Digestive Day 4 and Review

Digestion of Carbohydrates

  • Starch Definition: A polysaccharide which is one of the main forms of stored energy in plants.

  • Digestion Process:

    • Starts in the mouth with the enzyme amylase.

    • Starch is first broken down into polysaccharide, then into disaccharide, and finally into monosaccharide (glucose).

    • Use of cracker as an example:

    • When chewed, it becomes mushy due to enzymatic action from salivary amylase.

    • Limitations in the Stomach:

    • The acidic environment of the stomach (HCl) denatures amylase and pepsin digests it.

    • Thus, only about 50% of carbohydrate digestion occurs before entering the small intestine.

Role of the Pancreas and Small Intestine

  • Pancreatic Juice:

    • Contains pancreatic amylase which continues the breakdown of carbohydrates.

  • Brush Border Enzymes:

    • Important enzymes in the small intestine include:

    • Dextranase and Glucoamylase: Hydrolyze oligosaccharides into monosaccharides.

    • Maltase: Hydrolyzes maltose.

    • Sucrase: Hydrolyzes sucrose.

    • Lactase: Hydrolyzes lactose.

    • Final product of digestion is monosaccharides which can be absorbed.

Absorption of Monosaccharides

  • 80% of absorbed sugar is glucose.

  • Transport Mechanisms:

    • Glucose and galactose are absorbed via carrier proteins.

    • Fructose is absorbed via facilitated diffusion (requires a carrier, but not a transporter).

  • Osmolarity:

    • Increased osmolarity in the intestines draws water into the intestinal walls, enhancing absorption.

  • Transport Pathway:

    • Monosaccharides are transported into the bloodstream via the superior mesenteric vein to the hepatic portal vein, leading to the liver.

Lactose Intolerance

  • Definition: Lack of lactase enzyme leads to undigested lactose in the large intestine, causing increased osmolarity and diarrhea due to water retention.

  • Statistical Insights:

    • About 90% of Black Americans and nearly all Asians are lactose intolerant.

    • Fermented products like yogurt and cheese can often be consumed as they contain pre-digested lactose.

Digestion of Proteins

  • Proteolytic Enzymes: Include:

    • Pepsin: Begins digestion in the stomach under acidic conditions, breaking bonds in proteins.

    • Trypsin and Chymotrypsin: Continue digestion in the small intestine after pancreatic juice neutralizes the pH.

  • Further Breakdown: Oligopeptides are broken down into amino acids by:

    • Carboxypeptidase, Aminopeptidase, and Dipeptidase: All these enzymes break peptide bonds. The conversion process allows amino acids to be easily absorbed into the bloodstream.

Lipid Digestion

  • Role of Lipases:

    • Begins with lingual lipase in the mouth but is activated in the stomach with gastric lipase.

    • Only about 15% of lipids are digested in the stomach.

  • Emulsification: Emulsifies fat globules into smaller droplets without breaking bonds.

  • Bile and Pancreatic Lipase: Work together to further break down triglycerides into monoglycerides and free fatty acids.

  • Micelle Formation:

    • Micelles are formed with bile salts, making absorption of lipids more efficient.

Absorption of Fats

  • Chylomicron Formation:

    • Once in the intestinal cells, free fatty acids and monoglycerides are reconstituted into triglycerides and packed into chylomicrons in the Golgi apparatus.

    • Chylomicrons enter the lymphatic system through lacteals before entering the circulatory system.

  • Complex Absorption Process: Lipids are more complex compared to carbohydrates and proteins due to their hydrophobic nature.

Nutrient Summary and Additional Nutrient Absorption

  • Nucleic Acids: Hydrolyzed into nucleotides, sugars, and phosphates through pancreatic nucleases and then absorbed like other energies.

  • Fat-Soluble Vitamins (A, D, E, K): Absorbed along with dietary fats.

  • Minerals: Such as sodium, iron, calcium absorbed via specific transporters:

    • Iron: Absorbed as Fe²⁺, inhibited by hepcidin to prevent overload.

    • Calcium: Absorbed in the presence of Vitamin D.

Water Absorption and Conditions Affecting Absorption

  • Absorption primarily occurs through osmosis, following salt absorption.

  • Dehydration Disorders:

    • Diarrhea: Too little water absorbed, leading to loose feces. Common causes include lactose intolerance.

    • Constipation: Excessive water absorption leads to hard stools, caused by slow fecal movement through the intestines.

Key Concepts in the Digestive System

  • Digestive Process Overview:

    • Chemical digestion starts in the mouth (for carbs) and continues in the stomach (for proteins) and small intestine (for all). Different enzymes and conditions apply through each organ.

  • Transport Mechanisms:

    • Carbohydrates and proteins are absorbed directly into blood capillaries, whereas fats are absorbed indirectly via lymphatics.

    • Parathyroid Hormone: Regulates calcium levels by stimulating the release from bones and enhancing absorption via Vitamin D.

  • Digestive Enzyme Production: Various cells produce hydrochloric acid (parietal cells), pepsinogen (chief cells), bile (liver), and lipases (pancreas).

Questions from the Lecture

  • What stimulates gallbladder contraction?

    • Answer: Cholecystokinin (CCK).

  • Which enzyme catalyzes carbonic acid formation in red blood cells?

    • Answer: Carbonic anhydrase.

  • Types of epithelial tissue in digestive tract:

    • Simple columnar predominantly in the digestive tract; stratified squamous at the entry and exit points (mouth, pharynx, anus).