Aerobic and anaerobic respiration
Releasing energy with and without oxygen.
Review these first
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 12.1.1State the equations for aerobic and anaerobic respiration and compare their energy yields.
5 minute read
Aerobic and anaerobic respiration
Respiration is the set of chemical reactions in cells that break down nutrient molecules to release energy. It happens in every living cell, all the time — plants included.
Aerobic respiration
Respiration using oxygen, in the mitochondria. Word: glucose + oxygen → carbon dioxide + water Symbol: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O This releases a large amount of energy per glucose molecule, because glucose is broken down completely.
Anaerobic respiration
Respiration without oxygen, releasing a much smaller amount of energy because glucose is only partly broken down. In muscle: glucose → lactic acid In yeast: glucose → alcohol (ethanol) + carbon dioxide The yeast reaction is the basis of brewing and baking — the carbon dioxide makes bread rise, and the ethanol is the alcohol in beer and wine.
Why energy yield differs
In aerobic respiration glucose is fully oxidised to CO₂ and water, extracting nearly all the chemical energy. In anaerobic respiration the products — lactic acid or ethanol — still contain a great deal of unreleased energy, so far less is available to the cell.
The energy is used for
Muscle contraction, protein synthesis, cell division, active transport, and in mammals and birds, maintaining a constant body temperature.
Think of it like this
Aerobic respiration burns the log completely to ash; anaerobic respiration only chars it. The charred log still holds most of its energy — which is exactly why yeast can leave behind something as energy-rich as alcohol.
Worked examples
Method, step by step
A runner sprints hard for 30 seconds. Explain why lactic acid builds up in their muscles.
- 1During intense exercise, muscles need energy faster than oxygen can be delivered by the blood.
- 2With insufficient oxygen, muscle cells respire anaerobically as well as aerobically.
- 3Anaerobic respiration in muscle converts glucose to lactic acid, which accumulates.
Oxygen cannot be supplied fast enough for the energy demand, so muscles respire anaerobically, converting glucose into lactic acid, which builds up and causes fatigue.
Common misconceptions
- Thinking plants respire only at night. Plants respire constantly; during the day photosynthesis simply produces more oxygen than respiration consumes.
- Writing that anaerobic respiration produces "no" energy. It produces much less, not none.
- Confusing the two anaerobic products — lactic acid in animal muscle, ethanol and carbon dioxide in yeast.
In the exam
- Learn both anaerobic word equations separately. Writing the yeast equation for a muscle question is a common and costly slip.
- If asked to compare, the three comparison points are: oxygen required or not, relative energy released, and different products.