CO₂. Plant: ethanol and CO₂.WhereMakesmusclelactic acidyeastethanol and CO₂plantethanol and CO₂
What anaerobic respiration makes, by where it happens. CO₂ is carbon dioxide.

Now, why do you keep breathing hard after exercise? During hard exercise the muscles built up lactic acid. Oxygen is needed to break down the lactic acid, which the blood carries to the liver. The oxygen needed is called the oxygen debt.

You pay the debt back after you stop. Breathing and the heart stay fast for a while. The extra oxygen refills the blood and muscles. It also starts to break down the lactic acid. The fast breathing removes extra carbon dioxide. Most of the lactic acid is gone within an hour.

A line graph. The x axis is minutes after stopping, from 0 to 5 in steps of 1. The y axis is breaths per minute, from 0 to 40 in steps of 10. The points are at 0 minutes and 34 breaths per minute, 1 and 26, 2 and 20, 3 and 16, 4 and 14, and 5 and 14. The line falls and then goes flat at 14.012345010203040minutes after stoppingbreaths per minute
Breathing is fast just after exercise and falls back to the resting rate of 14 breaths per minute.

Worked example

At rest Ana takes 14 breaths per minute. Just after exercise she takes 34 breaths per minute. What is the percentage increase, to the nearest whole number?

  1. Change = 34 − 14 = 20.
  2. Percentage increase = change ÷ start × 100.
  3. 20 ÷ 14 × 100 = 142.9, which is 143 to the nearest whole number.

Answer: 143%

Worked example

Use the graph. At rest Ana takes 14 breaths per minute. How many extra breaths does she take in the first four readings, at 0, 1, 2 and 3 minutes?

  1. Take the resting rate, 14, from each reading.
  2. At 0 minutes: 34 − 14 = 20. At 1: 26 − 14 = 12. At 2: 20 − 14 = 6. At 3: 16 − 14 = 2.
  3. Add the extras: 20 + 12 + 6 + 2 = 40.
  4. She takes 40 extra breaths while she pays back the oxygen debt.

Answer: 40 extra breaths

Try it

Breathing before and after exercise

You need: a clock with a second hand, or a phone timer, paper and a pencil, a friend to count, or a way to count your own breaths.

Be safe: Do this only if you are well and have no heart or breathing problem. Run gently on flat ground and stop at once if you feel dizzy or have pain. An adult should be near the first time.

  1. Sit still for two minutes. Count your breaths for one minute. This is your resting rate. One breath is one in and one out.
  2. Stand and run on the spot, gently, for one minute.
  3. Sit down at once. Count your breaths for one minute and write the number.
  4. Keep counting one minute at a time and writing each number, until it is back to your resting rate.
  5. Draw a graph of the numbers against the minutes since you stopped.

What you will see: Your breathing is fast just after the run, then it slows in steps back to the resting rate. The graph in the picture is an example. Your numbers will differ.

If you cannot try it, this is what happens.
A line graph. The x axis is minutes after stopping, from 0 to 5 in steps of 1. The y axis is breaths per minute, from 0 to 40 in steps of 10. The points are at 0 minutes and 34 breaths per minute, 1 and 26, 2 and 20, 3 and 16, 4 and 14, and 5 and 14. The line falls and then goes flat at 14.012345010203040minutes after stoppingbreaths per minute

A wrong idea many people have

Plants photosynthesise, so they do not respire.

Why it feels right: We say animals breathe and plants make food, so it seems each does one thing.

The test that breaks it: In the dark a plant gives out carbon dioxide and takes in oxygen, as an animal does. A plant cell has mitochondria, where respiration happens.

What is true: Plants respire all the time, in the light and in the dark. In bright light they photosynthesise faster than they respire, so there is a net gain of oxygen.

Key words

respiration
the reaction in cells that releases energy from glucose.
aerobic respiration
respiration with oxygen. Glucose and oxygen make carbon dioxide and water.
anaerobic respiration
respiration without oxygen. It releases much less energy.
lactic acid
what muscles make in anaerobic respiration.
fermentation
anaerobic respiration in yeast and plants, making ethanol and carbon dioxide.
oxygen debt
the oxygen needed after exercise to break down lactic acid.

Back to your guess

The answer is B: My muscles made lactic acid, and oxygen is needed to break it down.

In hard exercise the muscles run short of oxygen and make lactic acid. Afterwards extra oxygen is needed to break the lactic acid down, so you keep breathing hard.

Check yourself

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

  1. Which statement about respiration is true?

    Tap your answer.

    1. Yes, that is right. All living cells need energy all the time, so all of them respire. This includes plant cells.

    2. Not this one. Breathing moves air in and out of the lungs. Respiration is the reaction in cells that releases energy. Go to S3.3.4. Try another.

    3. Not this one. Cells respire all the time. Exercise only makes them respire faster. Try another.

    See the answer

    It happens in every living cell, all the time.

    All living cells need energy all the time, so all of them respire. This includes plant cells.

    • If you chose B: Breathing moves air in and out of the lungs. Respiration is the reaction in cells that releases energy. Go to S3.3.4.
    • If you chose C: Cells respire all the time. Exercise only makes them respire faster.

  2. Which word equation is aerobic respiration?

    Tap your answer.

    1. Not this one. That is photosynthesis. Respiration goes the other way. Go to S4.1.6. Try another.

    2. Not this one. That is anaerobic respiration in muscle, with no oxygen. Try another.

    3. Yes, that is right. Aerobic respiration uses oxygen to break down glucose. The products are carbon dioxide and water.

    See the answer

    glucose + oxygen → carbon dioxide + water

    Aerobic respiration uses oxygen to break down glucose. The products are carbon dioxide and water.

    • If you chose A: That is photosynthesis. Respiration goes the other way. Go to S4.1.6.
    • If you chose B: That is anaerobic respiration in muscle, with no oxygen.

  3. What do muscles make in anaerobic respiration?

    Tap your answer.

    1. Not this one. Yeast and plants make these. Muscles make lactic acid. Try another.

    2. Yes, that is right. In anaerobic respiration glucose is only partly broken down. In muscles it becomes lactic acid.

    3. Not this one. Those are made by aerobic respiration, which uses oxygen. Try another.

    See the answer

    Lactic acid

    In anaerobic respiration glucose is only partly broken down. In muscles it becomes lactic acid.

    • If you chose A: Yeast and plants make these. Muscles make lactic acid.
    • If you chose C: Those are made by aerobic respiration, which uses oxygen.

  4. Why does bread dough rise?

    Tap your answer.

    1. Not this one. Yeast cannot make oxygen. It makes carbon dioxide. Try another.

    2. Not this one. Lactic acid is made in muscles. It is a liquid, not a gas. Try another.

    3. Not this one. Respiration makes water, but the gas that raises the dough is carbon dioxide. Try another.

    4. Yes, that is right. Yeast ferments the sugar in the dough. This makes ethanol and carbon dioxide. The gas forms bubbles that raise the dough.

    See the answer

    The yeast makes carbon dioxide, which forms bubbles in the dough.

    Yeast ferments the sugar in the dough. This makes ethanol and carbon dioxide. The gas forms bubbles that raise the dough.

    • If you chose A: Yeast cannot make oxygen. It makes carbon dioxide.
    • If you chose B: Lactic acid is made in muscles. It is a liquid, not a gas.
    • If you chose C: Respiration makes water, but the gas that raises the dough is carbon dioxide.

  5. Why do you keep breathing hard for a while after exercise?

    Tap your answer.

    1. Not this one. The lungs are not tired in that way. Your body needs extra oxygen. Try another.

    2. Yes, that is right. The oxygen debt is paid back after exercise. The extra oxygen breaks down the lactic acid, which the blood carries to the liver.

    3. Not this one. The lactic acid was made during the exercise. Afterwards it is broken down. Try another.

    See the answer

    Oxygen is needed to break down the lactic acid that built up in the muscles.

    The oxygen debt is paid back after exercise. The extra oxygen breaks down the lactic acid, which the blood carries to the liver.

    • If you chose A: The lungs are not tired in that way. Your body needs extra oxygen.
    • If you chose C: The lactic acid was made during the exercise. Afterwards it is broken down.

  6. How does the energy released from one glucose compare in aerobic and anaerobic respiration?

    Tap your answer.

    1. Yes, that is right. With oxygen the glucose is broken down fully, and far more energy is released than when it is only partly broken down.

    2. Not this one. Without oxygen the glucose is only partly broken down, so much less energy is released. Try another.

    3. Not this one. Anaerobic respiration can start fast, but it releases much less from each glucose. Try another.

    See the answer

    Aerobic respiration releases much more.

    With oxygen the glucose is broken down fully, and far more energy is released than when it is only partly broken down.

    • If you chose B: Without oxygen the glucose is only partly broken down, so much less energy is released.
    • If you chose C: Anaerobic respiration can start fast, but it releases much less from each glucose.

  7. From an earlier chapter

    Which process in a leaf takes in carbon dioxide and gives out oxygen?

    Tap your answer.

    1. Not this one. Respiration does the opposite. It uses oxygen and gives out carbon dioxide. Try another.

    2. Yes, that is right. In light, photosynthesis uses carbon dioxide and water and makes glucose and oxygen.

    3. Not this one. Osmosis is the movement of water across a membrane. It does not make oxygen. Try another.

    See the answer

    Photosynthesis

    In light, photosynthesis uses carbon dioxide and water and makes glucose and oxygen.

    • If you chose A: Respiration does the opposite. It uses oxygen and gives out carbon dioxide.
    • If you chose C: Osmosis is the movement of water across a membrane. It does not make oxygen.

  8. How many molecules of oxygen are needed to respire 5 molecules of glucose?

    See the answer

    30 molecules

    Each molecule of glucose needs 6 molecules of oxygen. 5 × 6 = 30.

  9. At rest a boy takes 16 breaths per minute. After running he takes 40. What is the percentage increase?

    See the answer

    150%

    40 − 16 = 24. 24 ÷ 16 × 100 = 150.

Keep it

Copy this into your exercise book.

Respiration is in every cell and releases energy from glucose. Aerobic: glucose + oxygen → carbon dioxide + water. Anaerobic: muscles make lactic acid. Yeast and plants make ethanol and carbon dioxide. Breathing hard repays the oxygen debt.

Draw: The three equations for respiration in three boxes, with the amount of energy released written beside each.

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

No internet tomorrow? Save the whole book as one file (1.0 MB). It opens on any phone.