9.1 Aerobic Respiration

A free GCSE Biology lesson for AQA (8461) from OBA School: what respiration is, the equations for aerobic respiration, mitochondria and how organisms use the energy transferred. 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 living cells
• Write the word and balanced symbol equations for aerobic respiration
• Explain why cells that need a lot of energy have many mitochondria
• Describe how organisms use the energy transferred by respiration
• Interpret experiments that show respiration releasing heat and carbon dioxide

Key words

• Respiration: A series of chemical reactions in cells that transfer energy from glucose. It is not the same as breathing.
• Aerobic: Using oxygen. Aerobic respiration needs oxygen.
• Exothermic: A reaction that transfers energy to the surroundings (often as heat).
• Mitochondria: Organelles in the cytoplasm where aerobic respiration takes place. Singular: mitochondrion.
• Metabolism: The sum of all the reactions in a cell or the body.

Respiration in cells

Cellular respiration is an exothermic reaction that happens continuously in every living cell, in plants, animals and microorganisms. It transfers the energy that cells need for living processes.

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

Key fact: Breathing moves air into and out of the lungs. Respiration is a chemical reaction in 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 (in animals) or from photosynthesis (in plants). Oxygen comes from the air, through the lungs and blood in mammals. The carbon dioxide and water are waste products.

Worked example: using the balanced equation

A muscle cell respires 40 molecules of glucose aerobically. How many molecules of oxygen does it use, and how many molecules of carbon dioxide does it make?
The equation shows 1 glucose : 6 O₂ : 6 CO₂.
Oxygen = 40 × 6 = 240. Carbon dioxide = 40 × 6 = 240.

Answer: 240 oxygen molecules used and 240 carbon dioxide molecules made

Mitochondria

Most of the reactions of aerobic respiration take place in the mitochondria. The inner membrane is folded, giving a large surface area for the enzymes that control respiration.

Cells that transfer a lot of energy have many mitochondria: for example muscle cells, sperm cells (to swim) and root hair cells (for active transport).

Using the energy, and showing respiration happens

• 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; beating cilia; moving chromosomes in cell division
• Keeping warm; Examples Birds and mammals keep a steady body temperature
• Active transport; Examples Root hair cells taking in mineral ions

Two simple experiments show that living organisms respire:

• Heat: germinating peas in a vacuum flask get warmer. Boiled (dead) peas washed in disinfectant stay at the same temperature. The disinfectant kills microorganisms, which would otherwise respire too.
• Carbon dioxide: living organisms in a sealed tube turn limewater cloudy, or turn hydrogencarbonate indicator from red to yellow.

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.
• 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: glucose + oxygen → carbon dioxide + water. The reactants are glucose and oxygen.

Question 2. True or false? Plants only respire at night.

Answer: False

Working: False. All living cells respire all the time. In daylight, plants photosynthesise as well as respire.

Question 3. Name the organelles where aerobic respiration takes place.

Answer: mitochondria

Working: Most of the reactions of aerobic respiration happen in the mitochondria.

Related lessons

• Previous lesson: 8.5 Required Practicals: Photosynthesis
• Next lesson: 9.2 The Response to Exercise
• In this chapter: 9.3 Anaerobic Respiration
• In this chapter: 9.4 Metabolism and the Liver

Continue in the full course

This lesson is section 9.1 (AGCBH0901) of our free GCSE Biology 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 Biology course.