Anaerobic Respiration (Cambridge O Level Biology)

• 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 (compared to aerobic respiration) 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 in 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 hearts/lungs can only deliver a finite amount of 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

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 causes the dough to rise
• And in brewing, where
  • The ethanol produced gives the beer its alcoholic nature
  • The carbon dioxide produced gives beer its fizz

Word equation for anaerobic respiration in yeast

The Balanced Chemical Equation for Anaerobic Respiration in Yeast

Balanced equation for anaerobic respiration in yeast

Comparing Types of Respiration Table

• Lactic acid builds up in muscle cells and lowers the pH of the cells (making them more acidic)
• Lactic acid buildup in muscles can also cause cramp, a severe but reversible condition

A soccer player experiencing cramp during a vigorous game; the referee is helping him by stretching out the player's calf muscle

beIN SPORTS Türkiye, CC BY 3.0, via Wikimedia Commons

• 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 (with deeper and faster breaths) and our heart rate remains high even after finishing the period of exercise
  • This is called the Excess Post-exercise Oxygen Consumption (EPOC) or 'oxygen debt'
  • 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
  • During the EPOC, the heart rate also remains high
• This is known as ‘repaying the oxygen debt’