MCAT Biological and Biochemical Foundations of Living Systems Flashcards: 3b Digestive System Absorption Regulation

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This deck focuses on 3b Digestive System Absorption Regulation, giving you a quick way to review the definitions, rules, and examples that matter most for MCAT Biological and Biochemical Foundations of Living Systems.

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Work through these flashcards in short sessions. Try to answer each prompt before flipping the card, then revisit any cards you miss until the explanation feels automatic.

Flashcard 1: What is the primary apical mechanism for amino acid absorption in the small intestine?

Answer: Na$^{+}$-dependent cotransport (secondary active transport). This mechanism leverages the sodium electrochemical gradient to efficiently transport amino acids into enterocytes for absorption.

Flashcard 2: What is the primary function of salivary amylase in the mouth?

Answer: Begins carbohydrate digestion by hydrolyzing starch to smaller sugars. Salivary amylase initiates the breakdown of complex carbohydrates into simpler forms during the initial phase of digestion in the oral cavity.

Flashcard 3: What type of epithelium lines the esophagus?

Answer: Nonkeratinized stratified squamous epithelium. This epithelium provides protection against mechanical stress from food passage while allowing flexibility in the esophagus.

Flashcard 4: What is the primary function of the lower esophageal sphincter (LES)?

Answer: Prevents gastric reflux into the esophagus. The LES acts as a valve to maintain unidirectional flow of food and prevent acidic stomach contents from damaging the esophageal lining.

Flashcard 5: What is the main function of parietal cells in the stomach?

Answer: Secrete hydrochloric acid and intrinsic factor. Parietal cells contribute to the acidic environment necessary for pepsin activation and facilitate vitamin $B_{12}$ absorption.

Flashcard 6: What is the main function of chief cells in the stomach?

Answer: Secrete pepsinogen. Chief cells release the inactive precursor pepsinogen, which is converted to pepsin for protein digestion in the acidic stomach environment.

Flashcard 7: What is the active protease formed from pepsinogen in the stomach?

Answer: Pepsin. Pepsinogen is activated by the low pH in the stomach to form pepsin, which cleaves peptide bonds in proteins.

Flashcard 8: What is the primary role of gastric mucus (from surface mucous cells)?

Answer: Protects gastric epithelium from acid and pepsin. Gastric mucus forms a protective barrier that neutralizes acid and prevents enzymatic degradation of the stomach lining.

Flashcard 9: What is the main function of intrinsic factor in digestion?

Answer: Enables vitamin $B_{12}$ absorption in the terminal ileum. Intrinsic factor binds to vitamin $B_{12}$, protecting it from degradation and facilitating its uptake via specific receptors in the ileum.

Flashcard 10: What is the main physiological trigger for gastrin release from G cells?

Answer: Peptides/amino acids in the stomach (and gastric distension). Gastrin release is stimulated by the presence of protein breakdown products and mechanical stretching to promote digestive processes.

Flashcard 11: What is the main effect of gastrin on the stomach?

Answer: Increases gastric acid secretion and gastric motility. Gastrin enhances the stomach's digestive capacity by stimulating parietal and chief cells and promoting peristalsis.

Flashcard 12: What is the main effect of secretin on pancreatic secretions?

Answer: Increases pancreatic $HCO_{3}^{-}$ secretion. Secretin promotes bicarbonate release to neutralize acidic chyme, creating an optimal pH for pancreatic enzyme activity in the duodenum.

Flashcard 13: What is the main stimulus for secretin release from the duodenum?

Answer: Acidic chyme (low pH) entering the duodenum. Low duodenal pH from gastric acid triggers secretin to regulate pancreatic bicarbonate secretion for acid neutralization.

Flashcard 14: What is the main effect of cholecystokinin (CCK) on digestion?

Answer: Stimulates pancreatic enzymes and gallbladder contraction. CCK coordinates the release of digestive enzymes and bile to emulsify and break down fats in the small intestine.

Flashcard 15: What is the main stimulus for CCK release from the duodenum?

Answer: Fatty acids and amino acids in the small intestine. The presence of fats and proteins in chyme signals CCK release to enhance lipid and protein digestion via pancreatic and biliary responses.

Flashcard 16: What is the primary role of bile salts in lipid digestion?

Answer: Emulsify fats and form micelles to aid absorption. Bile salts reduce surface tension of fat droplets and stabilize micelles, facilitating lipase action and lipid uptake by enterocytes.

Flashcard 17: What is the anatomical site where most nutrient absorption occurs?

Answer: Small intestine (primarily the jejunum). The jejunum's extensive surface area and vascular supply optimize the uptake of digested nutrients into the bloodstream.

Flashcard 18: What structural features increase small-intestinal absorptive surface area?

Answer: Plicae circulares, villi, and microvilli (brush border). These adaptations exponentially expand the mucosal surface, enhancing nutrient contact and absorption efficiency in the small intestine.

Flashcard 19: Which intestinal structure contains a lacteal for lipid absorption?

Answer: A villus. The villus integrates absorptive enterocytes, blood capillaries, and a central lacteal for efficient nutrient transport, especially lipids.

Flashcard 20: What is the major form in which dietary lipids leave enterocytes?

Answer: Chylomicrons entering lymphatic lacteals. Chylomicrons package triglycerides and are transported via lymphatics to bypass initial hepatic processing.

Flashcard 21: What is the main transporter mechanism for glucose uptake at the apical enterocyte membrane?

Answer: SGLT1 (Na$^{+}$-glucose cotransport; secondary active transport). SGLT1 utilizes the sodium gradient established by the Na$^{+}$-K$^{+}$ ATPase to drive glucose uptake against its concentration gradient.

Flashcard 22: What basolateral transporter exports glucose from enterocytes into blood?

Answer: GLUT2 (facilitated diffusion). GLUT2 allows passive exit of accumulated glucose down its concentration gradient into the bloodstream for systemic distribution.

Flashcard 23: What is the primary apical mechanism for fructose absorption in enterocytes?

Answer: GLUT5 (facilitated diffusion). GLUT5 enables fructose to enter enterocytes passively along its concentration gradient, independent of sodium.

Flashcard 24: Which segment of the colon is most associated with water and electrolyte reabsorption?

Answer: Large intestine (colon), especially proximal colon. The proximal colon maximizes water and electrolyte recovery from fecal matter to maintain fluid balance and form solid stool.

Flashcard 25: Which autonomic input generally increases gastrointestinal motility and secretion?

Answer: Parasympathetic (vagus and pelvic splanchnic nerves). Parasympathetic stimulation enhances digestive functions by promoting smooth muscle contraction and glandular secretion throughout the GI tract.