Carbohydrate Digestion in Small Intestine

Classification of Carbohydrates

• Carbohydrates can be classified into three types based on the number of sugar units:
  • Monosaccharides: simplest form of sugars, which cannot be further hydrolyzed
  • Disaccharides: consist of two monosaccharide units joined together by glycosidic linkage
  • Oligosaccharides: 2-10 monosaccharide units linked together
  • Polysaccharides: higher polymers of carbohydrates, containing > 10 monosaccharide units per molecule

Monosaccharides

• Monosaccharides contain the elements carbon, hydrogen, and oxygen, and have the general formula Cn(H2O)n, where n ≥ 3
• Monosaccharides cannot be broken down (non-hydrolysable) to smaller carbohydrates
• Classification of monosaccharides is based on:
  • Number of carbon atoms: trioses to heptoses
  • Functional groups: aldehyde (-CHO) or ketose group (-C=O)
• Cyclic structure of monosaccharides:
  • In aqueous solution, the aldehyde or ketone group spontaneously reacts with alcohol groups on the downstream carbon to form ring structures
  • New chiral carbon or anomeric center is formed
• Chirality:
  • A carbon atom that has four different atoms or groups of atoms bonded to it is called a chiral carbon atom or chiral center
  • All monosaccharides contain at least one asymmetrical (chiral) carbon atom and are therefore optically active, except dihydroxyacetone

Importance of Carbohydrates

• Carbohydrates are the major source of energy
• Precursors for many organic compounds (fats, amino acids)
• Constitute an important part of free nucleotides
• Prevent blood clots
• Carbohydrate polymers lubricate skeletal joints
• Carbohydrates are found on the surface of cells, participate in recognition and adhesion between cells
• ABO blood markers found on red blood cells are made up of carbohydrates

Digestion, Absorption, and Transport of Carbohydrates

• Carbohydrate digestion in the small intestine:
  • Food bolus enters into the duodenum, exposed to pancreatic juice, containing pancreatic bicarbonate and pancreatic amylase
• Absorption of carbohydrates:
  • Monosaccharides are almost completely absorbed from the small intestine
  • Two types of glucose transport: Na+-dependent glucose transporters and facilitative glucose transporters
  • Two mechanisms of glucose absorption: active absorption and facilitated absorption

Glucose Transport Proteins

• Glucose enters absorptive cells by binding to transport proteins that bind glucose on one side and release it on the opposite side
• Two types of glucose transport proteins:
  • Na+-dependent glucose transporters (SGLT 1 and SGLT 2)
  • Facilitative glucose transporters

Glycolysis

• Glycolysis is a cascade of reactions that converts glucose into:
  • Two pyruvate molecules
  • Two lactate molecules (mitochondria/O2 not available)
• Glycolysis can run independent of mitochondria/O2, it is mainly called anaerobic oxidation of glucose
• Intracellular site and tissue distribution: Glycolysis occurs in the cell cytoplasm of all tissues of the body