top of page

B9.1 Aerobic Respiration

GCSE Combined Science · AQA · Chapter B9: Respiration

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: what respiration is, the equations for aerobic respiration, mitochondria, energy flow and experiments that show organisms respire. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Describe cellular respiration as an exothermic reaction that happens continuously in all living cells
  • Write the word and balanced symbol equations for aerobic respiration
  • Explain why cells that need a lot of energy have many mitochondria
  • Compare respiration with photosynthesis, including the flow of energy
  • Interpret experiments that show respiration releasing heat and using oxygen

Key words

  • Respiration: A chemical reaction in cells that transfers energy from glucose. It is not the same as breathing (ventilation).
  • Aerobic: In the presence of oxygen. Aerobic respiration needs oxygen.
  • Oxidation: A reaction in which a substance combines with oxygen. Aerobic respiration is an oxidation reaction.
  • Exothermic: A reaction that transfers energy to the surroundings (often as heat).
  • Mitochondria: Organelles in the cytoplasm where aerobic respiration takes place. Singular: mitochondrion.

Respiration in cells

Cellular respiration is an exothermic reaction that happens continuously in every living cell, day and night, in plants, animals and microorganisms. It transfers the energy that the organism needs for living processes. If a cell stops respiring, it dies.

Respiration can be aerobic (using oxygen) or anaerobic (without oxygen, see B9.3). Aerobic respiration transfers much more energy from each glucose molecule.

Key fact: Breathing (ventilation) moves air into and out of the lungs. Respiration is a chemical reaction inside cells. Do not confuse them.

The equations

glucose + oxygen → carbon dioxide + water (energy is transferred to the surroundings)

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

Glucose comes from digested food (animals) or photosynthesis (plants). Oxygen comes from the air. Carbon dioxide and water are waste products: breathe on a cold window and the water vapour condenses.

Worked example: using the balanced equation

A liver cell respires 35 molecules of glucose aerobically. How many molecules of oxygen does it use, and how many molecules of water does it make?
The equation shows 1 glucose : 6 O₂ : 6 CO₂ : 6 H₂O.
Oxygen = 35 × 6 = 210. Water = 35 × 6 = 210.

Answer: 210 oxygen molecules used and 210 water molecules made

Mitochondria and uses of energy

Most aerobic respiration happens in the mitochondria. The folded inner membrane gives a large surface area for the enzymes involved.

Cells that need a lot of energy have many mitochondria, for example muscle cells (contraction) and sperm cells (swimming).

  • Chemical reactions that build larger molecules; Examples Making proteins from amino acids; making starch, glycogen or cellulose from glucose
  • Movement; Examples Muscle contraction in animals
  • Keeping warm; Examples Birds and mammals keep a steady body temperature

Respiration and photosynthesis

The reactants of respiration are the products of photosynthesis, and the other way round. But the energy flows in opposite directions: photosynthesis is endothermic (it takes in energy from light), and respiration is exothermic (it releases energy).

Showing that organisms respire

  • Heat: germinating seeds in a vacuum flask get warmer. Boiled (dead) seeds washed in disinfectant stay at the same temperature. The disinfectant kills microorganisms that would otherwise respire.
  • Oxygen use: in a respirometer, soda lime absorbs the carbon dioxide made, so as the animals use oxygen the gas volume falls and the coloured drop moves towards them.

Common mistakes

  • Saying respiration "makes energy". Energy cannot be made; respiration transfers energy.
  • Saying plants do not respire, or only respire at night. All living cells respire all the time.
  • Mixing up respiration and breathing.
  • Saying "aerobic" means "with air". It means "with oxygen".
  • Writing O instead of O₂, or forgetting to balance: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O.

Revision questions

Try these, then check your answers below each one.

Question 1. What are the reactants of aerobic respiration? (Choose from: Glucose and oxygen, Carbon dioxide and water, Glucose and carbon dioxide, Oxygen and water.)

Answer: Glucose and oxygen

Working: Aerobic respiration: glucose + oxygen → carbon dioxide + water. Glucose and oxygen are the reactants.

Question 2. True or false? Plant cells respire in the light and in the dark.

Answer: True

Working: True. Plants photosynthesise only in the light, but every living cell respires continuously, day and night.

Question 3. The diagram shows an animal cell. Which label points to the structure where most aerobic respiration takes place? (Choose from: A, B, C, D.)

Answer: C

Working: C is a mitochondrion, where most aerobic respiration happens. A is the cell membrane, B the nucleus and D the cytoplasm.

Related lessons

Continue in the full course

This lesson is section B9.1 (AGCSC0901) of our free GCSE Combined Science course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Combined Science course.

 

 

 

 

 

 

 

bottom of page