8 integration of metabolism

• Metabolism is a continuous process, with thousands of reactions simultaneously occurring in the living cell
• Biochemists prefer to present metabolism in the form of reactions and metabolic pathways for convenience
• Humans possess enormous capacity for food consumption
• Obesity is primarily a consequence of overconsumption
• Integration of major metabolic pathways of energy metabolism

  1. Glycolysis

  2. Fatty acid oxidation

  3. Degradation of amino acids

  4. Citric acid cycle

  5. Oxidative phosphorylation

  6. Hexose monophosphate shunt

  7. Gluconeogenesis

  8. Glycogen metabolism
• Glycolysis


  1. Degradation of glucose to pyruvate

  Generates 8 ATP

  2. Pyruvate converted to acetyl CoA
• Fatty acid oxidation


  1. Sequential degradation of fatty acids

  2. Release of 2-carbon fragment (acetyl CoA)

  3. Energy trapped in NADH and FADH2
• Degradation of amino acids

  1. Excess amino acids degraded

  2. Utilized to meet fuel demands

  3. Glucogenic amino acids serve as glucose precursors

  4. Ketogenic amino acids serve as acetyl CoA precursors
• Citric acid cycle


  1 Acetyl COA is the key & common 2 Acetyl CoA enters TCA cycle & Oxidized to CO2

  3. Final common metabolic pathway for oxidation of foodstuffs
• Oxidative phosphorylation


  1. NADH and FADH2 oxidized in ETC

  2. Coupled with oxidative phosphorylation to generate ATP
• Hexose monophosphate shunt


  1. Liberation of NADPH

  2. Production of ribose sugar
• Gluconeogenesis


  1. Synthesis of glucose from non-carbohydrate sources

  2. Precursors include pyruvate, glycerol, amino acids
• Glycogen metabolism


  1. Glycogen is storage form of glucose

  2. Degraded (glycogenolysis)

  3. Synthesized (glycogenesis)
• Regulation of metabolic pathways

  • Availability of substrates
  • Covalent modification of enzymes
  • Allosteric regulation
  • Regulation of enzyme synthesis
• The liver is the body's central metabolic clearing house
• Major metabolic functions of the liver in an absorptive state

  • Increased glycolysis
  • Increased glycogenesis
  • Increased hexose monophosphate shunt
  • Decreased gluconeogenesis
• Adipose tissue is regarded as the energy storage tissue
• Major metabolic functions of adipose tissue in an absorptive state

  • Increased glucose uptake
  • Increased glycolysis
  • Increased hexose monophosphate shunt
  • Increased synthesis of fatty acids and triacylglycerols
  • Inhibited degradation of triacylglycerols
• The metabolism of skeletal muscle is variable depending on its needs
• Important metabolic functions of skeletal muscle in an absorptive state

  • Higher glucose uptake
  • Increased glycogen synthesis
  • Fatty acids as fuel sources
  • Higher incorporation of amino acids into proteins
• The human brain constitutes about 2% of the body's weight
• The brain utilizes as much as 20% of the oxygen consumed by the body
• Major metabolic functions of the brain in an absorptive state

  • Glucose as the only fuel source
  • 120 g of glucose utilized per day
  • 50% of energy consumed by Na+-K+-ATPase
• Starvation may be due to food scarcity or the desire to rapidly lose weight
• Starvation is a metabolic stress which imposes certain metabolic compulsions on the organism
• Triacylglycerol (fat) of adipose tissue is the predominant energy reserve of the body
• Survival time on starvation is mostly dependent on fat stores
• Protein can meet the fuel demands of the body during starvation
• Starvation is associated with a decrease in insulin level


  Increase in glucagon
• Major metabolic functions of the liver in starvation

  • (Carbohydrate)Increased gluconeogenesis
  • (Lipid metabolism)Elevated glycogen degradation
• Major metabolic functions of adipose tissue in starvation

  • (Carbohydrate)Lowered glucose uptake
  • (Lipid metabolism)Elevated degradation of triacylglycerol
• Major metabolic functions of skeletal muscle in starvation

  • (Carbohydrate metabolism )Depressed glucose uptake
  • (Lipid metabolism)Utilization of fatty acids and ketone bodies by muscle
  • (Protein metabolism )Degradation of muscle proteins for glucose synthesis
• Major metabolic functions of the brain in starvation

  • Dependence on glucose during first 2 weeks
  • Dependence on ketone bodies after 3 weeks