8.2.1 Oxidation, Reduction & Phosphorylation

• 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

Table comparing oxidation and reduction

Oxidation and reduction in cell respiration

• Respiration involves a group of molecules called electron carriers which accept or donate their electrons
  • NAD⁺ is the primary electron carrier involved in respiration
  • FAD is another electron carrier used in respiration
• 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 and reduced FAD
  • 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

• Phosphorylation occurs when a phosphate ion is added to a molecule
  • E.g. the phosphorylation of ADP to make ATP
• This makes the molecule less stable and therefore more likely to react
  • We can say that phosphorylation activates a molecule because it makes it more reactive
• There are two main types of phosphorylation:
  • Substrate level phosphorylation where the phosphate ion is transferred from a donor compound
    • This takes place in glycolysis and the Krebs cycle
  • Oxidative phosphorylation where phosphorylation is coupled with oxidation
    • This takes place in the the electron transport chain
• Phosphorylation is an endergonic reaction whereas the removal of the phosphate ion by hydrolysis in dephosphorylation is an exergonic reaction
  • Remember that the hydrolysis of ATP to ADP releases energy, therefore dephosphorylation of ADP is exergonic
• The addition of a phosphate ion to one molecule occurs at the same time as the removal of a phosphate from another; this is known as the coupling of reactions
  • E.g. the exergonic dephosphorylation of ATP is coupled with the endergonic phosphorylation of glucose at the start of glycolysis

The cyclic formation of ATP from ADP by phosphorylation