Overview

• Mitochondria play a central role in eukaryotic aerobic metabolism.
• ATP production is not the only important mitochondrial function. They are host to the citric acid cycle, fatty acid beta oxidation pathway, and the pathways of amino acid oxidation.
• The mitochondria acts in thermogenesis, steroid synthesis, and apoptosis (programmed cell death).
• The discovery of the diverse and important roles of mitochondria has stimulated much current research on the biochemistry of this organelle.
• Mitochondria trace the evolutionary history of mitochondria to bacteria. Over 1.45 billion years ago, an endosymbiotic relationship arose between bacteria and a primitive eukaryote or eukaryotic progenitor.
• Mitochondria are ubiquitous in modern eukaryotes, and their bacterial origin is evident in almost every aspect of their structure and function.
• Electrons flow from electron donors (oxidizable substrates) through a chain of membrane bound carriers to a final electron acceptor with a large reduction potential. The final acceptor is molecular oxygen, O2.
• The appearance of oxygen in the atmosphere some 2.3 billion years ago, and its harnessing in living systems via the evolution of oxidative phosphorylation, made more complex forms of life possible.
• The free energy made available by downhill (exergonic) electron flow is coupled with the uphill transport of protons across a proton impermeable membrane. The free energy of fuel oxidation is thus conserved as transmembrane potential.
• The transmembrane flow of protons back down their electrochemical gradient through specific protein channels provides the free energy for the synthesis of ATP. This process is catalyzed by a membrane protein complex (ATP synthase) that couples proton flow to phosphorylation of ADP.
• The chemiosmotic theory is that transmembrane differences in proton concentration are the reservoir for the energy extracted from biological oxidation reactions.
• Chemiosmotic theory:

  1. Reduced substrate (fuel) donates electrons
  2. Electron carrier pump H+ out as electrons flow to O2
  3. Energy of electron flow is stored as electrochemical potential
  4. ATP synthase uses electrochemical potential to synthesize ATP.