Digestion and Absorption in the Small Intestine

Digestion of Nucleic Acids

• Nuclei of ingested cells in food contain DNA and RNA
• Pancreatic nucleases break down nucleic acids into nucleotide monomers
• Further broken down into free nitrogenous bases, pentose sugars, and phosphate ions
• Breakdown products are actively transported by special carriers in epithelium of villi

Absorption of Vitamins, Electrolytes, and Water

• Fat-soluble vitamins (A, D, E, and K) are absorbed by diffusion
• Water-soluble vitamins (C and B) are absorbed by passive or active transport
• Vitamin B12 binds with intrinsic factor and is absorbed
• In large intestine, vitamin K and B vitamins from bacterial metabolism are absorbed

Absorption of Electrolytes and Water

• Iron and calcium are absorbed in duodenum
• Iron and calcium absorption is related to need
• Ionic iron is stored in mucosal cells with ferritin
• When needed, iron is transported in blood by transferrin
• Ca2+ absorption is regulated by vitamin D and parathyroid hormone (PTH)
• 9 L of water from GI tract secretions enters small intestine
• 95% of water is absorbed in small intestine by osmosis
• Most of the rest is absorbed in large intestine

Mechanical Breakdown and Digestive Processes

• Mechanical breakdown: processes that physically mix or break down food into smaller fragments
• Segmentation: local constriction of intestine that mixes food with digestive juices
• Digestion: catabolic sets that involves enzymes that break down food molecules
• Absorption: passage of digested end products from intestinal mucosa into blood or lymph
• Defecation: elimination of undigested substances via anus in form of feces

Organization of Digestive System

• Peritoneum: serous membranes of abdominal cavity
• Visceral peritoneum: membrane on external surface of most digestive organs
• Parietal peritoneum: membrane that lines body wall
• Peritoneal cavity: fluid-filled space between two peritoneums
• Serous fluid lubricates mobile organs

The Pancreas

• Mostly retroperitoneal; deep to greater curvature of stomach; between spleen and small intestine
• Exocrine function: produce pancreatic juice
• Acini: clusters of secretory cells to produce pancreatic juices
• Ducts: secrete to duodenum via main pancreatic duct; smaller duct cells produce water and bicarbonate
• Endocrine function: secretion of insulin and glucagon by pancreatic islet cells
• Composition of pancreatic juice: watery, alkaline solution (pH 8) with electrolytes, digestive enzymes, and nucleases

Regulation of Bile and Pancreatic Secretions

• Bile and pancreatic juice secretions are stimulated by neural and hormonal controls
• Hormonal controls include: cholecystokinin (CCK) and secretin

Lipids

• Dietary requirements: unsaturated fats, saturated fats, and cholesterol
• Goal is to keep total cholesterol < 5.2 mmol/L
• 20-35% of calorie intake should be from fat

Proteins

• Dietary sources: animal products, soybeans, legumes, nuts, and cereals
• Complete proteins contain all essential amino acids
• Incomplete proteins lack some essential amino acids
• Uses in body: structural materials, functional molecules
• Three factors determine whether amino acids are used to synthesize proteins or burned as energy:
  • All-or-none rule
  • Adequacy of caloric intake
  • Hormonal controls

Carbohydrate Metabolism

• Carbohydrate metabolism is essentially glucose metabolism
• All food carbohydrates are transformed to glucose
• Glucose is converted to CO2 and water in various processes
• When cellular energy is high, glucose breakdown is inhibited
• Glucose is converted to glycogen (glycogenesis) or fat (lipogenesis)
• When blood glucose levels fall, glycogenolysis occurs

Lipid Metabolism

• Lipids provide a greater energy yield than glucose or protein catabolism
• Most products of fat digestion are transported in lymph as chylomicrons
• Broken down into fatty acids and glycerol

Lipogenesis

• Triglyceride synthesis occurs when cellular ATP and glucose levels are high
• Dietary glycerol and fatty acids not needed for energy are stored as triglycerides
• 50% is stored in adipose tissue; other 50% is deposited in other areas
• Glucose is easily converted to fat

Learn about the digestion process of nucleic acids into nucleotide monomers and the absorption of vitamins, electrolytes, and water in the small intestine. Topics include breakdown by pancreatic nucleases, transportation by epithelium of villi, and absorption of fat-soluble vitamins through diffusion.