Outline of Campbell Biology Chapter 9

II. Oxidation/Reduction Reactions in Metabolism

  1. Introduction
    1. Oxidation-reduction reactions = Chemical reactions which involve a partial or complete transfer of electrons from one reactant to another; called REDOX reactions for short.
    2. Oxidation = Partial or complete loss of electrons.
    3. Reduction = Partial or complete gain of electrons.
    4. Generalized Redox Reaction:
      1. Electron transfer requires both a donor and acceptor, so when one reactant is oxidized the other is reduced.

        Xe- + Y --> X + Ye-

        1. X = Substance being oxidized; acts as a reducing agent because it reduces Y.
        2. Y = Substance being reduced; acts an oxidizing agent because it oxidizes X.
      2. Not all REDOX reactions involve a complete transfer of electrons, but, instead, may just change the degree of sharing in covalent bonds. For example:

        CH4 + 2 O2 --> CO2 + 2 H2O + energy

         

        1. Covalent electrons of methane are equally shared because carbon and hydrogen have about the same affinity for electrons.
        2. When methane reacts with oxygen, electrons shift away from carbon and hydrogen to oxygen which is very electronegative.
        3. Oxygen is a powerful oxidizing agent because it is so electronegative.
        4. Since electrons lose potential energy when they shift toward a more electronegative atom, spontaneous REDOX reactions release energy.
    5. Respiration as a Oxidation/Reduction Process
      1. Respiration is a redox process that transfers hydrogen from sugar to oxygen.
        C6H12O6 + 6 O2 --> 6 CO2 + 6 H2O + energy

         

        1. Valence electrons of carbon and hydrogen lose potential energy as they shift toward electronegative oxygen.
        2. Released energy is used by cells to produce ATP.
        3. Organic molecules with abundant hydrogen are excellent fuels because they are rich in high-energy electrons. (Examples are carbohydrates and fats, the main energy foods.)
        4. A mole of glucose yields 686 Kcal of heat when burned in air.
        5. Cellular combustion of glucose is not a single explosive step, as the energy would be difficult to harness.
        6. In the cell, glucose is oxidized gradually in a series of enzyme-controlled steps that occur during glycolysis and the citric acid cycle (Krebs).
        7. Hydrogens stripped from glucose are not transferred directly to oxygen, but are first passed to a special electron acceptor - NAD+ or FAD.
    6. NAD+ and the Oxidation of Glucose
      1. Coenzyme = Small nonprotein organic molecule required for proper enzyme catalysts.
      2. Dinucleotide = A molecule consisting of two nucleotides joined together.
      3. Nicotinamide adenine dinucleotide (NAD+) = A dinucleotide which functions as a coenzyme in the redox reactions of metabolism.
        1. Found in all cells.
        2. Assists enzymes in electron transfer.
        3. Another such coenzyme is FAD (flavin adenine dinucleotide).
      4. During the oxidation of glucose, NAD+ functions as an oxidizing agent by trapping energy-rich electrons from glucose or food. These reactions are catalyzed by enzymes called dehydrogenases, which:
        1. Remove a pair of hydrogen atoms (2 electrons and 2 protons) from substrate.
        2. Deliver the two electrons and one proton to NAD+.
        3. Release the remaining proton into the surrounding solution.
          XH2 + NAD+ --> X + NADH + H+
          1. X = Various substrates oxidized by enzymatic transfer of electrons to NAD+.
          2. NAD+ = Oxidized coenzyme (net positive charge).
          3. NADH = Reduced coenzyme (electrically neutral).
      5. These high energy electrons transferred from substrate to NAD+ are then passed down the electron transport chain to oxygen, powering ATP synthesis (oxidative phosphorylation).