📌 Professor’s Notes: Quick Revision
High-yield revision notes for rapid review before university examinations.
- Digestion is the breakdown of complex food into absorbable molecules, while absorption is the transfer of nutrients, water, electrolytes, vitamins, and minerals from the intestinal lumen into the blood or lymph. The small intestine is the principal site for both processes because of its large surface area and brush border enzymes.
- Carbohydrate digestion begins in the mouth with salivary amylase and is completed by pancreatic amylase and brush border enzymes (maltase, sucrase, lactase, dextrinase, and glucoamylase). The final products are glucose, galactose, and fructose.
- Carbohydrate absorption occurs mainly in the duodenum and jejunum.
- Glucose and galactose are absorbed by SGLT-1 (secondary active transport with sodium).
- Fructose is absorbed by GLUT-5 (facilitated diffusion).
- All monosaccharides leave enterocytes through GLUT-2. Defects cause lactose intolerance, sucrase-isomaltase deficiency, and glucose-galactose malabsorption, leading to osmotic diarrhea.
- Protein digestion begins in the stomach with pepsin and continues in the small intestine by trypsin, chymotrypsin, elastase, and carboxypeptidases. Enteropeptidase (enterokinase) activates trypsinogen to trypsin, which activates other pancreatic enzymes. Brush border peptidases produce amino acids and small peptides for absorption.
- Lipid digestion occurs mainly in the duodenum. Bile salts emulsify fats, and pancreatic lipase (with colipase) converts triglycerides into free fatty acids and monoglycerides. These form micelles, enter enterocytes, are re-esterified to triglycerides, and are transported as chylomicrons through lymphatics. Absorption of vitamins A, D, E, and K depends on normal fat absorption. Defective lipid absorption causes steatorrhea.
- Water and electrolyte absorption:
- More than 99% of water is reabsorbed, mainly in the jejunum, ileum, and colon.
- Sodium is absorbed by secondary active transport driven by the Na⁺–K⁺ ATPase.
- Coupled sodium-glucose transport is the physiological basis of oral rehydration therapy (ORT).
- Potassium is absorbed mainly by passive diffusion.
- Chloride and bicarbonate absorption maintain electrolyte and acid-base balance. Excess chloride secretion causes secretory diarrhea, as in cholera.
- Vitamin and mineral absorption:
- Vitamin B12 binds intrinsic factor in the stomach and is absorbed in the terminal ileum. Intrinsic factor deficiency causes pernicious anemia.
- Fat-soluble vitamins (A, D, E, K) require normal fat absorption.
- Calcium is absorbed mainly in the duodenum under the influence of vitamin D.
📝 Practice MCQs
🎙️ Frequently Asked Viva Questions
High-yield viva questions with concise answers for rapid university examination preparation.
Q1. Why are only monosaccharides absorbed from the intestine?
Answer: Carbohydrates must be completely digested into glucose, galactose, and fructose, because larger sugars cannot cross the intestinal epithelium.
Q2. What is the role of the sodium–glucose cotransporter (SGLT1)?
Answer: SGLT1 transports glucose and galactose into enterocytes by secondary active transport coupled with sodium.
Q3. Why is oral rehydration solution (ORS) effective in diarrhea?
Answer: ORS contains glucose and sodium, which are absorbed together through SGLT1, enhancing water absorption even during diarrheal illness.
Q4. Why does lactose intolerance cause diarrhea and abdominal bloating?
Answer: Lactase deficiency leaves lactose undigested. It retains water in the intestine and is fermented by colonic bacteria, producing gas.
Q5. Why are pancreatic enzymes essential for protein digestion?
Answer: They break proteins into peptides, and trypsin activates other pancreatic proteases for complete protein digestion.
Q6. Why are bile salts essential for fat absorption?
Answer: Bile salts form micelles that transport fatty acids, monoglycerides, and cholesterol to the intestinal brush border for absorption.
Q7. Why are chylomicrons formed inside enterocytes?
Answer: Re-esterified triglycerides combine with proteins to form chylomicrons, which transport dietary lipids through lymphatics into the bloodstream.
Q8. Why does bile salt deficiency cause deficiency of fat-soluble vitamins?
Answer: Vitamins A, D, E, and K require normal fat digestion and micelle formation for efficient intestinal absorption.
Q9. Why does steatorrhea occur in pancreatic insufficiency?
Answer: Deficiency of pancreatic lipase impairs triglyceride digestion, resulting in bulky, pale, fatty stools.
Q10. Why does water absorption follow sodium absorption?
Answer: Active sodium absorption creates an osmotic gradient, causing water to move passively from the intestinal lumen into the blood.
Q11. Why is vitamin B₁₂ absorbed only in the terminal ileum?
Answer: The intrinsic factor–vitamin B₁₂ complex binds to specific receptors present in the terminal ileum for absorption.
Q12. Why does intrinsic factor deficiency cause pernicious anemia?
Answer: Without intrinsic factor, vitamin B₁₂ absorption is markedly reduced, leading to megaloblastic (pernicious) anemia.
Q13. Why does vitamin C enhance iron absorption?
Answer: Ascorbic acid converts ferric iron to the more soluble ferrous form, improving intestinal iron absorption.
Q14. Why does gastrectomy increase the risk of iron deficiency anemia?
Answer: Reduced gastric acid decreases conversion of ferric to ferrous iron, lowering iron absorption in the small intestine.
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