Chemical Digestion and Absorption Quiz

Chemical Digestion and Absorption

• Digestion is the process of breaking down complex food substances (carbohydrates, proteins, and lipids) into absorbable products.
• Absorption is the uptake of these products, where they are transported into cells in the small intestine.
• Several organs contribute to digestion and absorption, including the mouth, stomach, small intestine, liver, pancreas, and gallbladder.
• Each organ plays a specific role in degrading and/or absorbing food, utilizing enzymes and other substances for the process.

Digestion of Carbohydrates

• Dietary carbohydrate intake is 250-850 grams per day, contributing 50-60% of the diet.
• Major carbohydrates are polysaccharides (starch and glycogen), oligosaccharides (di-, tri-, tetra-, and pentasaccharides), and monosaccharides (glucose and fructose).
• In the mouth, salivary amylase converts starch to maltose.
• Stomach acid denatures salivary amylase, temporarily stopping carbohydrate digestion.
• In the small intestine, pancreatic amylase digests starch to maltose, maltriose, and dextrins within 15-30 minutes of chyme entry.
• Pancreatic amylase functions best in an alkaline environment, aided by bile salts.
• Brush border enzymes (maltase, sucrase, lactase) further break down disaccharides into monosaccharides (glucose, fructose, and galactose).
• Carbohydrate digestion is complete in the small intestine, primarily the jejunum and proximal ileum.
• Glucose accounts for 80% of end products, while galactose and fructose combine for the remaining 20%.
• Pentoses are also formed from nucleic acids and partial digestion of pentosans.

Absorption of Monosaccharides

• Monosaccharides are absorbed by enterocytes (intestinal cells) via various transport mechanisms.
• Glucose and galactose are absorbed via active transport with sodium.
• Fructose is absorbed by facilitated diffusion through GLUT5.
• Absorbed monosaccharides enter the bloodstream via the portal vein and travel to the liver.
• Diagrams show the active transport process with illustrations of SGLT1 and GLUT2, highlighting the crucial role of Na⁺ transport.

Fate of Glucose in the Body

• About 5% of absorbed glucose is stored as glycogen in the liver and muscles.
• Approximately 50-60% of absorbed glucose is catabolised to produce energy, yielding 4 kcal/gram when completely oxidized.
• Some intermediary products of glucose breakdown can be converted to amino acids.
• About 30-40% of absorbed glucose is converted to fat and stored.

Chemical Digestion: Proteins

• Protein digestion involves enzymes like pepsin in the stomach and various pancreatic and brush border enzymes in the small intestine.
• Pancreatic enzymes (trypsin, chymotrypsin, carboxypeptidase) break down polypeptides into smaller peptides.
• Brush border enzymes (aminopeptidases, carboxypeptidases, and dipeptidases) further break down these peptides to amino acids.

Absorption of Amino Acids & Dipeptides

• Amino acids and dipeptides are absorbed into epithelial cells through active transport using sodium or hydrogen ions.
• They then move out of the epithelial cells into the blood via diffusion.

Digestion of Fats

• Dietary fats include simple fats (triglycerides and cholesterol), compound fats (phospholipids), and associated fats (steroids and fat-soluble vitamins).
• Emulsification is the process where fat globules are broken down into smaller droplets by bile salts, increasing their surface area for lipolytic enzyme action.
• Lipases, aided by colipase, break down triglycerides into monoglycerides and free fatty acids.
• Phospholipids are digested by pancreatic phospholipase. Free cholesterol is absorbed intact.
• Micelles are formed from fatty acids, bile salts, monoglycerides, diglycerides, phospholipids and cholesterol.
• Micelles aid the absorption of fats by transporting them to the brush border of the small intestine for absorption.

Absorption of Fats

• Small fatty acids enter cells via simple diffusion and then enter the bloodstream.
• Larger fats enter cells as micelles.
• Bile salts are reabsorbed into the blood, reforming bile.
• Fat-soluble vitamins are absorbed along with fats, as they are found within the micelles.
• Inside epithelial cells, fats are rebuilt and coated with proteins to form chylomicrons.
• Chylomicrons exit the cells via exocytosis and travel through the lymphatic system to the bloodstream before being used by liver and fat tissues.

Digestion of Nucleic Acids

• Pancreatic juice contains nucleases (ribonuclease and deoxyribonuclease) that digest RNA and DNA, respectively.
• Nucleotides are further digested by brush border enzymes (nucleosidease and phosphatase) into pentose, phosphate, and nitrogenous bases.
• Nucleotides are absorbed by active transport.

Absorption of Electrolytes

• Electrolytes are absorbed from GI secretions and ingested foods and liquids.
• Sodium and potassium are absorbed through active transport via Na⁺/K⁺ pumps.
• Chloride, iodide, and nitrate are passively absorbed.
• Iron, magnesium, and phosphate ions are absorbed through active transport.
• Intestinal calcium absorption requires vitamin D and parathyroid hormone.

Absorption of Vitamins

• Fat-soluble vitamins travel in micelles and are absorbed by simple diffusion.
• Water-soluble vitamins are absorbed by diffusion.
• Vitamin B12 combines with intrinsic factor before absorption.
• Receptor-mediated endocytosis is another method of Vitamin uptake.

Absorption of Water

• About 9 liters of fluid enter the GI tract daily.
• The small intestine absorbs 8 liters, and the large intestine absorbs 90% of the remaining liter.
• Water absorption occurs via osmosis through cell walls into vascular capillaries within the villi.