Outline of Campbell's Biology Text: Chapter 9

I. An Introduction to Respiration

  1. Review of ATP/ADP cycle.
    1. Hydrolysis of ATP's unstable phosphate bonds is exergonic; that is, energy is released:
    2. Direct ATP hydrolysis would release energy as heat, a form unavailable for cellular work.
    3. Instead, enzymes catalyze the transfer of the terminal phosphate group from ATP to another molecule.
    4. The molecule receiving the phosphate group is said to be phosphorylated or activated and becomes more reactive in the process.
    5. The phosphorylated substance loses its phosphate group as cellular work is performed.
    6. Cells must replenish the ATP supply to continue cellular work.
    7. Respiration provides the energy for ATP synthesis from ADP and inorganic phosphate.
  2. An Overview of Cellular Respiration
    1. There are three metabolic stages of cellular respiration:
      1. Glycolysis
      2. Krebs Cycle
      3. Electron transport chain and oxidative phosphorylation
    2. Glycolysis is a catabolic pathway that:
      1. Occurs in the cytosol.
      2. Partially oxidizes glucose (6C) into two pyruvic acid (3C) molecules.
    3. The Krebs Cycle is a catabolic pathway that:
      1. Is located within the mitochondrial matrix.
      2. Completes glucose oxidation by breaking down a pyruvic acid derivative (acetyl CoA) into carbon dioxide.
    4. Glycolysis and the Krebs Cycle:
      1. Directly produce a small amount of ATP.
      2. Supply energized electrons that indirectly drive most ATP production by oxidative phosphorylation.
    5. Oxidative phosphorylation accounts for most ATP production during respiration.
      1. Process includes an electron transport chain made of electron-carrier molecules built into the inner mitochondrial membrane.
      2. Oxygen pulls energized electrons harvested during glycolysis and the Krebs Cycle, down the electron transport chain to a lower energy state.
      3. This exergonic slide of electrons is coupled to ATP synthesis.
    6. For each molecule of glucose oxidized to carbon dioxide and water, the cell makes about 36 to 38 ATP molecules.
  3. Substrate-Level Phosphorylation
    1. There are two basic mechanisms that couple the exergonic oxidation of glucose to the endergonic synthesis of ATP: substrate-level phosphorylation and chemiosmosis.
    2. Substrate-level phosphorylation = The direct enzymatic transfer of phosphate to ADP from an intermediate substrate in catabolism.
    3. Reaction is energetically possible because the phosphate bonds of the intermediate are more unstable than those of ATP.
    4. An example is the hydrolysis of PEP (phosphoenolpyruvate) during glycolysis:
      1. PEP + H2O --> pyruvic acid + P Delta-G = -14.8 kcal/mol
      2. ADP + P --> ATP + H2O Delta-G = +7.5 kcal/mol
      3. Total Rsn is PEP + ADP + P --> ATP + Pyruvic Acid Delta-G = -7.5 kcal/mol
    5. Only a small percentage of ATP is produced this way: most ATP is produced by oxidative phosphorylation.
  4. Chemiosmotic Coupling: The Basic Principle
    1. The mechanism for coupling exergonic electron flow from the oxidation of food to endergonic ATP production is chemiosmosis.
    2. Chemiosmosis
      1.  
      2. The coupling of exergonic electron flow down an electron transport chain to endergonic ATP production by the creation of an electrochemical proton gradient across a membrane. The proton gradient drives ATP synthesis as protons diffuse back across the membrane.
      3. Proposed by British biochemist, Peter Mitchell (1961).
      4. The term chemiosmosis emphasizes a coupling between (1) chemical reactions and (2) transport processes.
      5. Applies to oxidative phosphorylation, photophosphorylation and other cases of cellular work.
    3. Membrane structure plays a prominent functional role in chemiosmosis:
      1. Integral membrane proteins translocate H+ across a membrane, creating a proton (of pH) gradient.
      2. The membrane's phospholipid bilayer is impermeable to H+, so it counteracts the tendency for protons to leak back across the membrane by diffusion.
      3. Transmembrane protein complexes called ATP synthase use the potential energy stored in a proton gradient to make ATP by allowing H+ to diffuse down the gradient, back across the membrane. As protons diffuse through the ATP synthase complex, ATP synthase phosphorylates ADP.
    4. The energy required to create the proton gradient comes from:
      1. Light - during the energy-capturing reactions of photosynthesis.
      2. Oxidation of glucose - during glycolysis and the Krebs Cycle of respiration.
    5. During respiration, chemiosmosis occurs across the inner membrane of the mitochondria.
      1. Using energy from the oxidation of glucose, the electron transport chain translocates H+ from the mitochondrial matrix, across the inner membrane to the intermembrane space.
      2. Cristae or infoldings of the inner mitochondrial membrane, increase the surface area available for chemiosmosis to occur.