Digestion and Absorption of Nutrients
Key transporters, enzymes and clinical links for carbohydrate, protein and fat absorption in the small intestine.
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Questions Covered in This Set
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What is the four-step choreography of nutrient absorption?
Luminal digestion by pancreatic enzymes → brush-border digestion → apical transport into the enterocyte → intracellular processing → basolateral export to blood or lymph.
Why does α-amylase not produce free glucose?
It cleaves only interior α-1,4 bonds and cannot touch α-1,6 branch points or terminal bonds, so it yields maltose, maltotriose and α-limit dextrins.
How are glucose and galactose absorbed apically?
Via SGLT1, a secondary active Na⁺-symporter (2 Na⁺ per sugar) powered by the basolateral Na⁺/K⁺-ATPase.
How do fructose entry and all monosaccharide exit occur?
Fructose enters apically via GLUT5 (facilitated diffusion, no Na⁺); glucose, galactose and fructose all exit basolaterally via GLUT2.
Why does SGLT1 explain oral rehydration solution?
Glucose absorption drags Na⁺, and Na⁺ drags water, so hydration continues even when cholera toxin has crippled secretion control.
What triggers the pancreatic protease cascade?
Brush-border enteropeptidase (enterokinase) converts trypsinogen to trypsin, which autoactivates and activates chymotrypsinogen, proelastase and procarboxypeptidases.
Which is absorbed faster: free amino acids or di/tripeptides?
Di- and tripeptides, via the apical H⁺-coupled transporter PepT1 (proton gradient maintained by the Na⁺/H⁺ exchanger).
Contrast Hartnup disease and cystinuria.
Hartnup = loss of neutral amino acid carrier (pellagra-like rash from tryptophan loss); cystinuria = loss of dibasic amino acid carrier (kidney stones). Neither causes protein malnutrition because PepT1 rescues peptide uptake.
What does pancreatic lipase need and what does it produce?
It needs colipase to anchor on bile-salt-coated droplets; it cleaves positions 1 and 3 to give 2-monoglyceride + 2 free fatty acids.
Why is lactase clinically special?
It is the last brush-border enzyme expressed and the first lost after mucosal injury, causing lactose intolerance and post-gastroenteritis osmotic/fermentative diarrhea.
