Why are pancreatic proteases (trypsin, chymotrypsin, elastase) secreted as inactive zymogens rather than as active enzymes?
ATo reduce the metabolic energy required for their synthesis
BTo prevent the pancreas from digesting itself before the enzymes reach the intestinal lumen
CBecause active enzymes would be destroyed by stomach acid before reaching the duodenum
DTo allow easier transport through the narrow pancreatic duct
Proteolytic enzymes that are active in the secretory cell would digest the cell proteins that produce them — a catastrophic autodigestion. The zymogen design is a safety mechanism: the enzymes are inert until enteropeptidase on the duodenal brush border activates trypsinogen, and active trypsin then cascades to activate the remaining zymogens. Option C is wrong because zymogens would also be stable in acid; the point is protecting the pancreas, not surviving the stomach.
Question 2 Multiple Choice
A patient has a genetic defect that abolishes enteropeptidase (enterokinase) activity on the duodenal brush border. Which consequence best follows?
AAll protein digestion fails because pepsin also requires enteropeptidase for activation
BOnly trypsin fails to activate; chymotrypsin and elastase self-activate from stomach acid
CPancreatic proteases including trypsin, chymotrypsin, and elastase remain as inactive zymogens in the duodenum
DProtein digestion is delayed but ultimately complete once pancreatic acid activates the zymogens
Enteropeptidase activates trypsinogen to trypsin, and trypsin is the master activator that converts all the other pancreatic zymogens in a cascade. Without enteropeptidase, the cascade never starts, leaving all pancreatic proteases inactive. Option A is wrong because pepsin is activated by gastric acid and autocatalysis — it has nothing to do with enteropeptidase. Option D is wrong because the pancreatic zymogens are designed to resist acid activation; they need trypsin.
Question 3 True / False
Salivary amylase stops digesting starch once it reaches the stomach because pepsin degrades it.
TTrue
FFalse
Answer: False
Salivary amylase is inactivated by the low pH of the stomach (pH 1.5–3.5), not by pepsin. Amylase has an optimal pH range of about 6–7 and is denatured by acid. This pH sensitivity is actually a design feature: the acidic stomach environment is required for pepsin's optimal function and for denaturing dietary proteins to expose their peptide bonds. The handoff from amylase to pepsin is pH-mediated, not protease-mediated.
Question 4 True / False
Pepsinogen and trypsinogen both exemplify the zymogen mechanism: each protects the cell that secretes it by remaining inactive until it reaches its target compartment.
TTrue
FFalse
Answer: True
Yes — both are inactive precursors for the same reason. Pepsinogen is secreted by gastric chief cells and activated by hydrochloric acid (and autocatalytically by pepsin) once in the stomach lumen. Trypsinogen is secreted by the pancreas and activated only after enteropeptidase in the duodenum initiates the cascade. In both cases, the zymogen design prevents the enzyme from destroying the cell that made it.
Question 5 Short Answer
Why must bile salts emulsify dietary fat before pancreatic lipase can digest it efficiently, and what structural feature of fat makes emulsification necessary?
Think about your answer, then reveal below.
Model answer: Dietary fats are hydrophobic and coalesce into large globules that minimize surface area. Lipase is a water-soluble enzyme that can only act at the water-fat interface. Large globules have very little surface area relative to their volume, severely limiting lipase access. Bile salts are amphipathic molecules that insert their hydrophobic ends into fat droplets and their hydrophilic ends into water, breaking large globules into microscopic droplets — dramatically increasing the surface area available for lipase action.
Colipase, secreted alongside pancreatic lipase, also plays a role by anchoring lipase to the lipid-bile salt interface. This is a general principle: enzymes that act on insoluble substrates require mechanisms to maximize the interface between the aqueous enzyme and the substrate. Emulsification is the digestive system's solution to this geometry problem.