Chapter 9: The Digestive System
🎯 Diagnostic: Test Your Starting Level 24 questions (2 per section). No prior reading required, see what you already know.
Aim to answer every question before checking. Missed questions point you to the sections you need most.
1. (9.1) The GI tract is continuous from:
C. The alimentary canal is one long hollow tube. Accessory organs secrete into it through ducts but are not part of the tube.
2. (9.1) The wall of the GI tract, from lumen outward, is:
B. Mucosa (epithelium + lamina propria + muscularis mucosae), then submucosa (Meissner's plexus), muscularis externa (Auerbach's plexus), and serosa (or adventitia).
3. (9.2) Salivary amylase begins the digestion of:
A. Salivary amylase hydrolyzes α-1,4 bonds in starch. Lingual lipase begins fat digestion to a small degree. Humans cannot digest cellulose (β-1,4 bonds).
4. (9.2) Mastication (chewing) serves to:
D. Chewing produces a bolus of small particles that amylase and later enzymes can act on efficiently.
5. (9.3) Once food enters the pharynx, swallowing becomes:
B. The oral phase is voluntary; the pharyngeal and esophageal phases are reflexive. The swallowing center in the medulla coordinates the 40+ muscle pairs involved.
6. (9.3) The esophagus propels food to the stomach via:
A. Coordinated circular and longitudinal muscle contractions push food regardless of posture, which is why you can swallow upside down.
7. (9.4) Parietal cells of the stomach secrete:
C. Intrinsic factor is required for vitamin B12 absorption in the terminal ileum. Loss of parietal cells (pernicious anemia) causes B12 deficiency.
8. (9.4) Chief cells secrete:
D. Pepsinogen is a zymogen. Low gastric pH autocatalyzes it to active pepsin, which cleaves proteins at aromatic residues.
9. (9.5) Most chemical digestion and absorption occurs in the:
B. Duodenum (digestion), jejunum (absorption), and ileum (terminal absorption plus B12 and bile salts) handle nearly all nutrient uptake.
10. (9.5) Villi and microvilli of the small intestine function to:
A. Combined with circular folds (plicae circulares), villi and microvilli multiply the absorptive surface ~600-fold, up to ~250 m².
11. (9.6) Brush-border enzymes (maltase, sucrase, lactase) are anchored on:
D. Brush-border disaccharidases finish carbohydrate digestion at the site of absorption. Lactase deficiency is the most common adult enzyme deficiency.
12. (9.6) Enterokinase (enteropeptidase) converts:
C. Enterokinase on duodenal brush border kicks off a cascade: trypsin activates chymotrypsinogen, procarboxypeptidase, and proelastase.
13. (9.7) Bile salts function in digestion by:
A. Bile salts are amphipathic steroids (derived from cholesterol) that break large lipid droplets into small micelles. They do not hydrolyze bonds themselves.
14. (9.7) Bile is stored and concentrated in the:
D. The liver makes bile continuously; the gallbladder stores and concentrates it. CCK triggers gallbladder contraction when fats reach the duodenum.
15. (9.8) The exocrine pancreas secretes:
B. Acinar cells secrete the enzymes; duct cells secrete the bicarbonate-rich fluid that buffers incoming gastric acid.
16. (9.8) Pancreatic bicarbonate neutralizes gastric chyme in the duodenum to:
C. Secretin (released by duodenal S cells in response to acid) drives the bicarbonate output, raising duodenal pH from about 2 to 8.
17. (9.9) Glucose is absorbed across enterocytes via:
D. Secondary active transport uses the Na⁺ gradient built by the basolateral Na⁺/K⁺ pump. Fructose uses GLUT5 (facilitated, non-Na⁺-coupled).
18. (9.9) Long-chain fatty acids and monoglycerides absorbed by enterocytes are:
A. Chylomicrons enter lacteals because they are too big for fenestrated capillaries. Lymph eventually joins venous blood at the thoracic duct, bypassing the portal system.
19. (9.10) The large intestine is primarily responsible for:
C. The colon reclaims most residual water (hence watery diarrhea if transit is too rapid) and compacts waste for elimination.
20. (9.10) Gut microbiota in the colon:
B. The gut is the densest microbial community in the body. Disruption by broad-spectrum antibiotics can lead to C. difficile overgrowth.
21. (9.11) Gastrin, secreted by G cells in the antrum of the stomach, primarily:
A. Gastrin triggers acid release directly and via histamine from enterochromaffin-like cells. Zollinger-Ellison syndrome (gastrinoma) produces severe ulcers.
22. (9.11) Cholecystokinin (CCK) is released in response to:
D. CCK ("cholecyst" = gallbladder, "kinin" = move) also slows gastric emptying so the duodenum is not overwhelmed.
23. (9.12) The enteric nervous system:
B. Meissner's (submucosal) and Auerbach's (myenteric) plexuses contain more neurons than the spinal cord and can coordinate peristalsis without CNS input.
24. (9.12) Parasympathetic stimulation of the GI tract via the vagus nerve (CN X):
C. The vagus is the dominant parasympathetic nerve to the gut (down to the splenic flexure). Sympathetic stimulation does the opposite, slowing digestion during stress.
You eat a slice of pizza. Twenty-four hours later, the cheese has become part of your blood, the crust is fueling your brain, and the pepperoni’s fats are insulating your nerves. How does a slab of dough, cheese, and meat transform into you?
Your digestive system is a disassembly line. A factory assembly line builds complex products from simple parts. Your GI tract does the opposite - it takes a complete pizza and systematically dismantles it into parts small enough to absorb: amino acids from the cheese protein, glucose from the crust starch, and fatty acids from the pepperoni fat. Each station along the 30-foot tube has specialized tools (enzymes) designed for a specific job.
The MCAT loves the digestive system because it bridges biology and biochemistry. You will see passages about enzyme deficiencies, hormone cascades, clinical conditions like pancreatitis and celiac disease, and experimental setups testing absorption. This chapter follows your pizza through every station of the disassembly line - from the first bite to the final exit.