Step by Step
Carb
Carbohydrate digestion
Salivary amylase (in the mouth) and pancreatic amylase (in the duodenum) break starch down into maltose and dextrins. Brush border enzymes (maltase, sucrase, lactase) complete the job, producing monosaccharides. Glucose and galactose are absorbed via SGLT1 (using sodium cotransport); fructose is absorbed via GLUT5 (facilitated diffusion); all three exit the enterocyte into the blood via GLUT2.
Prot1
Protein digestion begins in the stomach
Pepsin, active only in the stomach's very acidic environment (pH below 2), begins breaking down proteins.
Prot2
The pancreatic protease activation cascade
Pancreatic proteases are secreted as inactive zymogens. Enterokinase (an enzyme on the duodenal brush border) activates trypsinogen into active trypsin — and trypsin then activates chymotrypsinogen, proelastase, and procarboxypeptidase, setting off a full activation cascade.
Prot3
Final protein absorption
Brush border peptidases and cytoplasmic peptidases break proteins down further into free amino acids, which are absorbed via sodium cotransport into the portal blood. Note: lactase deficiency leaves lactose undigested, causing osmotic and fermentation-driven diarrhea.
Applied Walkthrough
1
A meal containing bread (starch) and meat (protein) is eaten. Salivary amylase begins breaking down the starch immediately in the mouth, continuing in the duodenum with pancreatic amylase — eventually producing maltose and dextrins, then monosaccharides via brush border enzymes.
2
Meanwhile, pepsin in the highly acidic stomach environment begins breaking down the protein from the meat.
3
As this partially digested material reaches the duodenum, enterokinase activates trypsinogen into trypsin, which then activates a cascade of additional pancreatic proteases — continuing protein breakdown.
4
Finally, brush border and cytoplasmic peptidases finish converting protein fragments into free amino acids, which are absorbed via sodium cotransport into the portal blood — while the monosaccharides from the starch are absorbed via SGLT1, GLUT5, and GLUT2 depending on which specific sugar they are.
Exam Application
Exams test whether you can trace the full carbohydrate digestion pathway (amylase → brush border enzymes → specific transporters) and the full protein digestion cascade (pepsin → enterokinase-triggered trypsin cascade → peptidases), and whether you understand what happens when lactase is deficient.
⚠ Common Trap
The most common trap is forgetting that pancreatic proteases are secreted as INACTIVE zymogens that require enterokinase (and then trypsin itself) to activate — if this activation happened prematurely inside the pancreas, it would cause autodigestion (as seen in acute pancreatitis).
✓ Quick Self-Check
1. What enzyme initiates protein digestion in the stomach?
Pepsin.
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2. What enzyme activates trypsinogen into active trypsin?
Enterokinase, at the duodenal brush border.
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3. How is glucose absorbed into the enterocyte?
Via SGLT1, using sodium cotransport.
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4. How is fructose absorbed into the enterocyte?
Via GLUT5, by facilitated diffusion.
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5. What happens when lactase is deficient?
Undigested lactose causes osmotic and fermentation-driven diarrhea.
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