Anaerobic respiration equations: lactic acid, or ethanol and carbon dioxide
There are two anaerobic respiration equations, one for animals and one for plants and yeast. In muscle cells: glucose → lactic acid. In plant and yeast cells: glucose → ethanol + carbon dioxide. Anaerobic means without oxygen, so oxygen appears in neither equation. The cell only partly breaks down the glucose, so it gets much less energy than from aerobic respiration.
When cells run short of oxygen
Cells respire anaerobically when there is little or no oxygen. OCR Twenty First Century (J257) gives three examples:
- your muscle cells during hard exercise
- plant root cells in soil full of water
- bacteria inside a deep cut, which scientists call a puncture wound
In each case the cell still needs energy. It gets a little by breaking glucose part of the way down, without oxygen. You can read about the full version on the aerobic respiration equation page.
In your muscles: glucose → lactic acid
This equation has one reactant and one product. There is no carbon dioxide and no water. AQA prints it as glucose → lactic acid, in words only. J257 says some bacteria make lactic acid too.
Lactic acid builds up in the muscle. AQA says that after a long spell of hard exercise, the muscles become fatigued. They get tired and stop contracting well.
The lactic acid also creates what AQA calls an oxygen debt. Higher tier pupils also learn two more facts. Your blood carries the lactic acid to the liver, which turns it back into glucose. The oxygen debt is the extra oxygen your body needs after exercise to react with the lactic acid and clear it from your cells.
In yeast and plants: glucose → ethanol + carbon dioxide
Yeast is a microorganism. Without oxygen it turns glucose into ethanol, a kind of alcohol, and carbon dioxide gas. AQA calls this fermentation.
AQA points out that fermentation matters to industry, because people use it to make bread and alcoholic drinks.
Plant cells can respire this way too, as in waterlogged roots. J257 says some other microorganisms do the same. Our bacterial cell diagram shows the parts of one kind of microorganism.
Board by board
AQA (8461 and 8464) prints both word equations. It gives no symbol versions for these two, so neither does this page.
Edexcel (1BI0 and 1SC0) prints no equation. It asks you to compare aerobic with anaerobic respiration. OCR Gateway (J247) asks you to compare the conditions, the substances used, the products and how much ATP each kind gives, in plants, fungi and animals.
At KS3 you give a word summary of anaerobic respiration in humans and microorganisms, including fermentation. The respiration equation page puts all three word equations together.
Common mistakes
- Writing lactic acid + carbon dioxide for muscles. Anaerobic respiration in animal cells gives lactic acid alone.
- Giving yeast the muscle product. Yeast makes ethanol and carbon dioxide.
- Adding oxygen to the left side. Anaerobic respiration happens without oxygen.
- Saying anaerobic respiration releases more energy. It transfers much less, because the glucose is not fully broken down.
Respiration Equations mixes wrong tiles into each bank, so you must pick lactic acid or ethanol for the right cell. See more on the biology hub.
Questions people ask
Why do muscles respire anaerobically?
During hard exercise the blood cannot bring oxygen to the muscles fast enough. The muscle cells keep releasing some energy without it.
Is fermentation the same as anaerobic respiration?
Fermentation is the name AQA gives to anaerobic respiration in yeast cells. KS3 includes it within anaerobic respiration in microorganisms.
Does anaerobic respiration make carbon dioxide?
Only in plants, yeast and some other microorganisms. The muscle equation, glucose → lactic acid, has no carbon dioxide in it.