• 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