Lecture Notes: Carbohydrate Metabolism

Overview of Digestion

• Polysaccharides and Disaccharides:
  • Broken down into monosaccharides.
  • Monosaccharides include glucose, fructose, and galactose.
• Absorption Process:
  • Absorbed through capillaries into villi.
  • Transported to the liver via hepatic portal vein.

Role of the Liver

• Conversion in the Liver:
  • Fructose and galactose are converted into glucose.
• Primary Focus:
  • Carbohydrate metabolism mainly involves glucose.

Storage and Usage

• Glycogen Storage:
  • Stored in the liver and skeletal muscle.
• Glucose Utilization:
  • Produces energy.
  • Can be stored as triglycerides (fat) through lipogenesis if excess.
  • Converted to amino acids for protein synthesis or stored as glycogen (glycogenesis).

Glucose Transport and Absorption

• GI Tract Absorption:
  • Through secondary active transport using sodium glucose importers.
• Cellular Entry:
  • Through facilitated diffusion via insulin-regulated glucose transporters (GLUT).
  • Insulin increases GLUT insertion, enhancing glucose uptake.

Cellular Metabolism

• Phosphorylation:
  • Glucose converted to glucose-6-phosphate upon cell entry.
• Brain and Liver Considerations:
  • High concentration of GLUT receptors critical for brain function.

Cellular Respiration Steps

1. Glycolysis:

   • Occurs in the cytoplasm.
   • Anaerobic (does not require oxygen).
   • Breakdown of glucose (6C) into 2 pyruvic acid molecules (3C).
   • Produces 2 net ATP (uses 2 ATP, produces 4 ATP).
   • Pyruvic acid converted to lactic acid if no oxygen (then converted back to pyruvate in the liver).

2. Krebs Cycle:

   • Occurs in the mitochondria.
   • Pyruvic acid converted to acetyl-CoA.
   • Involves decarboxylation and coenzyme A (from pantothenic acid).
   • Produces CO2, NADH, FADH2, and 2 ATP.

3. Electron Transport Chain:

   • Part of aerobic respiration occurring in mitochondria.
   • Uses the NADH and FADH2 from Krebs Cycle to produce ATP.

Summary

• Energy from glucose is transferred to coenzymes through glycolysis and Krebs Cycle.
• Each glucose molecule results in formation of ATP and high-energy electron carriers (NADH, FADH2).
• Krebs Cycle also known as Citric Acid Cycle or TCA Cycle, involves multiple decarboxylation and redox reactions.