Human Physiology: Glucose as Fuel

Glucose as the Key Fuel for Humans

• Constant glucose supply is essential for human life.
• Brain's preferred energy source: Glucose is the primary fuel for brain function.
• Required for cells with limited or no mitochondria: Red blood cells (RBCs) rely solely on glucose for energy as they lack mitochondria.
• Essential for exercising muscles: During physical activity, muscles utilize glucose through anaerobic glycolysis, generating energy even in the absence of oxygen.

Glucose Transport Mechanisms

• Na+-independent facilitated diffusion (GLUT):
  • Glucose moves across cell membranes following its concentration gradient.
  • Does not require energy (ATP).
  • Facilitated by a family of at least 14 glucose transporters (GLUT-1 to 14).
• Na+-monosaccharide cotransporter system (SGLUT):
  • Glucose is transported against its concentration gradient, moving from low to high concentrations.
  • Requires energy.
  • The movement of glucose is coupled with the transport of sodium ions (Na+) into the cell.
  • This mechanism operates in the epithelial cells of the intestines and renal tubules.

Sources of Blood Glucose

• Dietary carbohydrates: Main source of glucose after meals. Excess glucose is stored as glycogen in the liver and muscles.
• Glycogen degradation (Glycogenolysis):
  • Liver glycogen is broken down to release glucose during fasting.
  • Muscle glycogen is broken down during exercise to provide energy for muscle activity.
• Gluconeogenesis (Glucose synthesis):
  • Process of producing glucose from non-carbohydrate sources (amino acids).
  • Occurs during prolonged fasting.

Dietary Carbohydrate Sources for Glucose

• Free Monosaccharides:
  • Primarily glucose and fructose.
  • Fructose is converted into glucose in the liver.
• Disaccharides:
  • Sucrose (glucose + fructose)
  • Lactose (glucose + galactose)
  • Maltose (glucose + glucose)
  • Digested into monosaccharides by disaccharidases (sucrase, lactase, maltase) in the small intestine.
  • Fructose and galactose are converted into glucose in the liver
• Polysaccharides:
  • Starch (plant source, e.g., rice, potato, flour)
  • Glycogen (animal source)
  • Digested by amylase into maltose, then by maltase into glucose in the gastrointestinal tract (GIT).

Fates of Blood Glucose

• Catabolic Pathways (Break down):
  • Energy production (ATP):
    • Glycolysis followed by the Krebs cycle
    • Anaerobic glycolysis: Produces lactate
    • Aerobic glycolysis: Produces pyruvate
  • Synthesis of other products:
    • Hexose monophosphate pathway (HMP): Generates NADPH and ribose 5-phosphate.
• Synthetic Pathways (Build up):
  • Glycogen synthesis
  • Lipogenesis (fat synthesis)
  • Protein synthesis
  • Other carbohydrates

Glucose Metabolism Summary

• Hexose Monophosphate Pathway:
  • Involved in NADPH and ribose-5-phosphate production.
• Glycogen Synthesis:
  • Process of storing glucose as glycogen.
• Glycolysis:
  • Process of breaking down glucose to pyruvate.
  • Can occur with or without oxygen.
  • Leads to lactate production in anaerobic conditions.
• Krebs Cycle (Citric Acid Cycle):
  • Further breakdown of pyruvate to yield NADH, FADH2 and ATP.
• Electron Transport Chain:
  • Utilizes NADH and FADH2 to drive oxidative phosphorylation and produce ATP.