1.5

• Biochemical energy production process

  1. Digestion

  2. Acetyl group formation

  3. Citric acid cycle

  4. Electron transport chain and oxidative phosphorylation
• Digestion


  • Begins in the mouth (saliva contains starch-digesting enzymes)
  • Continues in the stomach (gastric juices)
  • Completed in the small intestine (the majority of digestive enzymes and bile salts)
• End products of digestion

  • Glucose and other monosaccharides from carbohydrates
  • Amino acids from proteins
  • Fatty acids and glycerol from fats and oils
• Acetyl group formation


  • Involves numerous reactions, some in the cytosol of cells and some in cellular mitochondria
  • Small molecules from digestion are further oxidized
• Primary products of acetyl group formation

  • Two-carbon acetyl units (which become attached to coenzyme A to give acetyl CoA)
  • Reduced coenzyme NADH
• Citric acid cycle


  • Occurs inside mitochondria
  • Acetyl groups are oxidized to produce CO2 and energy
  • Some energy released is lost as heat, some is carried by NADH and FADH2 to the fourth stage
• Electron transport chain and oxidative phosphorylation

  • Occurs inside mitochondria
  • NADH and FADH2 supply the "fuel" (hydrogen ions and electrons) needed for the production of ATP molecules
  • Molecular O2, inhaled via breathing, is converted to H2O in this stage
• The reactions in stages 3 and 4 are the same for all types of foods (carbohydrates, fats, proteins)
• Common metabolic pathway


  The sum total of the biochemical reactions of the citric acid cycle, the electron transport chain, and oxidative phosphorylation
• The reactions of stages 1 and 2 of biochemical energy production differ for different types of foodstuffs
• The Chemistry at a Glance feature on page 790 summarizes the four general stages in the process of production of biochemical energy from ingested food
• The diagram is a very simplified version of the "energy generation" process that occurs in the human body