Chapter 8 Digestion & Absorption of Carbohydrates, Proteins, Lipids & Nucleic Acids

Welcome to this comprehensive MBBS Biochemistry lecture on Chapter 8: Digestion and Absorption. This deep dive covers carbohydrates, proteins, lipids, and nucleic acids—from the moment food enters the mouth to the point where nutrients enter the bloodstream as monosaccharides, amino acids, monoglycerides, fatty acids, nucleotides, and finally chylomicrons. Perfect for MBBS, BDS, Nursing, Pharmacy, and NEET-PG preparation. 📘 1️⃣ DIGESTION & ABSORPTION OF CARBOHYDRATES Digestion in the Mouth Carbohydrate digestion begins with salivary α-amylase (ptyalin), which breaks α(1→4) glycosidic bonds in starch and glycogen. This forms maltose, maltotriose, and small branched fragments called limit dextrins. The enzyme stops working in the stomach due to acidity. Digestion in the Small Intestine In the duodenum, pancreatic α-amylase continues digestion, again breaking α(1→4) linkages to produce maltose, isomaltose, and dextrins. Brush Border Enzymes Specialised enzymes on the intestinal mucosa complete digestion: Maltase breaks maltose into glucose. Isomaltase breaks α-limit dextrins. Sucrase converts sucrose into glucose and fructose. Lactase splits lactose into glucose and galactose. Trehalase hydrolyses trehalose into glucose. Absorption of Monosaccharides Glucose and galactose are absorbed by SGLT-1, a sodium-dependent transporter. Fructose is absorbed by GLUT-5 via facilitated diffusion. All monosaccharides exit the enterocyte into the blood via GLUT-2. Clinical Notes Lactase deficiency causes lactose intolerance. A defect in SGLT-1 results in glucose-galactose malabsorption. 📘 2️⃣ DIGESTION & ABSORPTION OF PROTEINS Digestion in the Stomach Protein digestion begins with hydrochloric acid, which denatures proteins and activates pepsinogen to pepsin. Pepsin breaks internal peptide bonds, initiating protein breakdown. Digestion in the Small Intestine Pancreatic proteases are activated by enteropeptidase, which converts trypsinogen into trypsin. Trypsin then activates other enzymes, including chymotrypsin, elastase, and carboxypeptidases. Each cleaves specific peptide bonds, reducing proteins into oligopeptides. Brush Border and Intracellular Peptidases Aminopeptidases, dipeptidases, and tripeptidases further digest peptides into amino acids, dipeptides, and tripeptides. Absorption Amino acids use sodium-dependent transporters. Dipeptides and tripeptides use an H⁺-dependent PEPT1 transporter and are hydrolysed inside the enterocyte. All end products enter the portal blood as free amino acids. Clinical Notes Cystinuria: defective transporter for cystine and basic amino acids. Hartnup disease: defective absorption of neutral amino acids, especially tryptophan. 📘 3️⃣ DIGESTION & ABSORPTION OF LIPIDS — From Starch to Chylomicrons Emulsification in the Small Intestine Bile salts break large fat globules into small droplets, increasing surface area for enzymatic digestion. Action of Pancreatic Enzymes Pancreatic lipase, with its cofactor colipase, converts triglycerides into 2-monoglycerides and free fatty acids. Phospholipase A₂ converts lecithin into lysolecithin. Cholesterol esterase converts cholesterol esters into cholesterol and fatty acids. Micelle Formation Fatty acids, monoglycerides, cholesterol, and fat-soluble vitamins combine with bile salts to form micelles. Micelles deliver lipids to the intestinal mucosa for absorption. Absorption and Re-esterification Inside enterocytes, monoglycerides and fatty acids are re-esterified to form triglycerides. Cholesterol is converted back to cholesterol esters. Chylomicron Formation (VERY HIGH-YIELD) Triglycerides, cholesterol esters, and phospholipids combine with ApoB-48 to form chylomicrons. Chylomicrons enter lacteals (intestinal lymphatics) and eventually reach the bloodstream via the thoracic duct. Clinical Notes Pancreatic insufficiency (e.g., cystic fibrosis) leads to fat malabsorption. Bile salt deficiency results in steatorrhea. Abetalipoproteinemia prevents chylomicron formation. 📘 4️⃣ DIGESTION & ABSORPTION OF NUCLEIC ACIDS Pancreatic Digestion Pancreatic DNase and RNase break DNA and RNA into nucleotides. Brush Border Digestion Nucleotidases remove phosphate groups to form nucleosides, and nucleosidases then break them into nitrogenous bases, pentose sugars, and phosphates. Absorption and Fate Pentose sugars are absorbed like other monosaccharides. Nitrogenous bases are absorbed and enter salvage pathways or degradation. 📚 REFERENCES: U. Satyanarayana & U. Chakrapani – Biochemistry (Latest Edition) #DigestionAndAbsorption #MBBSBiochemistry #CarbohydrateDigestion #ProteinDigestion #LipidDigestion #Chylomicrons #NucleicAcidDigestion #USatyanarayana #Enzymes #Metabolism digestion and absorption, protein digestion, lipid digestion, chylomicron formation, nucleic acid digestion, amylase pepsin lipase, sodium glucose transporter, enzyme activation digestion, micelles and bile salts, MBBS biochemistry chapter 8