When oxygen is available, cells break glucose down completely to carbon dioxide and water. This aerobic respiration releases far more energy per glucose molecule than respiration without oxygen, which is why active tissues are packed with mitochondria and why we breathe faster when we exercise. The balanced chemical equation is Extended content.
Aerobic respiration is the chemical reactions in cells that use oxygen to break down nutrient molecules to release energy. Word equation: glucose + oxygen → carbon dioxide + water. Extended: the balanced chemical equation is C6H12O6 + 6O2 → 6CO2 + 6H2O.
Fig. 1: glucose and oxygen are used in the mitochondrion; carbon dioxide and water are released, with a large amount of energy
Section 01
Glucose plus oxygen.
In the Respiration tab choose aerobic. A glucose molecule (orange hexagon) and six oxygen molecules (blue) arrive at the mitochondrion; carbon dioxide and water molecules leave, and a shower of yellow sparks shows the energy released.
Definition: aerobic respiration is the chemical reactions in cells that use oxygen to break down nutrient molecules to release energy.
Word equation: glucose + oxygen → carbon dioxide + water.
Where: most of the reactions happen in the mitochondria, so very active cells (muscle, sperm, root hair cells) have many mitochondria.
Energy: glucose is broken down completely, so the sim's "Energy per glucose" card reads a lot (about 19 times more) than anaerobic respiration.
Other fuels: fats and proteins can also be respired, but exam equations use glucose.
Examiner trap
Oxygen is a reactant, not a product, and water is a product. Students often swap them when writing the word equation in a hurry. Plants respire aerobically all the time too, in light and in darkness.
Section 02
Balancing the equation.
Extended. Switch the sim to balanced equation (Extended). Each glucose molecule needs six oxygen molecules and gives six carbon dioxide and six water molecules.
Atom
Left: C6H12O6 + 6O2
Right: 6CO2 + 6H2O
Carbon
6
6
Hydrogen
12
12
Oxygen
6 + 12 = 18
12 + 6 = 18
Ratio: 1 glucose : 6 oxygen : 6 carbon dioxide : 6 water. The number of oxygen molecules used equals the number of carbon dioxide molecules released.
Link to photosynthesis: the balanced equation for photosynthesis is the same equation written the other way round, with light energy and chlorophyll.
Examiner trap
Write the formulae exactly: C6H12O6, O2, CO2, H2O. "6O" or "H2O6" loses the mark, and energy is not a substance, so it is not written as a product in the balanced equation.
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Quick check+10 XP
In the Respiration tab, which organelle appears only when aerobic is chosen?
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Which molecules arrive at the mitochondrion with glucose in the aerobic animation?
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What does the "Energy per glucose" card show for aerobic respiration?
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In the Exercise tab, what happens to breathing when you change from resting to sprinting?
Section 03
More work, more respiration.
Working muscles respire faster, so they need more oxygen and make more carbon dioxide. In the Exercise tab pick walking, jogging or sprinting and watch the breathing rate and depth rise.
Rest to sprinting: in the sim the rate goes from about 12 to 40 breaths per minute and each breath from about 0.5 to 2.6 dm3.
Why: faster aerobic respiration in muscle cells uses more oxygen and releases more carbon dioxide, so more air must be exchanged.
Heart: the heart rate rises too, delivering more glucose and oxygen to the muscles and removing carbon dioxide faster.
Stage 2 · Exam
Exam-style questions
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
Finish the four checks above to unlock the exam questions
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Which is the word equation for aerobic respiration?
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Extended: which is the balanced chemical equation for aerobic respiration?
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Which description of aerobic respiration is correct?
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Extended: a cell respires 2 molecules of glucose aerobically. How many molecules of oxygen does it use?
Skill unlocked
Aerobic respiration, mastered.
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