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Digestion is controlled by hormonal and neural signals that coordinate enzyme secretion with food arrival. Salivary amylase initiates starch digestion; pepsin in the acidic stomach hydrolyzes protein; pancreatic enzymes (trypsin, amylase, lipase) in the small intestine complete carbohydrate, protein, and fat digestion. Cholecystokinin and secretin coordinate gallbladder contraction with pancreatic secretion and bile flow.
From your study of enzyme kinetics, you know that enzymes are biological catalysts — proteins with active sites shaped to bind specific substrates, lower activation energy, and speed reactions without being consumed. Digestive enzymes apply these principles to a logistical problem: a large, mixed bolus of food must be broken into absorbable monomers (glucose, amino acids, fatty acids) as it travels through a tube roughly nine meters long. The system solves this by staging different enzymes along the tract, each tuned to the pH and substrate conditions of its specific region.
Salivary amylase begins starch hydrolysis in the mouth, cleaving α-1,4 glycosidic bonds between glucose units. It stops working when it reaches the acidic stomach — this is not a flaw but a design feature, since the stomach's low pH is needed for the next enzyme. Pepsinogen, secreted by chief cells in the gastric mucosa, is a zymogen — an inactive precursor — that is converted to active pepsin by hydrochloric acid and by pepsin itself (autocatalytic activation). This is a critical safety mechanism: you cannot store active protein-cleaving enzymes in the cells that secrete them without destroying those cells. The acid environment (pH 1.5–3.5) denatures most dietary proteins, unfolding them and making their peptide bonds accessible to pepsin.
When partially digested chyme enters the duodenum, it triggers the release of two hormones you know from your prerequisite on hormone-receptor signaling: secretin, released in response to acid, stimulates the pancreas to secrete bicarbonate-rich fluid that neutralizes the acid; cholecystokinin (CCK), released in response to fats and protein, stimulates pancreatic enzyme secretion and gallbladder contraction. The pancreas responds by releasing a suite of zymogens — trypsinogen, chymotrypsinogen, proelastase — plus active amylase and lipase. Trypsinogen is activated by enteropeptidase (enterokinase) on the duodenal brush border, and active trypsin then activates the other zymogens in a cascade. This cascade structure ensures enzymes are not activated until they reach the intestinal lumen, protecting the pancreas from autodigestion.
Pancreatic lipase presents a unique challenge: fats are hydrophobic and form droplets that minimize surface area, but enzymes work at surfaces. Bile salts from the gallbladder emulsify fat globules into microscopic droplets, dramatically increasing surface area and allowing lipase to work efficiently. Colipase, a cofactor secreted with pancreatic lipase, anchors lipase to the lipid-bile salt interface. The integrated result of this orchestrated system is that by the mid-jejunum, carbohydrates are reduced to monosaccharides, proteins to dipeptides and amino acids, and fats to monoglycerides and fatty acids — all in forms ready for transport across the intestinal epithelium.