IB Biology SL

Revision Notes

2.7.1 Cellular Respiration

Cellular Respiration Defined

  • Cell respiration is the controlled release of energy from organic compounds to produce ATP
  • Respiration is a series of chemical reactions that happens in every cell
  • Its purpose is to release energy in usable forms from the chemical energy stored in food e.g. glucose
  • Respiration is a catabolic process
  • Glucose is the main respiratory fuel used in cells
    • Lipids and proteins can also be used
  • Organic food substances contain a lot of chemical energy
  • This energy cannot be released in one, uncontrolled step in cells, which would cause cell damage and tissue death
  • Enzymes control the release of energy through a series of chemical reactions called a pathway
  • This ends in the production of ATP (adenosine triphosphate)
    • To make ATP, a phosphate group is linked to adenosine diphosphate (ADP)
    • This process requires energy which comes from the breakdown of organic molecules
  • The energy that is released is used for
    • Fuelling anabolic processes
    • Muscle contraction
    • Fuelling active transport
    • Moving molecules around the cell
    • Generating heat to maintain body temperature in warm-blooded animals

Exam Tip

Respiration is often confused with breathing, but remember, respiration is a chemical process, breathing is a method of moving air in and out of the body


  • ATP is a source of energy for cellular processes
  • The energy can be released immediately, exactly when it is required
  • All organisms require a constant supply of energy to maintain their cells and stay alive
  • This energy is required:
    • In anabolic reactions – synthesizing larger molecules from smaller molecules
    • To move molecules across the cell membrane (active transport)
    • To move substances and organelles within the cell
    • In animals, energy is required:
      • For muscle contraction – to coordinate movement at the whole-organism level
      • In the conduction of nerve impulses, as well as many other cellular processes
  • In all known forms of life, ATP from respiration is used to transfer energy in all energy-requiring processes in cells
  • ATP is converted to ADP and phosphate when releasing its energy
    • ADP and phosphate can then be re-converted to ATP during respiration
  • Organisms require a constant supply of ATP because much of the energy is dissipated (lost to the surroundings) as heat

Cycling of ATP, downloadable IB Biology revision notes

The constant cycling of ATP and ADP+Pi within a cell

Author: Lára

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.

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