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IB DemystifiedMYP Sciences

Respiration

Every heartbeat, every thought and every step you take is powered by respiration, a chemical reaction happening in every living cell, all the time. It releases energy from glucose, and when oxygen runs short, cells can switch to a quicker but less efficient backup.

Recommended for MYP 3 · About 3 lessons · Criteria A, B, C and D

hard exerciserecovery (rest)0102030405004812time / minuteslactic acid / mmol per L
Figure 1. Lactic acid builds up during hard exercise and is removed during recovery.
On this page
  1. Learning objectives
  2. Before you start
  3. Key vocabulary
  4. Understanding the ideas
  5. Aerobic v anaerobic
  6. Respiration in the real world
  7. Worked examples
  8. Assessment tips
  9. Check your understanding
  10. Practice questions
  11. Investigation
  12. Criterion-linked questions
  13. Challenge questions
  14. Topic check
  15. Review your mistakes
  16. Your progress

Learning objectives

By the end of this topic you should be able to:

  • describe aerobic respiration with word and symbol equations
  • describe anaerobic respiration in muscles and in yeast
  • explain how organisms use the energy released
  • explain the body's response to exercise, including oxygen debt
  • analyse respiration data and design investigations
  • discuss sports and energy drinks and fuel ethanol

Before you start

You will use these skills. If any feel shaky, review them first.

  • photosynthesis and glucose (see Photosynthesis)
  • enzymes and temperature (see Enzymes)
  • breathing and gas exchange (see Breathing and gas exchange)

Key vocabulary

Respiration
A chemical reaction in cells that releases energy from glucose.
Aerobic respiration
Respiration using oxygen, producing carbon dioxide and water.
Anaerobic respiration
Respiration without oxygen, producing lactic acid (animals) or ethanol and carbon dioxide (yeast).
Mitochondria
Organelles where most aerobic respiration takes place.
Oxygen debt
The extra oxygen needed after exercise to break down lactic acid.
Fermentation
Anaerobic respiration in yeast.

Understanding the ideas

  1. Aerobic respiration

    Respiration is a chemical reaction that releases energy from glucose. It happens in every living cell, in plants and animals, day and night. Aerobic respiration uses oxygen: glucose + oxygen → carbon dioxide + water, mostly in the mitochondria.

  2. Anaerobic respiration in muscles

    When muscles work hard and oxygen cannot be delivered fast enough, they also respire anaerobically: glucose → lactic acid. This releases much less energy, and the lactic acid causes fatigue. Afterwards, extra oxygen (the oxygen debt) is needed to break it down.

  3. Fermentation in yeast

    Yeast respires anaerobically too, producing ethanol and carbon dioxide. This fermentation is used to make bread rise and to produce ethanol for fuel and industry.

  4. Using the energy

    The energy from respiration is used for movement, keeping warm, growth and repair, and active transport. Breathing is not respiration: breathing moves air in and out of the lungs to supply the oxygen that respiration uses.

  5. What does it connect to?

    Respiration links to photosynthesis, enzymes, breathing, the circulatory system and the carbon cycle.

Aerobic v anaerobic

  • Aerobic: glucose + oxygen → carbon dioxide + water; lots of energy; C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O.
  • Anaerobic in muscles: glucose → lactic acid; little energy; causes oxygen debt.
  • Anaerobic in yeast (fermentation): glucose → ethanol + carbon dioxide.
  • All living cells respire all the time; plants photosynthesise only in light.
  • Breathing ≠ respiration.

Respiration in the real world

Athletes train to improve their oxygen supply and reduce lactic acid build-up. Bakers use yeast to make bread and naan rise. Sugar mills turn molasses into ethanol by fermentation.

Worked examples

Example 1: percentage change

Heart rate rises from 70 to 140 beats per minute. Calculate the percentage increase.

  1. Increase = 140 − 70 = 70.
  2. 70 ÷ 70 × 100 = 100%.

Example 2: choosing the equation

A sealed tank of yeast and sugar produces a flammable liquid. Which process is happening?

  1. There is no oxygen, so respiration is anaerobic.
  2. Yeast makes ethanol (flammable) and carbon dioxide: fermentation.

Assessment tips

Respiration questions often use equations, exercise data and yeast experiments. Expect to:

  • Write word and symbol equations for aerobic and anaerobic respiration.
  • Explain the effects of exercise and oxygen debt.
  • Analyse data, including percentage changes and anomalies.
  • Design fair and ethical investigations.

Common mistakes: confusing respiration with breathing; saying plants only respire at night; mixing up lactic acid (muscles) with ethanol (yeast); and saying respiration makes energy (it releases energy).

Check your understanding

Quick questions on the ideas above. Try each one before using a hint.

Practice questions

Show

Investigation: do seeds respire?

Partially guided investigation · set up 20 minutes, readings over 3 days · groups

Research question
Do germinating seeds release more heat than boiled (dead) seeds?
Scientific background
Respiration releases energy, some of it as heat. Living, germinating seeds respire quickly; boiled seeds are dead and cannot respire.
Hypothesis
Write your own prediction, with a scientific justification.
Variables
Identify your independent, dependent and control variables, and explain how you will control them.
Apparatus
Two vacuum flasks, soaked peas or beans (germinating), boiled and cooled peas, dilute disinfectant (to kill microbes on the seeds), cotton wool, two thermometers.
Method
  1. Rinse both sets of seeds in dilute disinfectant.
  2. Half fill one flask with germinating seeds and the other with boiled seeds.
  3. Plug each with cotton wool around a thermometer, and turn the flasks upside down.
  4. Record the temperature in each flask twice a day for three days.

Safety. Wear gloves and eye protection when using disinfectant, and wash hands after handling seeds. Handle glass thermometers and flasks carefully.

Then evaluate: why were the seeds washed in disinfectant, and why use boiled seeds as a control?

Criterion-linked questions

Criterion B: inquiring and designing

Criterion C: processing and evaluating

Criterion D: reflecting on the impacts of science

Challenge questions

Harder problems in unfamiliar contexts. Plan before you calculate.

Topic check

Five questions picked at random from the whole topic. Take a new set whenever you like.

Review your mistakes

Questions you got wrong on this device appear here so you can try them again. Answer one correctly and it leaves the list.

Your progress

Tracked separately for each skill, on this device only.

SkillCorrectStatus

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