Aerobic Respiration: The Equation and Where It Happens
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Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Key concept
Aerobic respiration is the chemical reaction that releases energy from glucose using oxygen. The word equation is: glucose + oxygen → carbon dioxide + water. It occurs mainly in the mitochondria of cells and provides the energy for all life processes.
Respiration is not breathing. This distinction is tested repeatedly. Respiration is a chemical reaction inside cells that releases energy from glucose. Breathing (ventilation) is the physical process that brings oxygen to the lungs. Never use the two words interchangeably.
The equations
Word equation: glucose + oxygen → carbon dioxide + water (+ energy released)
Chemical equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
Energy is released during the reaction, not produced. The energy was stored in the chemical bonds of glucose and is transferred during respiration to be used by the cell. This energy is not shown as a product in the balanced equation but is an essential outcome of the reaction.
Where aerobic respiration occurs
Most of the reactions of aerobic respiration take place in the mitochondria. Cells with high energy demands: such as muscle cells, sperm cells, root hair cells and liver cells: contain many mitochondria. The number of mitochondria in a cell is an indicator of how much energy that cell needs.
Aerobic respiration happens in all living cells, all the time: in plants and animals, day and night. It is the primary source of energy for life processes.
What the energy is used for
The energy released by respiration drives all cellular activities:
Muscle contraction for movement
Active transport of ions and molecules against concentration gradients
Synthesis of large molecules: building proteins from amino acids, starch from glucose
Cell division (mitosis and meiosis)
Maintaining body temperature in mammals and birds
Nerve impulse transmission
When an exam question asks "why is respiration important?", name specific uses of the energy rather than just saying "the body needs energy".
Try a focused question (3 marks)
State the word equation for aerobic respiration and describe where it takes place in the cell.
Show the answer and marking guidance
Glucose + oxygen → carbon dioxide + water.
Energy is released during the reaction.
Aerobic respiration takes place mainly in the mitochondria.
Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.
Common mix-ups to avoid
Check these points against your own answer before moving on.
Confusing respiration with breathing: respiration is a chemical reaction in cells.
Writing the photosynthesis equation instead (the same reactants and products but reversed).
Saying energy is produced rather than released or transferred.
Claiming respiration only happens in animals: all living cells respire.