Overview
This lecture explains how proteins are digested into amino acids, absorbed through the small intestine, transported to the liver, and used to build essential body components.
Protein Structure and Function
- Proteins are large molecules made of one or more chains of amino acids.
- Amino acids are the basic building blocks of proteins.
- The body uses amino acids to build enzymes, antibodies, hormones, muscle proteins, and collagen.
Digestion of Proteins
- Protein digestion begins in the stomach with the enzyme pepsin.
- Pepsin breaks peptide bonds, forming polypeptide chains (long chains of amino acids).
- Polypeptides move to the small intestine for further digestion.
Enzymatic Breakdown in the Small Intestine
- Pancreatic enzymes (trypsin, chymotrypsin, carboxypeptidase) continue digestion in the small intestine.
- Brush border enzymes on the microvilli finish breaking polypeptides into smaller peptides and individual amino acids.
- Amino acids, being small, are absorbed through the lining of the small intestine into the bloodstream.
Absorption and Transport to the Liver
- Capillaries in the intestinal wall pick up amino acids and other nutrients.
- Nutrients travel to the liver via the hepatic portal system (a network of veins).
- The hepatic portal vein carries nutrient-rich blood from the digestive tract to the liver.
Distribution of Amino Acids in the Body
- The liver takes nutrients it needs first before blood enters general circulation.
- Remaining amino acids circulate to body cells for building specialized proteins.
Key Terms & Definitions
- Protein — Large molecule made of one or more amino acid chains.
- Amino acid — Basic building block of protein.
- Pepsin — Stomach enzyme that begins protein digestion by breaking peptide bonds.
- Polypeptide — Chain of many amino acids linked together.
- Peptide — Two or more amino acids linked together.
- Brush border enzymes — Enzymes on the small intestine microvilli that complete protein digestion.
- Hepatic portal system — Network of veins directing blood from the digestive tract to the liver.
- Hepatic portal vein — Vein transporting blood from the digestive tract to the liver.
Action Items / Next Steps
- Review the roles of digestive enzymes (pepsin, trypsin, chymotrypsin, carboxypeptidase).
- Study the function of the hepatic portal system in nutrient transport.