Respiration is a Chemical Process (OCR Gateway GCSE Biology: Combined Science)

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Biology Project Lead

Respiration is a Chemical Reaction

Cellular respiration

  • Cellular respiration is the process of transferring energy from the breakdown of glucose
    • It is a universal chemical process which is continuously occurring in all living cells
    • Every process happening in your body requires energy
  • The energy transferred by respiration cannot be immediately or directly used by the cell
    • Energy is stored in the form of a special molecule of called ATP
    • ATP can then be used in cellular processes where energy is required
    • An analogy of ATP is a rechargeable battery which can store potential energy and release it as kinetic energy
      • Once the energy has been depleted the battery can be recharged
  • Organisms need energy for:
    • Chemical reactions to build larger molecules from smaller molecules
    • Muscle contraction to allow movement
    • Keeping warm (to maintain a constant temperature suitable for enzyme activity)
  • The energy transferred supplies all the energy needed for metabolic processes to occur within cells and organisms as a whole

Uses of energy in the human body, IGCSE & GCSE Chemistry revision notes

Uses of the energy released from respiration

  • Although most of the energy generated through respiration uses glucose as a substrate, organisms can break down other molecules to be used in respiration
    • Proteins, lipids (fats) and other carbohydrates can all be utilised
  • Cellular respiration can either be aerobic (in the presence of oxygen) or anaerobic (in the absence of oxygen)
  • Cellular respiration is controlled by enzymes, consequently the rate can be affected by temperature and pH

Aerobic Respiration

  • Aerobic respiration requires oxygen
    • It is defined as the chemical reaction in cells that uses oxygen to break down nutrient molecules to release energy
    • Most aerobic respiration takes place in the mitochondria
  • Aerobic respiration is the complete breakdown of glucose to release a relatively large amount of energy for use in cell processes and reactions
  • Carbon dioxide and water are produced as waste products as well as releasing useful cellular energy

Word equation for aerobic respiration, IGCSE & GCSE Chemistry revision notes

Word equation for aerobic respiration

  • This equation can also be shown as a balanced symbol equation
    • One molecule of glucose combines with six molecules of oxygen to produce six molecules of carbon dioxide and six molecules of water

Balanced equation for aerobic respiration, IGCSE & GCSE Chemistry revision notes

The balanced symbol equation for aerobic respiration

Respiration is Exothermic

  • Cellular respiration is an exothermic reaction as it transfers energy to the environment in the form of heat
    • This is one of the reasons why our bodies heat-up when we exercise
  • The heat generated through aerobic respiration can be demonstrated using germinating beans

Practical investigation: demonstrating the production of heat


  • Vacuum Flasks
  • Thermometer
  • Cotton wool
  • Germinating seeds
  • Dead/boiled seeds


  • Set up the flasks as shown in the diagram
    • Flask A with the germinating seeds
    • Flask B with the dead seeds
  • Make sure the cotton wool is plugging the top of each flask
  • Hold the thermometer in place with the cotton wool
  • Invert the flask
  • Record the initial temperature
  • After 4 days, record the final temperature

Production of heat in respiration, downloadable IGCSE & GCSE Biology revision notes

Experiment to demonstrate the production of heat by living material during respiration


  • The thermometer in the flask with the germinating seeds (Flask B) should show an increase in temperature
  • Flask A should remain at room temperature
  • This is because the seeds in flask B are respiring and producing heat energy in the process
  • This shows that respiration is an exothermic reaction
  • The seeds in flask A are not respiring because they are dead, so the temperature remains the same

Exam Tip

Remember that cellular respiration is not breathing; it is a chemical process of transferring energy from glucose in all living cells.

There are usually 3 marks given for the aerobic respiration chemical equation in an exam:

  • One for getting the correct formula for glucose and oxygen
  • One for getting the correct formula for carbon dioxide and water
  • One for balancing the equation correctly

So make sure you can do all three to gain maximum marks!

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Author: Phil

Phil has a BSc in Biochemistry from the University of Birmingham, followed by an MBA from Manchester Business School. He has 15 years of teaching and tutoring experience, teaching Biology in schools before becoming director of a growing tuition agency. He has also examined Biology for one of the leading UK exam boards. Phil has a particular passion for empowering students to overcome their fear of numbers in a scientific context.