Respiration: Grade 9 Understanding for IGCSE Biology 2.34 2.36 2.37 2.38

I can’t believe that it is over year since I started posting about iGCSE Biology misconceptions and yet I have never written about Respiration.  If there is one topic that students misunderstand more than any other (apart perhaps from genetics), this must be it….  So I am going to try to explain in a straightforward way what respiration is and why it is so important for life.

Life requires energy.  Living cells are constantly doing things that use up energy: pumping molecules across their cell membranes, moving organelles around the cell, cell division, nerve cells sending electrical impulses around the body, muscle fibres contracting etc. etc.  In every case, this energy comes from a metabolic process called Respiration.  It is a series of chemical reactions, catalysed by enzymes and in some way, it happens in all cells.

So let’s start with a good definition.  [Examiners are simple souls and often start questions with the classic “What is Respiration?”]

Respiration is a series of chemical reactions that happens inside cells in which food molecules (for example glucose) are oxidised to release energy for the cell.

My definition has to be a little vague because although glucose is found in all the equations for respiration, other food molecules can certainly be respired.  And oxygen is only used in aerobic respiration.  Many organisms can only respire without oxygen (anaerobic respiration) and some, such as humans can switch between aerobic and anaerobic depending on the conditions.


Aerobic Respiration happens for the most part in tiny organelles in the cytoplasm called Mitochondria.  The diagram above shows the structure of a mitochondrion (I wouldn’t worry about learning it but perhaps you should be able to recognise the characteristically folded inner membrane?)

What are the differences between aerobic and anaerobic respiration in humans?

Well we have mentioned two already and there are others…..:

  • Aerobic respiration requires oxygen, anaerobic does not.
  • Aerobic respiration takes place in mitochondria, anaerobic only occurs in the cytoplasm.
  • Aerobic respiration produces much more energy per glucose molecule than anaerobic – it is a more complete oxidation of the glucose, so much more energy is released.
  • Anaerobic respiration produces lactic acid as a waste product (in humans) whereas in aerobic, carbon dioxide and water are the products

The summary equations for the processes are different as well.

Aerobic respiration:

word equation                       Glucose + Oxygen ——> Carbon Dioxide + Water

balanced chemical equation            C6H12O6 + 6O2  ——> 6CO2 + 6H20

Anaerobic respiration in humans:

Glucose —–> Lactic Acid

Anaerobic respiration in Yeast (a single celled fungus):

Glucose —–> Ethanol and Carbon Dioxide

A couple of final points to note:

Anaerobic respiration in muscle cells does not produce carbon dioxide as a waste product (see the equation above…) Lactic acid is the only waste product.  But lactic acid will accumulate in muscles and stop the muscle functioning properly so after a period of intense activity, lactic acid needs to be removed.  How does this happen?

Lactic acid moves from the muscle in the blood and is transported to the liver.  In the liver, the lactic acid is metabolised in an aerobic pathway that uses oxygen.  This is why sprinters will always be breathing fast after the race, even when they are standing still.  Their body needs extra oxygen to oxidise the lactic acid they have produced during the race.  This extra oxygen is termed an oxygen debt and is the oxygen needed in the liver to fully oxidise lactic acid to carbon dioxide and water.

Finally, respiration is not the same as breathing.  Our American cousins sometimes muddle these processes up but in this one case, the British way is much better….  Use the term ventilation for breathing – moving air in and out of the lungs – and reserve respiration for the chemical reactions that happen inside the cells to release energy.


Please leave a comment below if you find this post helpful or ask me about anything that isn’t clear….



  1. Philip

    I am a little confused with the concept of energy here. Is energy always released in respiration? Sometimes I have read that energy is used (up) by organisms and indeed can pass through an ecosystem, but with respiration is energy required as well as released from oxidation of food molecules? I would like to be really sure that I have a clear conceptual understanding especially in an exam!

  2. ArJay

    Thank you so so much for this blog. I found this after my IGCSE’s ( where i failed biology unfortunately) and because of this blog i feel so much more familiar with everything now. Its given me hope that i could actually do well in my re sits in January. Once again, thank you very much! You’ve helped me a long with a lot of other people.

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