Starch Digestion

Overview

• Focus on starch digestion.
• Review the structure of starch.
• Discuss the steps involved in starch digestion.

Structure of Starch

• Starch is found in foods like bread.
• Composed of two forms of glucose polymers:
  • Amylose: Linear chain of glucose linked by Alpha 1,4 glycosidic bonds.
  • Amylopectin: Branched chains of glucose with branch points linked by Alpha 1,6 glycosidic bonds.

Ingestion and Initial Digestion

• Mouth:
  • Physical breakdown by jaws, teeth, and tongue.
  • Chemical breakdown by saliva containing salivary alpha amylase.
  • Salivary alpha amylase hydrolyzes Alpha 1,4 glycosidic bonds partially.
• Stomach:
  • Starch is partially hydrolyzed to oligosaccharides and shorter polysaccharides.
  • Salivary amylase is inactivated by the stomach's acidic environment.

Small Intestine Digestion

• Pancreas:
  • Secretes pancreatic alpha amylase into the small intestine.
  • Continues breaking down Alpha 1,4 glycosidic bonds.
• Intestinal Cells (Enterocytes):
  • Contain brush border enzymes.
  • Maltase: Hydrolyzes maltose (two glucose molecules).
  • Sucrase-Isomaltase: Hydrolyzes both Alpha 1,4 and 1,6 glycosidic bonds.

Glucose Absorption

• Sodium-Glucose Linked Transporters (SGLTs):
  • Located on apical surface of enterocytes.
  • Co-transport sodium ions and glucose into cells.
• Glucose Transport to Bloodstream:
  • GLUT2 transporter facilitates movement into the bloodstream.
  • Glucose used for energy or stored as glycogen in the liver.

Resistant Starch

• Not fully digested in the small intestine.
• Reaches the colon (large intestine) and undergoes fermentation by gut microbiota.
  • Produces short-chain fatty acids, used by the body.
• Remaining starch becomes waste and is excreted.

Conclusion

• Starch digestion involves breakdown and absorption processes starting from the mouth to the small intestine, with some reaching the colon for fermentation.
• Glucose produced is a key energy source or stored as glycogen.