Revision Notes

12.3 Anaerobic Respiration

Anaerobic Respiration: Basics

  • Anaerobic respiration does not require oxygen and is defined as the chemical reactions in cells that break down nutrient molecules to release energy without using oxygen
  • It is the incomplete breakdown of glucose and releases a relatively small amount of energy for use in cell processes
  • It produces different breakdown products depending on the type of organism it is taking place in
  • You need to know the equations for anaerobic respiration in humans (animals) and the microorganism yeast

Anaerobic Respiration in Animals

  • Anaerobic respiration mainly takes place in muscle cells during vigorous exercise
  • When we exercise vigorously, our muscles have a higher demand for energy than when we are resting or exercising normally. Our bodies can only deliver so much oxygen to our muscle cells for aerobic respiration
  • In this instance, as much glucose as possible is broken down with oxygen, and some glucose is broken down without it, producing lactic acid instead
  • There is still energy stored within the bonds of lactic acid molecules that the cell could use; for this reason, less energy is released when glucose is broken down anaerobically


Word equation for anaerobic respiration in animals, IGCSE & GCSE Chemistry revision notesWord equation for anaerobic respiration in animals

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Lactic Acid & The Oxygen Debt

  • Lactic acid builds up in muscle cells and lowers the pH of the cells (making them more acidic)
  • This could denature the enzymes in cells so it needs to be removed
  • Cells excrete lactic acid into the blood. When blood passes through the liver, lactic acid is taken up into liver cells where it is oxidised, producing carbon dioxide and water (Lactic acid reacts with oxygen – this is actually aerobic respiration with lactic acid as the nutrient molecule instead of glucose)
  • So the waste products of lactic acid oxidation are carbon dioxide and water
  • This is the reason we continue to breath heavily and our heart rate remains high even after finishing exercise – we need to transport the lactic acid from our muscles to the liver, and continue getting larger amounts of oxygen into the blood to oxidise the lactic acid
  • This is known as ‘repaying the oxygen debt’

Exam Tip

It’s easy to get confused about the products of anaerobic respiration in animals – the ONLY product made is lactic acid.

Carbon dioxide is NOT one of the products made in anaerobic respiration in animals – it is made in aerobic respiration!

Anaerobic Respiration in Yeast

  • We take advantage of the products of anaerobic respiration in yeast by using it in bread making (where the carbon dioxide produced helps dough to rise) and in brewing (where the ethanol produced makes beer)


Word equation for anaerobic respiration in yeast, IGCSE & GCSE Chemistry revision notesWord equation for anaerobic respiration in yeast

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Anaerobic Respiration Balanced Equation in Yeast

  • The balanced chemical equation for anaerobic respiration in yeast is:


Balanced equation for anaerobic respiration in yeast, IGCSE & GCSE Chemistry revision notesBalanced equation for anaerobic respiration in yeast

Comparison of Aerobic & Anaerobic Respiration

Comparing types of respiration:

Anaerobic Respiration table, IGCSE & GCSE Biology revision notes

Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.

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