Anaerobic Respiration (Cambridge International IGCSE Biology)

Anaerobic respiration - respiration without oxygen

Spec point — Describe anaerobic respiration as the chemical reactions in cells that break down nutrient molecules to release energy without using oxygen State that anaerobic respiration releases much less energy per glucose molecule than aerobic respiration State the word equation for anaerobic respiration in yeast as:glucose → alcohol + carbon dioxide State the word equation for anaerobic respiration in muscles during vigorous exercise as:glucose → lactic acid

Anaerobic respiration - respiration without oxygen

• 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
• There is an incomplete breakdown of glucose when respiration occurs anaerobically and so a relatively small amount of energy is released for use in cell processes (compared to the relatively large amount of energy released when aerobic respiration occurs)
• Anaerobic respiration produces different breakdown products depending on the type of organism it is taking place in

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
• The word equation for anaerobic respiration in animals is:

$glu\mathrm{cos}e\rightarrow lactic acid$

Anaerobic respiration in yeast

• The products of anaerobic respiration in yeast are used in bread making, where the carbon dioxide produced causes dough to rise

• And in brewing, where:

  • the ethanol produced gives the beer its alcoholic nature
  • the carbon dioxide produced gives beer its fizz

• The word equation for anaerobic respiration in yeast is:

$glu\mathrm{cos}e\rightarrow alcohol+carbon dioxide$

Balanced chemical equation for anaerobic respiration: Extended

Spec point — State the balanced chemical equation for anaerobic respiration in yeast as: CHO → CHOH + CO

• When one molecule of glucose (C₆H₁₂O₆) is broken down anaerobically in yeast cells, two molecules of alcohol (in this case, ethanol: C₂H₅OH) and two molecules of carbon dioxide (CO₂) are produced
• The balanced chemical equation for anaerobic respiration in yeast is:

$C_{6}H_{12}O_{6}\rightarrow2C_{2}H_{5}OH+2CO_{2}$

Anaerobic respiration & oxygen debt: Extended

Spec point — State that lactic acid builds up in muscles and blood during vigorous exercise causing an oxygen debt Outline how the oxygen debt is removed after exercise, limited to: (a) continuation of fast heart rate to transport lactic acid in the blood from the muscles to the liver (b) continuation of deeper and faster breathing to supply oxygen for aerobic respiration of lactic acid (c) aerobic respiration of lactic acid in the liver

Anaerobic respiration & oxygen debt: Extended

• 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 breathe 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 Hint It's easy to get confused about the products of anaerobic respiration in animals - the only product is lactic acid. Carbon dioxide is not one of the products formed when anaerobic respiration occurs in animals.

Comparing aerobic & anaerobic respiration

Spec point — nan

• There are some important points of comparison that can be made between the two types of respiration