B2.6 Respiration

B2.6 Respiration


Respiration in cells can take place aerobically or anaerobically. The energy released is used in a variety of ways. The human body needs to react to the increased demand for energy during exercise.


  • Definition: The process of transferring energy from food molecules in every living cell.
  • Aerobic respiration – uses oxygen
  • Anaerobic respiration – uses no oxygen
  • All chemical reactions inside cells are controlled by enzymes.

Aerobic respiration

  • Glucose reacts with oxygen, producing carbon dioxide and water as waste products.
  • This takes place continuously in animals and plants.

Word equation:

Chemical equation (do not need to learn!):


  • Respiration actually involves a series of many small reactions.
  • Each reaction is controlled by an enzyme.



  • Most of the reactions in respiration happen in the mitochondria.
  • The inner surface of the mitochondria is highly folded to increase the surface area for enzymes.


Energy use

The energy that is released during respiration is used:

  • To build up larger molecules using smaller ones.
  • In animals, to enable muscles to contract.
  • In mammals and birds, to maintain a steady body temperature in colder surroundings.
  • In plants, to build up sugars, nitrates and other nutrients into amino acids which are then built up into proteins.


The role of respiration during exercise

  • Muscles contract to move the bones in our bodies.
  • Respiration releases energy, which is used to contract the muscles:
  • When we exercise, our muscles contract more quickly and with more force.
  • This requires more energy.
  • This requires more glucose and oxygen.
  • Also, more carbon dioxide is created which needs to be removed.
  • The human body needs to react to the increased demand for energy during exercise.


Changes during exercise


  • During exercise a number of changes take place: o The rate and depth of breathing increases.
  • This increases the rate of gaseous exchange.
  • More oxygen is taken into the blood.
  • More carbon dioxide is removed from the blood. o The heart rate increases.
  • This increases rate of blood flow to the muscles
  • All of these changes increase the blood flow to the muscles and so increase the supply of sugar and oxygen and increase the rate of removal of carbon dioxide.



  • Glucose is stored as glycogen is in the muscles.
  • During exercise, glycogen is broken down into glucose in the muscles.
  • This increases the amount of glucose that can be respired.


Anaerobic respiration

  • During exercise, if insufficient oxygen is reaching the muscles they use anaerobic respiration to obtain energy.
  • Anaerobic respiration is the incomplete breakdown of glucose and produces lactic acid.
  • As the breakdown of glucose is incomplete, much less energy is released than during aerobic respiration.

  • However, lactic acid is poisonous. We can only tolerate small amounts in our body.


  • If muscles are subjected to long periods of vigorous activity they become fatigued, ie they stop contracting efficiently.
  • One cause of muscle fatigue is the build up of lactic acid in the muscles.
  • Blood flowing through the muscles removes the lactic acid.
  • During and after exercise, we breathe heavily to take in extra oxygen to oxidise the lactic acid:

  • The extra oxygen is called the oxygen debt.
  • The heart continues to pump faster.
  • The breathing rate remains high.
  • This delivers the extra oxygen to the muscles.
  • This pays back the oxygen debt.