Respiration equations: aerobic and anaerobic, in words
The respiration equation most people mean is the one for aerobic respiration: glucose + oxygen → carbon dioxide + water. Glucose is a sugar from food. Oxygen comes from the air you breathe in. Cells break the glucose down to release energy, and carbon dioxide and water are left over. Without oxygen, muscles turn glucose into lactic acid, and yeast turns it into ethanol and carbon dioxide.
Three equations, one purpose
Respiration is a chemical reaction that goes on in every living cell, day and night. It releases energy from glucose. Your body spends that energy on moving, keeping warm and building large molecules from small ones.
There are two kinds. Aerobic respiration uses oxygen. Anaerobic respiration happens without oxygen. AQA prints three word equations for them:
- Aerobic: glucose + oxygen → carbon dioxide + water
- Anaerobic, in muscles: glucose → lactic acid
- Anaerobic, in plant and yeast cells: glucose → ethanol + carbon dioxide
The aerobic one has a full page of its own, with the aerobic respiration equation in symbols and a count of the atoms. The two without oxygen share a page on the anaerobic respiration equation.
Respiration and breathing are different things
Breathing moves air in and out of your lungs. Scientists call that ventilation. Respiration is the reaction inside your cells that uses the oxygen once your blood has carried it there. OCR Gateway lists this as a mix-up that many learners make.
Plants respire too. They do it all the time, alongside photosynthesis. The photosynthesis equation looks like the aerobic one written backwards, which is a handy way to check both.
Where in the cell it happens
OCR Twenty First Century (J257) gives the clearest answer. In animal and plant cells, respiration happens in the cytoplasm and in the mitochondria. In bacteria, which have no mitochondria, it happens in the cytoplasm. You can find the mitochondria on our animal cell diagram.
OCR also names ATP, a molecule that respiration makes. J257 says cells need ATP for jobs such as muscle contraction and building new molecules. AQA and Edexcel talk about energy being transferred or released, without naming ATP.
How the boards set it out
AQA (8461 and 8464) prints all three word equations. For the aerobic one it also lists the formulae C₆H₁₂O₆, O₂, CO₂ and H₂O for you to recognise.
Edexcel (1BI0 and 1SC0) prints no respiration equation. It asks you to compare aerobic and anaerobic respiration in your own words.
OCR Gateway (J247) asks you to recall the word equation for respiration but does not print it. J257 describes the products in sentences: water and carbon dioxide from aerobic respiration, lactic acid in animal cells, and ethanol with carbon dioxide in plants and yeast.
At KS3 you give a word summary for aerobic and for anaerobic respiration. No symbol equation is needed at that stage.
Common mistakes
- Saying respiration means breathing. Breathing is ventilation, and respiration is the reaction in your cells.
- Swapping the anaerobic products. Lactic acid comes from muscle cells, and ethanol with carbon dioxide comes from yeast and plants.
- Leaving oxygen out of the aerobic equation. Aerobic means with oxygen, so it must appear on the left.
- Thinking only animals respire. Plant cells respire all the time as well.
In Respiration Equations you build each word equation from tiles, then say where in the cell it runs. More topics sit on the biology hub.
Questions people ask
Is respiration exothermic or endothermic?
Exothermic. AQA, Edexcel and OCR all describe it as a reaction that gives out energy.
Which type of respiration releases more energy?
Aerobic. In anaerobic respiration the glucose is only partly broken down, so much less energy is transferred.
What is the respiration equation in symbols?
For aerobic respiration the usual balanced form is C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O. No GCSE board prints this line in its specification, so check which form your teacher uses.