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First exams 2025

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Oxidation & Reduction (HL) (HL IB Biology)

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Oxidation & Reduction in Cell Respiration

  • Oxidation and reduction are commonly known as redox reactions
  • These reactions occur at the same time and involve the transfer of electrons between molecules
    • Oxidation is the loss of electrons
    • Reduction is the gain of electrons
  • Redox reactions also involve hydrogen, oxygen and energy transfer
    • Oxidation is also the loss of hydrogen, gain of oxygen and releases energy to the surroundings (exergonic)
    • Reduction is also the gain of hydrogen, loss of oxygen and absorbs energy from the surroundings (endergonic)
  • Molecules that have a strong tendency to lose/donate their electrons, are known as reducing agents
  • Molecules that that have a strong tendency to gain electrons, are known as oxidising agents
  • Oxidation and reduction reactions feature in cellular respiration and photosynthesis

Comparison of Oxidation and Reduction Table

Oxidation Reduction
Loss of electrons Gain of electrons
Loss of hydrogen Gain of hydrogen
Gain of oxygen Loss of oxygen
Exergonic (releases energy) Endergonic (absorbs energy)

Oxidation and reduction in cell respiration

  • Respiration involves a group of molecules called electron carriers which accept or donate their electrons
    • NAD+ (nicotinamide adenine dinucleotide) is the primary electron carrier involved in respiration
    • FAD (flavin adenine dinucleotide) is another electron carrier used in respiration
  • NAD and FAD are both coenzymes which serve as links between redox reactions
  • Both NAD and FAD serve as oxidising agents:
    • NAD+ and FAD gain electrons and also gain one or more hydrogen ions (from molecules involved in respiration), switching to a slightly different form called reduced NAD (NADH) and reduced FAD (FADH2)
    • NAD+ + 2e- + 2H+ --> NADH + H+
    • FAD + 2e- + 2H--> FADH2
  • These electron carriers are used to transport the electrons they have gained to other reactions in respiration
  • When they lose these electrons they return to their original form releasing their electrons in the process
    • NADH -->  NAD+ + 2e- + 2H+
    • FADH2 --> FAD + 2e- + 2H
  • This is an example of a redox reaction

Examiner Tip

To help you remember which way around loss and gain of electrons is from redox reactions, think OILRIG:

  • Oxidation Is Loss
  • Reduction Is Gain

NAD is a collective term for the different forms NAD takes; NAD exists in an oxidised and a reduced form:

  • NAD+ is the oxidised form and acts as an oxidising agent
  • NADH is the reduced form and acts a reducing agent

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Marlene

Author: Marlene

Expertise: Biology

Marlene graduated from Stellenbosch University, South Africa, in 2002 with a degree in Biodiversity and Ecology. After completing a PGCE (Postgraduate certificate in education) in 2003 she taught high school Biology for over 10 years at various schools across South Africa before returning to Stellenbosch University in 2014 to obtain an Honours degree in Biological Sciences. With over 16 years of teaching experience, of which the past 3 years were spent teaching IGCSE and A level Biology, Marlene is passionate about Biology and making it more approachable to her students.