Chapter 8: How Cells Make ATP: Energy-Releasing Pathways

Redox Reactions

  • Most eukaryotes and prokaryotes carry out aerobic respiration, a form of cellular respiration requiring molecular oxygen (O2).
    • During aerobic respiration, nutrients are catabolized to carbon dioxide and water.
    • Most cells use aerobic respiration to obtain energy from glucose, which enters the cell through a specific transport protein in the plasma membrane
  • It appears that CO2 is produced by the removal of hydrogen atoms from glucose.
    • Conversely, water seems to be formed as oxygen accepts the hydrogen atoms.
    • Because the transfer of hydrogen atoms is equivalent to the transfer of electrons, this process is a redox reaction in which glucose becomes oxidized and oxygen becomes reduced
  • The products of the reaction would be the same if the glucose were simply placed in a test tube and burned in the presence of oxygen.
    • However, if a cell were to burn glucose, its energy would be released all at once as heat, which not only would be unavailable to the cell but also would actually destroy it.
    • For this reason, cells do not transfer hydrogen atoms directly from glucose to oxygen.
  • Aerobic respiration includes a series of redox reactions in which electrons associated with the hydrogen atoms in glucose are transferred to oxygen in a series of steps.
  • Energy released during aerobic respiration is used to produce up to 36 to 38 atps per molecule of glucose.

The Four Stages of Aerobic Respiration

  • The chemical reactions of aerobic respiration occur in four stages: glycolysis, formation of acetyl CoA, the citric acid cycle, and the electron transport chain and chemiosmosis.
  • During glycolysis a molecule of glucose is degraded to 2 molecules of pyruvate.
    • Substrate-level phosphorylation produces 2 ATP molecules during glycolysis, and 4 hydrogen atoms are removed and used to produce 2 NADH.
  • During the formation of acetyl CoA, the 2 pyruvate molecules each lose a molecule of carbon dioxide, and the remaining acetyl groups each combine with coenzyme A, producing 2 molecules of acetyl CoA; 1 NADh is produced per pyruvate.
  • Each acetyl group from acetyl CoA enters the citric acid cycle by combining with a four-carbon compound, oxaloacetate, to form citrate, a six-carbon compound.
    • Two acetyl CoA molecules enter the cycle for every glucose molecule.
    • For every 2 carbons that enter the cycle as part of an acetyl CoA molecule, 2 leave as carbon dioxide.
    • For every acetyl CoA, hydrogen atoms are transferred to 3 NAD+ and 1 FAD; only 1 ATP is produced by substrate-level phosphorylation.
  • Hydrogen atoms (or their electrons) removed from fuel molecules are transferred from one electron acceptor to another down an electron transport chain located in the mitochondrial inner membrane; ultimately, these electrons reduce molecular oxygen, forming water.
    • In oxidative phosphorylation the redox reactions in the electron transport chain are coupled to synthesis of ATP through the mechanism of chemiosmosis.
  • Glycolysis occurs in the cytosol, and the remaining stages of aerobic respiration take place in the mitochondria.
  • In glycolysis each glucose molecule produces 2 NADH and 2 ATPs (net).
    • The conversion of 2 pyruvates to acetyl CoA results in the formation of 2 NADH.
    • In the citric acid cycle, the 2 acetyl CoA molecules are metabolized to form 6 NADH, 2 FADH2, and 2 ATPs.
    • To summarize, we have 4 ATPs, 10 NADh, and 2 FADH2.
  • When electrons donated by the 10 NADh and 2 FADH2 pass through the electron transport chain, 32 to 34 ATPs are produced by chemiosmosis.
    • Therefore, each glucose molecule yields a total of up to 36 to 38 ATPs.
  • In chemiosmosis some of the energy of the electrons in the electron transport chain is used to pump protons across the inner mitochondrial membrane into the intermembrane space.
    • This pumping establishes a proton gradient across the inner mitochondrial membrane.
    • Protons (H+) accumulate within the intermembrane space, lowering the ph.
  • The diffusion of protons through channels formed by the enzyme ATP synthase, which extends through the inner mitochondrial membrane from the intermembrane space to the mitochondrial matrix, provides the energy to synthesize ATP.

Energy Yield of Nutrients Other Than Glucose

  • Amino acids undergo deamination, and their carbon skeletons are converted to metabolic intermediates of aerobic respiration.
  • Both the glycerol and fatty acid components of lipids are oxidized as fuel.
    • Fatty acids are converted to acetyl CoA molecules by the process of b-oxidation.

Anaerobic Respiration and Fermentation

  • In anaerobic respiration electrons are transferred from fuel molecules to an electron transport chain that is coupled to ATP synthesis by chemiosmosis; the final electron acceptor is an inorganic substance such as nitrate or sulfate, not molecular oxygen.
  • Fermentation is an anaerobic process that does not use an electron transport chain.
    • There is a net gain of only 2 ATPs per glucose; they are produced by substrate-level phosphorylation during glycolysis.
    • To maintain the supply of NAD+ essential for glycolysis, hydrogen atoms are transferred from NADH to an organic compound derived from the initial nutrient.
  • Yeast cells carry out alcohol fermentation, in which ethyl alcohol and carbon dioxide are the final waste products.
  • Certain fungi, prokaryotes, and animal cells carry out lactate (lactic acid) fermentation, in which hydrogen atoms are added to pyruvate to form lactate, a waste product.