Outline of Campbell Biology Chapter 9

V. THE ELECTRON TRANSPORT CHAIN AND OXIDATIVE PHOSPHORYLATION

  1. Introduction
    1. The electron transport chain:
      1. Is made of electron carriers embedded in the inner mitochondrial membrane.
      2. Passes electrons from reduced coenzyme (NADH and FADH2) to oxygen.
      3. The Krebs Cycle occurs only under aerobic conditions, it does not use oxygen directly. The electron transport chain and oxidative phosphorylation require oxygen.)
      4. Uses electron flow to create a proton gradient across the inner mitochondrial membrane.
    2. Oxidative phosphorylation is coupled to the electron transport chain by the potential energy stored in the proton gradient.
      1. ATP synthase catalyzes the phosphorylation of ADP. The energy required to power this process is released when protons diffuse down the gradient through the enzyme complex.
      2. The overall function of oxidative phosphorylation is to transform electron energy of glucose into energy stored in the phosphate bonds of ATP.
  2. Electron carrier molecules transfer electrons down the electron transport chain.
    1. Prosthetic groups of electron carriers shift between reduced and oxidized states as they accept and donate electrons.
    2. Most electron carriers are proteins. Ubiquinone (Q) is the only electron carrier of the transport chain that is not bound to a protein.
    3. Electron carriers of the transport chain, which are proteins include:

      Protein Electron Carriers

      Prosthetic Group

      flavoprotein

      flavin mononucleotide (FMN)

      iron-sulfur protein

      iron and sulfur

      cytochromes

      heme group
    4. Heme group = Prosthetic group composed of four organic rings surrounding a single iron atom.
    5. Cytochromes = Type of protein molecule which contains a heme prosthetic group and which functions as an electron carrier in the electron transport chains of mitochondria and chloroplasts.
    6. There are several types of cytochromes, each a different protein with a heme group.
    7. It is the iron of cytochromes that transfers electrons.
    8. Sequence of Electron Transfers Along the Electron Transport Chain:
    9. Electron transfer from NADH to oxygen is exergonic (Delta-G = -53 kcal/mol).
    10. Free energy change does not occur in one explosive step. Instead, electrons lose a small amount of energy as they cascade down the chain from carrier to carrier.
    11. Oxygen, the terminal electron acceptor, has a great affinity for electrons and pulls electrons down the chain.
    12. FADH2 also donates electrons to the electron transport chain, but those electrons are added at a lower energy level than NADH.
    13. The electron transport chain does not make ATP directly. It generates a proton gradient across the mitochondrial membrane, which stores potential energy that can be used to phosphorylate ADP.
  3. Generation of the Proton Gradient
    1. The electron transport chain generates a proton gradient by transporting protons from the mitochondrial matrix to the intermembranal space.
      1. Some electron carriers of the transport chain accept and release protons along with the electrons.
      2. Other carriers transport only electrons.
    2. Proton translocation is based on spatial organization of the electron transport chain in the membrane.
    3. Electron carriers are organized into three complexes:
      1. NADH dehydrogenase complex.
      2. When reduced, the complex also accept two protons from the matrix (one from NADH and one from solution).
      3. When oxidized, the complexes passes on electrons and also releases two protons into the intermembrane space.
      4. Cytochrome b-c1 complex.
      5. This complex may translocate additional protons from the matrix to the intermembrane space.
      6. It passes electrons to the mobile carrier, cytochrome c.
      7. Cytochrome oxidase complex.
      8. This complex passes electrons to oxygen.
    4. Mobile carriers transfer electrons between complexes. These mobile carriers are:
      1. Ubiquinone (Q).
        1. Near the matrix, Q accepts electrons from the NADH dehydrogenase complex and two protons from solution.
        2. Q diffuses across the lipid bilayer and releases two protons to the intermembrane space.
        3. As protons are released, Q passes electrons to the cytochrome b- c1 complex.
      2. Cytochrome c (Cyt c).
        1. Cyt c is reduced as it accepts electrons from the cytochrome b-c1 complex.
        2. As it is oxidized, Cyt c conveys its electrons to the cytochrome oxidase complex.
    5. When the transport chain is operating:
      1. The pH in the intermembrane space is one or two pH units lower than in the matrix.
      2. The pH in the intermembrane space is the same as the pH of the cytosol because the outer mitochondrial membrane is permeable to protons.
  4. The Proton-Motive Force and ATP Synthesis
    1. Proton motive force = Potential energy stored in the proton gradient created across biological membranes that are involved in chemiosmosis.
      1. This force is an electrochemical gradient with two components:
        1. Concentration gradient of protons (chemical gradient).
        2. Voltage across the membrane because of a higher concentration of positively charged protons on one side (electrical gradient).
      2. It tends to drive protons across the membrane back into the matrix, but the inner mitochondrial membrane is not very permeable to protons.
      3. Protons reenter the matrix by passing through an ATP-synthesizing protein complex that spans the inner mitochondrial membrane. This complex is ATP synthase.
    2. Multiple copies of ATP synthase stud the inner mitochondrial membrane. They function to couple the exergonic passage of protons with the endergonic phosphorylation of ADP.
    3. This complex of several proteins has two main components:
      1. F0 - This part spans the membrane and channels proton diffusion.
      2. F1 - Attached to F0 on the matrix side of the inner mitochondrial membrane, this spherical part catalyzes ADP phosphorylation. How the F1 enzyme uses energy from the proton current is still unknown.
    4. Effects of three classes of respiratory poisons provide evidence for chemiosmosis and its dependence upon the structural organization of the mitochondrial membrane.
      1. Poisons that block electron flow.
        1. Cyanide blocks electron flow from Cyta3 to oxygen. This stops the electron transport chain so it cannot pump protons. Without the proton gradient, ATP is not produced.
      2. Poisons that make the inner mitochondrial membrane leaky to protons.
        1. These poisons, such as dinitrophenol, are called uncouplers. By allowing protons to leak back across the membrane, they uncouple the process of proton pumping with ATP production.
      3. Poisons that inhibit ATP synthase.
        1. This class of poisons, which includes the antibiotic oligomycin, directly inhibits ATP synthase.
        2. The proton gradient becomes greater than normal and yet the potential energy of the gradient cannot be tapped to produce ATP.
  5. The ATP Ledger for Respiration
    1. The net ATP yield from the oxidation of a glucose molecule is influenced by several factors:
      1. For each high energy electron pair that travels from NADH down the electron transport chain to oxygen, enough proton-motive force is created to produce a maximum of three ATPs.
      2. FADH2 is worth a maximum of only two ATPs, since it donates electrons at a lower energy level to the electron transport chain.
      3. In most eukaryotic cells, the ATP yield is lower from an NADH produced during glycolysis. The mitochondrial membrane is impermeable to NADH, so its electrons must be shuttled across the membrane. These electrons are received inside the mitochondrion by FAD, a process which downgrades the energy level of those electrons.
      4. There is a debit of two ATPs from the preparatory steps of glycolysis, and everything is doubled after the sugar-splitting step of glycolysis.
      5. This tally only estimates the ATP yield from respiration. Some variables that affect ATP yield include:
        1. Mitochondrial membranes may differ in permeability to protons.
        2. The proton motive force may be used to drive other kinds of work such as active transport.
        3. The estimate of 36 ATPs produced per glucose is contingent upon an adequate oxygen supply.
    2. Maximum ATP yield for cellular respiration in a eukaryotic cell.

Process

Substrate Level
Phosphorylation

Reduced
Coenzyme

Oxidative
Phosphorylation

Total

Glycolysis

Net 2 ATP

2 NADH

4 to 6 ATP

6-8

Oxidation
of Pyruvic
Acid

------

2 NADH

6 ATP

6

Krebs
Cycle

2 ATP

6 NADH
2 FADH2

18 ATP
4 ATP

24

Total 36-38