Digestion of Proteins and Carbohydrates

Protein Digestion

• Begins with pepsin in the stomach due to the stomach's acidic environment (pH 1-2).
• Pepsin and trypsin (both endopeptidases) break down polypeptides into dipeptides.
• Dipeptides move to the Brush Border Enzymes (BBE) on the microvilli.
• BBE convert dipeptides into two amino acids.
• A sodium-potassium pump (\text{Na}^+ / \text{K}^+ pump) operates in the background, driven by ATP.
• Amino acids are cotransported with sodium (\text{Na}^+) via secondary active transport.
• Amino acids then exit the enterocyte through facilitated diffusion to the capillary of the villus.
• The hepatic portal vein transports the amino acids to the liver.

Carbohydrate Digestion

• Begins in the mouth with salivary amylase breaking down carbohydrates.
• No carbohydrate digestion occurs in the stomach.
• In the small intestine, pancreatic amylase further breaks down carbohydrates into disaccharides.
• Brush border enzymes (on microvilli) convert disaccharides into monosaccharides.
• A sodium-potassium pump (\text{Na}^+ / \text{K}^+ pump) operates in the background, utilizing ATP.
• Monosaccharides are cotransported with sodium (\text{Na}^+) via secondary active transport.
• Monosaccharides uses facilitated diffusion to exit into site.
• The hepatic portal vein transports the monosaccharides to the liver.

Similarities in Protein and Carbohydrate Digestion

• Both involve Brush Border Enzymes (BBE).
• Both utilize secondary active transport.
• Both result in the transport of nutrients to the liver via the hepatic portal vein.