The word equation for aerobic respiration is glucose + oxygen → carbon dioxide + water (+ energy released). The balanced symbol equation is C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (+ energy released). Learn both, because GCSE Biology questions ask for either, and the marks usually go to students who can also say what each substance is doing in the reaction.
This guide gives you both versions, shows you how to check that the symbol equation is balanced, explains why energy appears in it, compares it with anaerobic respiration and gives you memory tricks that work under exam pressure.
TL;DR
- Word equation: glucose + oxygen → carbon dioxide + water (+ energy released).
- Symbol equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (+ energy released).
- Glucose and oxygen are the reactants. Carbon dioxide and water are the products. Energy is transferred, not made.
- Aerobic respiration needs oxygen. Anaerobic respiration does not, and it transfers less energy.
- Never confuse respiration (a reaction in cells) with breathing (moving air in and out of the lungs).
Table of Contents
- The word equation for aerobic respiration
- The balanced symbol equation
- How to check that the equation is balanced
- Why energy is in the equation
- Aerobic and anaerobic respiration compared
- Common mistakes that lose marks
- How to remember the equation
- Worked examples
- Our perspective
- Revise bioenergetics with live teaching
- Frequently asked questions
- Sources
The word equation for aerobic respiration
Here it is in the form most mark schemes expect:
glucose + oxygen → carbon dioxide + water (+ energy released)
BBC Bitesize's AQA GCSE Biology guide states the equation in exactly this form. The AQA GCSE Biology specification represents aerobic respiration as glucose + oxygen → carbon dioxide + water, and treats respiration as an exothermic reaction that transfers energy for living processes. So the energy part is something you should always be ready to include or explain.
The arrow means "react to produce". Everything on the left is used up. Everything on the right is made. Say it that way in an answer and you will rarely go wrong.
Which version should you write?
If the question says "word equation", write the words. If it says "balanced symbol equation" or "chemical equation", write the symbols and the number of molecules. If it just says "write the equation for aerobic respiration", the safest answer is the word equation, with the symbol version added if you have time and are confident. A correct word equation is never marked wrong for lacking symbols when the question did not ask for them.
The balanced symbol equation
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (+ energy released)
The same Bitesize guide gives this as the symbol equation, and the Edexcel version of the Bitesize guide gives the same balanced reaction. AQA lists C₆H₁₂O₆, O₂, CO₂ and H₂O as the chemical symbols students should recognise for respiration, so these four formulae are worth knowing by heart.
What each symbol stands for
| Formula | Substance | Side of the equation | Role |
|---|---|---|---|
| C₆H₁₂O₆ | Glucose | Reactant | The fuel, oxidised to release energy |
| O₂ | Oxygen | Reactant | Needed to oxidise the glucose completely |
| CO₂ | Carbon dioxide | Product | Waste gas, removed from the body |
| H₂O | Water | Product | Product of the reaction |
| Energy | Energy transferred | Product | Used for living processes |
Bitesize describes glucose as the respiratory substrate that is oxidised to release energy, with carbon dioxide and water as the products.
How to check that the equation is balanced
A balanced equation has the same number of each type of atom on both sides. This is a check you can do in thirty seconds in an exam, and it is the best way to stop yourself writing 6O₂ as just O₂ or forgetting a 6 on the water.
Left-hand side
- Carbon: glucose has 6 carbon atoms, so 6.
- Hydrogen: glucose has 12 hydrogen atoms, so 12.
- Oxygen: glucose has 6 oxygen atoms, and 6O₂ is 6 × 2 = 12 more. Total 18.
Right-hand side
- Carbon: 6CO₂ gives 6 carbon atoms.
- Hydrogen: 6H₂O gives 6 × 2 = 12 hydrogen atoms.
- Oxygen: 6CO₂ gives 6 × 2 = 12, and 6H₂O gives 6 × 1 = 6. Total 18.
Carbon 6 and 6, hydrogen 12 and 12, oxygen 18 and 18. Balanced.
Balancing it from scratch
If you ever forget the numbers, you can rebuild them in four steps, starting from the unbalanced skeleton C₆H₁₂O₆ + O₂ → CO₂ + H₂O.
- Balance carbon first. Glucose has 6 carbons, so you need 6CO₂.
- Balance hydrogen next. Glucose has 12 hydrogens, so you need 6H₂O (6 × 2 = 12).
- Count the oxygen on the right: 12 from the carbon dioxide plus 6 from the water is 18.
- Take away the 6 oxygen atoms already in glucose. That leaves 12 oxygen atoms to come from O₂, which means 6O₂.
This order, carbon then hydrogen then oxygen, works for many combustion and respiration equations, because oxygen appears in several places and is best left until last.
Why energy is in the equation
Respiration is not the same as breathing. The WJEC guide on Bitesize describes respiration as chemical reactions in cells, while the respiratory system is what exchanges the gases involved. Your lungs bring the oxygen in and take the carbon dioxide out. The reaction itself happens in your cells.
AQA says the energy supplied by respiration is used for building larger molecules from smaller ones, for movement and for keeping warm. That is why you will sometimes see the exam question "Why do mammals need energy?" attached to this topic. Link the answer to those three uses and you have the marks.
Two wording points matter here:
- Energy is transferred, not "made" or "created". Energy cannot be created. Glucose stores chemical energy, and respiration moves it to forms the cell can use, and some goes to the surroundings as heat.
- Respiration is exothermic. It transfers energy to the surroundings, which is why you feel warm after exercise.
Some courses present the summary equation with the energy shown as ATP, the molecule cells use to carry energy. The WJEC guide gives the summary as glucose + oxygen → carbon dioxide + water + energy in the form of ATP. If your board is AQA or Edexcel, "energy released" is the standard wording, but knowing ATP is the carrier helps if a longer question asks where the energy goes.
Aerobic and anaerobic respiration compared
Examiners love asking you to compare the two. The AQA specification sets out the difference: aerobic respiration uses oxygen, while anaerobic respiration does not. Because glucose is only incompletely oxidised in anaerobic respiration, it transfers less energy.
| Feature | Aerobic respiration | Anaerobic respiration in muscles | Anaerobic respiration in plants and yeast |
|---|---|---|---|
| Oxygen needed? | Yes | No | No |
| Word equation | glucose + oxygen → carbon dioxide + water | glucose → lactic acid | glucose → ethanol + carbon dioxide |
| Energy transferred | Relatively large amount | Less | Less |
| Glucose oxidation | Complete | Incomplete | Incomplete |
Two things to notice. First, the anaerobic equations have no oxygen on the left. Second, only the aerobic equation has water as a product. If a question asks you to spot which equation is which, check the left side for oxygen first.
The yeast version is also called fermentation, and it is the reaction behind bread rising and brewing. Muscles switch to the lactic acid version during hard exercise, when the blood cannot deliver oxygen fast enough. It is a common 6-mark link between this topic and the exercise topic, so make sure you can write both equations.
Common mistakes that lose marks
Writing "energy" on the wrong side. Energy is released, so it belongs on the right with the products. Putting it on the left makes it look like a reactant.
Saying respiration "makes energy". Use "releases energy" or "transfers energy". Examiners are strict on this.
Mixing up respiration and photosynthesis. The two equations are mirror images. Photosynthesis takes in carbon dioxide and water, and makes glucose and oxygen. Respiration takes in glucose and oxygen, and makes carbon dioxide and water. If you have written the left side of one and the right side of the other, you have a muddle.
Forgetting the numbers. C₆H₁₂O₆ + O₂ → CO₂ + H₂O is not balanced. You need 6O₂, 6CO₂ and 6H₂O.
Saying only plants photosynthesise, so only animals respire. All living organisms respire, plants included, all the time. Plants photosynthesise in the light and respire day and night.
Using breathing and respiration as synonyms. Breathing is ventilation of the lungs. Respiration is the chemical reaction in cells.
How to remember the equation
There is nothing to be gained from rote memorising a string of symbols you do not understand. These methods stick because they build the equation from ideas.
1. Remember the logic, then the words
You breathe in oxygen and you eat food. Those are the inputs. You breathe out carbon dioxide and lose water. Those are the outputs. The word equation is just those four items in order, plus energy for movement and warmth. Say it out loud as a sentence: "Glucose and oxygen react to give carbon dioxide and water, and release energy."
2. Use the mirror trick with photosynthesis
Write photosynthesis on one line, then reverse it for respiration. If you know one, you know the other, with the energy source changed (light in photosynthesis, released energy from respiration).
3. Count with the numbers 6, 12, 6
Glucose is C₆H₁₂O₆. The numbers climb 6, 12, 6, and the products are 6CO₂ and 6H₂O. Notice that the "6" appears in glucose, in the oxygen, in the carbon dioxide and in the water. The only number that does not match is the 12 hydrogens, which become 6 waters with two hydrogens each.
4. Rebuild it by balancing
If you cannot recall the numbers, use the four-step method above. Two minutes of practice makes it automatic.
5. Cover, write, check
Cover the equation, write it from memory, check it, and repeat the next day, then a week later. Spaced repetition beats rereading. Add it to a flashcard with the word equation on one side and the symbol equation on the other.
6. Write it in a question, not just on a card
Practise with exam-style questions. Short answers such as "Write the word equation for aerobic respiration" are worth 1 or 2 marks, but the same equation turns up inside longer questions about exercise, the heart rate and breathing rate. The exam questions by topic bank is a good place to find questions in the style of the real papers, and the equation sheets and specification page is useful to keep next to your notes.
Worked examples
Question 1 (2 marks): Write the word equation for aerobic respiration.
Answer: glucose + oxygen → carbon dioxide + water (1 mark for the correct reactants, 1 mark for the correct products). Adding "+ energy released" is good practice.
Question 2 (2 marks): Complete the balanced symbol equation: C₆H₁₂O₆ + ___O₂ → ___CO₂ + 6H₂O
Answer: 6O₂ and 6CO₂. Check by counting oxygen: 6 + 12 = 18 on the left, and 12 + 6 = 18 on the right.
Question 3 (3 marks): Explain why a sprinter's muscles may switch to anaerobic respiration.
Answer: During a sprint, the muscles need energy faster than the blood can supply oxygen. Without enough oxygen, the cells break down glucose without it, producing lactic acid and transferring less energy. The word equation is glucose → lactic acid.
Question 4 (2 marks): State two uses of the energy released by respiration.
Answer: any two from building larger molecules from smaller ones, movement, and keeping warm.
If you can answer all four without looking, you have this part of the specification covered.
Our perspective
Every year we see the same pattern with this equation. Students can recite it, yet lose marks because they cannot say what it means: which side the energy sits on, why oxygen matters, and how it differs from the anaerobic version. Memory alone is not enough at GCSE.
That is why we teach it live. When a teacher writes the equation on screen and asks a student to balance it, then asks why anaerobic respiration releases less energy, misunderstandings surface in the moment and are fixed in the moment. A recorded video or an app that marks an answer right or wrong cannot do that.
We build every lesson and resource around the exam boards students actually sit, AQA and Edexcel, and our content is written by qualified teachers with classroom experience. We pair live teaching with revision notes, quizzes and flashcards, so a student can go from a live lesson on bioenergetics to a quiz on the same topic the same evening. Parents can see how that practice is going through the dashboard.
If we had to pick one habit for any student reading this: stop rereading the equation and start writing it, balancing it and explaining it, until it feels boring. Boring means secure.
the author
Revise bioenergetics with live teaching
If respiration is one of those topics that never quite sticks, regular live lessons can change that. The Complete GCSE Science Membership costs £20 a month and includes 24+ live lessons every week in Biology, Chemistry and Physics for Years 9 to 11, plus a library of revision notes, quizzes and flashcards, past papers and mark schemes, and a revision timetable generator. It covers both Combined and Triple Science on AQA and Edexcel.
You can see how the lessons and tools fit together on how HeadstartScience works, or go straight to the Complete GCSE Science Membership. If you would like to start with something free, the free resources are open to everyone, and the revision blog has more study guides in the same style as this one.
Frequently asked questions
What is the word equation for aerobic respiration?
Glucose + oxygen → carbon dioxide + water (+ energy released). Glucose and oxygen are the reactants on the left, and carbon dioxide and water are the products on the right. Add the energy released if you have the space, as most mark schemes accept it.
What is the balanced symbol equation for aerobic respiration?
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (+ energy released). You can check it by counting atoms: 6 carbon, 12 hydrogen and 18 oxygen on each side. Remember the numbers go in front of the formulae and you never change the small subscript numbers.
Is the aerobic respiration equation the same for AQA and Edexcel?
The reaction is the same on both boards. AQA's specification gives the word equation and expects you to recognise the symbols, and Edexcel's guidance on Bitesize gives the same balanced symbol equation. Always check your own specification for how it wants the energy term written.
What is the difference between aerobic and anaerobic respiration?
Aerobic respiration uses oxygen and transfers a relatively large amount of energy, producing carbon dioxide and water. Anaerobic respiration does not use oxygen and transfers less energy, because glucose is only partly broken down. In muscles it produces lactic acid, and in plants and yeast it produces ethanol and carbon dioxide.
Is respiration the same as breathing?
No. Breathing moves air in and out of the lungs so that gases can be exchanged. Respiration is the series of chemical reactions in cells that release energy from glucose. Breathing supplies the oxygen that aerobic respiration needs.
Sources
- Aerobic and anaerobic respiration, AQA GCSE Biology, BBC Bitesize
- AQA GCSE Biology 8461 specification: Bioenergetics
- Aerobic and anaerobic respiration, Edexcel GCSE, BBC Bitesize
- Aerobic respiration, WJEC GCSE, BBC Bitesize