Digestive System — Set 1

Correct Answer: B. Salivary Amylase

• **Salivary Amylase** = also called Ptyalin, it breaks the glycosidic bonds in starch, converting it into maltose and dextrins, making it the first enzyme to act in chemical digestion. • **Optimal pH ~6.8** — it works best in the slightly acidic–neutral environment of the mouth but is inactivated once food reaches the acidic stomach. • 💡 Option A (Lipase) is wrong because lipase acts on fats, not carbohydrates; Option C (Trypsin) is wrong because trypsin is a pancreatic protease that breaks down proteins; Option D (Pepsin) is wrong because pepsin is a gastric enzyme that digests proteins in the stomach.

Correct Answer: A. Emulsification of fats

• **Emulsification of fats** = bile salts (sodium glycocholate and sodium taurocholate) break large fat globules into tiny droplets, dramatically increasing the surface area available for lipase to act upon. • **No enzymatic activity** — bile does not digest fats chemically; it only emulsifies them, which is a physical/mechanical action that facilitates enzymatic digestion. • 💡 Option B (Neutralization of saliva) is wrong because bile neutralizes the acidic chyme from the stomach using bicarbonates, not saliva; Option C (Absorption of vitamins) is wrong because while bile aids fat-soluble vitamin absorption indirectly, that is not its primary role; Option D (Digestion of proteins) is wrong because proteins are digested by pepsin and trypsin, not bile.

Correct Answer: C. Small Intestine

• **Small Intestine** = its inner wall is lined with millions of finger-like projections called villi, each covered with microvilli (brush border), creating an enormous surface area estimated at 250–300 m² for efficient nutrient absorption into blood and lymph. • **Specialized segments** — the duodenum receives digestive juices, while the jejunum and ileum are the primary sites where glucose, amino acids, fatty acids, vitamins, and minerals are absorbed. • 💡 Option A (Esophagus) is wrong because it is merely a transport tube with no absorptive function; Option B (Large Intestine) is wrong because it mainly absorbs water and salts, not nutrients; Option D (Stomach) is wrong because the stomach primarily digests proteins and absorbs very little — mostly alcohol and certain drugs.

Correct Answer: D. Hydrochloric Acid

• **Hydrochloric Acid (HCl)** = secreted by oxyntic (parietal) cells, it lowers the stomach pH to 1.5–3.5, creating the acidic environment required to convert inactive pepsinogen into active pepsin and to kill most ingested pathogens. • **Protein denaturation** — HCl unfolds the tertiary structure of proteins, exposing peptide bonds for enzymatic cleavage, which accelerates digestion. • 💡 Option A (Acetic Acid) is wrong because acetic acid is the acid in vinegar and is not secreted by the body; Option B (Sulphuric Acid) is wrong because it is a strong industrial acid with no physiological role in digestion; Option C (Nitric Acid) is wrong because it is used in explosives manufacturing, not present in gastric secretions.

Correct Answer: A. Peristalsis

• **Peristalsis** = coordinated, involuntary waves of circular muscle contraction followed by relaxation propagate along the entire GI tract from the esophagus to the rectum, propelling the food bolus or chyme in one direction. • **Autonomic control** — peristalsis is governed by the enteric nervous system and smooth muscle; it works even in the absence of voluntary effort or gravity, which is why swallowing upside-down is possible. • 💡 Option B (Mastication) is wrong because mastication is the chewing of food in the mouth — a voluntary mechanical process; Option C (Segmentation) is wrong because segmentation is a mixing movement in the intestines that churns food without propelling it forward; Option D (Deglutition) is wrong because deglutition refers to the act of swallowing, not to wave-like propulsion.

Correct Answer: C. Vermiform Appendix

• **Vermiform Appendix** = a narrow, worm-shaped tube (~9 cm) attached to the cecum of the large intestine; it is considered vestigial in humans because it has lost the cellulose-digesting function it served in herbivorous ancestors. • **Immune tissue** — it contains lymphoid tissue and may play a minor role in gut immunity, but inflammation of the appendix (appendicitis) requires surgical removal, confirming its dispensability. • 💡 Option A (Pancreas) is wrong because the pancreas is an active gland producing digestive enzymes and hormones like insulin; Option B (Gallbladder) is wrong because it stores and concentrates bile, an essential digestive secretion; Option D (Spleen) is wrong because the spleen is a lymphoid organ involved in blood filtration and immunity, not the digestive tract.

Correct Answer: C. Increasing surface area for absorption

• **Increasing surface area for absorption** = each villus is a finger-like mucosal projection (~0.5–1.6 mm tall) bearing microvilli on its surface; together they amplify the absorptive area of the small intestine by about 600-fold compared to a plain tube. • **Lacteals inside villi** — each villus contains a central lymph capillary called a lacteal that absorbs dietary fats and fat-soluble vitamins, while blood capillaries absorb glucose and amino acids. • 💡 Option A (Secretion of bile) is wrong because bile is secreted by the liver and stored in the gallbladder, not by villi; Option B (Churning of food) is wrong because churning is performed by the smooth muscle of the stomach; Option D (Production of insulin) is wrong because insulin is produced by the beta cells of the Islets of Langerhans in the pancreas.

Correct Answer: D. Pepsin

• **Pepsin** = the chief protease of the stomach, secreted as inactive pepsinogen by chief (zymogenic) cells and activated by HCl; it cleaves peptide bonds between specific amino acids, breaking proteins into shorter polypeptides. • **Protective mechanism** — secreting pepsin as the inactive precursor pepsinogen prevents the stomach from digesting its own lining; the protective mucus layer further guards against self-digestion. • 💡 Option A (Amylase) is wrong because amylase digests carbohydrates, not proteins, and salivary amylase is inactivated in the acidic stomach; Option B (Maltase) is wrong because maltase is a brush-border enzyme that splits maltose into glucose; Option C (Lipase) is wrong because gastric lipase has a minor role in fat digestion and does not act on proteins.

Correct Answer: D. Gallbladder

• **Gallbladder** = a pear-shaped sac located under the liver that stores and concentrates bile produced by the liver (up to 10-fold concentration) until fat-containing food enters the duodenum, triggering its release via the common bile duct. • **Cholecystokinin (CCK)** — this hormone, released from the duodenal wall when fat enters, stimulates the gallbladder to contract and expel bile, coordinating fat digestion. • 💡 Option A (Pancreas) is wrong because the pancreas secretes pancreatic juice containing digestive enzymes, not bile; Option B (Liver) is wrong because the liver produces bile but does not store it — that is the gallbladder's role; Option C (Duodenum) is wrong because the duodenum receives bile from the gallbladder via the common bile duct but does not store it.

Correct Answer: C. Enamel

• **Enamel** = composed of 96% hydroxyapatite (calcium phosphate crystals) arranged in highly organized prisms, making it the hardest biologically produced substance in the human body — harder even than bone. • **Avascular and acellular** — unlike bone, mature enamel contains no living cells and cannot repair itself naturally once worn away, which is why dental cavities are permanent damage. • 💡 Option A (Cementum) is wrong because cementum is the calcified tissue covering the tooth root, which anchors it to the jawbone via the periodontal ligament; Option B (Bone) is wrong because bone is about 70% mineral and can remodel/repair itself; Option D (Dentine) is wrong because dentine, though harder than bone, lies beneath the enamel and is softer than enamel.