Oxidation & Reduction (HL) (HL IB Biology)

• 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 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