• at this point, molecules of NADH and FADH2 account for most of the energy extracted from the glucose molecule
  • these two electron carriers link glycolysis and the citric acid cycle to oxidative phosphorylation
  • oxidative phosphorylation uses the energy released by the electron transport chain to power ATP synthesis

ELECTRON TRANSPORT CHAIN

  • the electron transport chain is a collection of molecules embedded in the inner membrane of the mitochondrion in eukaryotic cells (in prokaryotes it is in the plasma membrane)
  • the folding of the inner membrane increases its surface area, providing space for thousands of copies of the chain in each mitochondria
  • most components of the chain are proteins
  • these proteins exist in multiprotein comnplexes numbered I through IV
  • tightly bound to these proteins are prosthetic groups - nonprotein components that are essential for the catalytic functions of certain enzymes
  • during the electron transport chain, electron carriers alternate between reduced and oxidized states as they accept and donate electrons
  • each component of the chain becomes REDUCED when it accpets an electron from its "uphill" neighbor which has a lower affinity for electrons
  • it then returns to its OXIDIZED form as it passes the electron to its "downhill" neighbor which is more electronegative
  • PASSING THROUGH COMPLEX I
    • electrons removed from glucose by NAD+ (during glycolysis and the citric acid cycle) are transferred from NADH to the first molecule of the electron transport chain in complex I
    • this molecule is a flavoprotein - the name comes from its prosthetis goup called flavin momonucleotide (FMN)
    • in the next redox reaction, the flavoprotein returns to its oxidized form as it passes electrons to an iron-sulfur protein
    • the iron-sulfur protein then passes the electrons to a compound called ubiquinone (Q)
    • this electron carrier is a small hydrophobic molecule - the only one in the electron transport chain that is not a protein
    • Ubiquinone is individually mobile within the membrane rather than residing in a specific complex
  • most of the remaining electron carriers between ubiquinone and oxygen are cytochromes
    • these proteins have a prosthetic group - called a heme group - has an iron atom that accepts and donates electrons
    • the electron transport chain contains several different types of cytochromes - each is a different protein with a slightly different electron-carrying group
    • the last cytochrome of the chain - cyt a3 - passes its electrons to oxygen which is VERY ELECTRONEGATIVE
    • oxygen also receives a pair of hydrogen ions from the aqueous solution - this forms water
  • another source of electrons for the electron transport chain is FADH2
    • this is the other reduced product of the citric acid cycle
    • FADH2 adds its electrons to the electron transport chain at complex II - thus, at a lower energy level than NADH does
    • although FADH2 and NADH both contribute 2 electrons, the chain provides about 1/3 less energy for ATP synthesis when FADH2 is used rather than NADH
  • the electron transport chain makes no ATP directly
  • it eases the fall of electrons from food to oxygen - thus breaking the large free-energy drop into a series of small steps to release energy in manageable amounts

CHEMIOSMOSIS

  • chemisosmosis is an energy-coupling machanism that uses energy stored in the form of an H+ gradient across a membraneto drive cellular work
  • in the inner membrane of the mitochondria, there are many copies of a protein complex called ATP synthase - the enzyme that actually makes ATP from ADP and inorganic phosphate
  • under the conditions of cellular respiration, ATP synthase uses the energy of n existing ion gradient to power ATP synthesis
  • the power source for ATP synthase is the difference in H+ concentration (pH) on opposite sides of the inner mitochondrial membrane
  • this process - in which energy is stored in the form of a hydrogen ion gradient across a membrane is used to drive cellular work such as the synthesis of ATP - is called chemiosmosis
  • ATP synthase is a multisubunit complex with four main parts - each one is made up of multiple polypeptides
    • protons move one by one into the binding sites on one of the parts (the rotor) causing it to spin in a way that catalyzes ATP production from ADP and inorganic phosphate
  • ATP synthase is the smallest rotary motor known in nature
  • the H+ gradient used in chemiosmosis is established by the electron transport chain
    • the chain is an energy converter that uses the exergonic flow of electrons from NADH and FADH2 to pump H+ across the membrane from the mitochondrial matrix into the intermembrane space
    • H+ has a tendency to move back across the membrane, diffusing down its gradient
    • the ATP sythases are the only site where the H+ can navigate through the membrane
    • H+ passage through an ATP synthase uses the exergonic flow of H+ to drive the phosphorylation of ADP
    • energy stored in an H+ gradient across a membrane couples the redox reactions of the electron transport chain to ATP synthesis - this is an example of chemiosmosis
    • certain members of the electron transport chain accept and release protons (H+) along with electrons
    • at certain steps in the chain, electron transfers cause H+ to be taken up and released into the surrounding solution
    • in eukaryotic cells the electron carriers are spatially arranged in the membrane in such a way that H+ is accepted from the mitochondiral matrix and deposited in the intermembrane space
    • the H+ gradient that results is called proton-motive force - emphasizing the ability of the gradient to perform work
    • this force drives the H+ back across the membrane through the H+ channels provided by ATP sythases
  • in mitochondria the energy for gradient formation comes from exergonic redox reactions and ATP synthesis is performed
  • chemiosmosis also occurs in the chloroplast to generate ATP during photosynthesis
  • prokaryotes generate the H+ gradient across their plasma membranes. then they use the proton-motive force to make ATP in the cell as well as to rotate the flagella and pump nutrients and waste products across the membrane



THE 4 PARTS OF ATP SYNTHASE

external image 09_14ATPSynthase.jpg

CHEMIOSMOSIS COUPLES THE ELECTRON TRANSPORT CHAIN TO ATP SYNTHESIS

external image c8.9x16.chemiosmosis.jpg

ACCOUNTING FOR ATP PRODUCED BY CELLULAR RESPIRATION

  • during respiration, most of the energy follows this sequence: glucose --> NADH --> electron transport chain --> proton-motive force --> ATP
  • calculating the amount of ATP is not exact - this is for three reasons
    1. phosphorylation and the redox reactions are not directly coupled to each other so the ratio of number of NADH molecules to number of ATP molecules is not a whole number.
      • it is known that 1 NADH results in 10 H+ being transported across the inner mitochondrial membrane and that between 3 and 4 H+ must reenter the mitochondrial matrix through aTP synthase to generate 1 ATP
      • therefore, a single molecule of NADH generates enough proton-motive force for the synthesis of 2.5 to 3.3 ATP - it is generally rounded to 3 ATP
      • the citric acid cycle supplies electrons through FADH2 but since it enters the chain later, it only accounts for enough H+ for the synthesis of 1.5 to 2 ATP
      • these numbers also take into account the slight energetic cost of moving the ATP formed in the mitochondria to the cytoplasm
    2. the ATP yield varies depending on the type of shuttle used to transport the electrons from the cytosol to the mitochondria
      • the inner membrane of the mitochondria is impermeable to NADH so 2 electrons from NADH captured in glycolysis must be conveyed to the mitochondrion using one of several electron shuttle systems
      • depending on the shuttle used, the electrons are passed to either NAD+ or FAD in the mitochondrial matrix - FAD yields about 2 ATP but NAD+ yields about 3 ATP
    3. proton-motive force generated by the redox reactions of respiration is used to drive other types of work
      1. the proton-motive force powers the mitochondrion's uptake of pyruvate from the cytosol, BUT if all the proton-motive from the electron transport chain were used to drive ATP synthesis then one glucose molecule could generate a max of 38 ATP (34 from ocidative phosphorylation and 4 from substrate-level phosphorylation)
  • a rough estimate can be made of the efficiency of respiration = the percentage of energy from glucose transferred to ATP
    • complete oxidation of a mole of glucose releases 686kcal of energy uner standard conditions
    • phosphorylation of ADP to ATP stores at least 7.3 kcal per mole of ATP times 38 moles of ATP per mole of glucose divided by 686 kcal per mole of glucose
      • 7.3 kcal x 38 mol ATP /686 kcal = .4
      • .4 x 100 = 40%
    • therefore 40% of the potential energy in glucose has been transferred to ATP
      • actual percentage is most likely lower because of the cellular conditions
    • the rest of the energy is lost as heat - used to maintain body temperature and is also dissipated through sweat

ATP YIELD PER MOLECULE OF GLUCOSE AT EACH STAGE OF CELLULAR RESPIRATION

external image 09_17ATPYieldPerGlucose-L.jpg

NEXT: FERMENTATION and CONNECTIONS TO METABOLIC PATHWAYS