Digestive System – NDA Biology PYQs

NDA 2025-II

Q. 1. The optimum pH at which the enzymes pepsin and trypsin of the human alimentary canal works are

(a) pH 2·0 and 7·9, respectively

(b) pH 4·5 and 3·5, respectively

(c) pH 9·5 and 4·5, respectively

(d) pH 7·0 and 8·0, respectively

Answer: (a) pH 2·0 and 7·9, respectively

Explanation: Pepsin is a protease secreted by the stomach. It works best at a strongly acidic pH of approximately 2.0. This acidity is provided by hydrochloric acid (HCl) secreted by parietal cells of the stomach wall. Trypsin is a protease secreted by the pancreas into the small intestine. It functions optimally at an alkaline pH of approximately 7.5–8.0. Option (a) correctly pairs pepsin with pH 2.0 and trypsin with pH 7.9. Concept Tested: Optimum pH of digestive enzymes: pepsin ~2.0 (acidic), trypsin ~7.9 (alkaline)

★ JOVIK Exam Insight NDA frequently tests enzyme pH optima. Pepsin works in the acidic stomach (pH ~2). Trypsin works in the alkaline small intestine (pH ~8). The mnemonic: Pepsin = acidic stomach; Trypsin = alkaline intestine.

NDA 2024-I

Q. 2. In the human digestive system, which one among the following is the role of the pancreas?

(a) Secretion of surfactants to break up lipid droplets

(b) Storage and regulated release of bile

(c) Secretion of lipase, amylase and protease

(d) Neutralizing stomach acids

Answer: (c) Secretion of lipase, amylase and protease

Explanation: The pancreas is a mixed gland. Its exocrine function is to secrete pancreatic juice into the duodenum. Pancreatic juice contains three major enzyme types: lipase (digests fats), amylase (digests starch/carbohydrates), and proteases such as trypsin and chymotrypsin (digest proteins). The gall bladder stores and releases bile, not the pancreas. Bile is produced by the liver. Concept Tested: Pancreatic exocrine function: secretion of lipase, amylase, and proteases into the duodenum

NDA 2022-II

Q. 3. The digestion of fat in human intestine is performed by

(a) trypsin

(b) bile and lipase

(c) bile and amylase

(d) bile and pepsin

Answer: (b) bile and lipase

Explanation: Fat digestion in the small intestine requires two components. Bile (produced by the liver, stored in the gall bladder) emulsifies large fat globules into smaller droplets, increasing the surface area. Lipase (from the pancreas) then breaks these fat droplets into fatty acids and glycerol. Neither amylase nor pepsin acts on fats: they target starch and proteins respectively. Trypsin is a protease. Concept Tested: Fat digestion: bile emulsifies fats; lipase hydrolyses them into fatty acids and glycerol

★ JOVIK Exam Insight The bile + lipase pair for fat digestion is a common NDA question. Bile does not digest fat: it only emulsifies it (breaks it into droplets). The actual chemical digestion is done by lipase.

Q. 4. The protein-digesting enzyme secreted by the stomach wall in case of mammals is called

(a) chitinase

(b) amylase

(c) pepsin

(d) trypsin

Answer: (c) pepsin

Explanation: Pepsin is the main protein-digesting enzyme secreted by the stomach. It is produced by chief cells (peptic cells) of the gastric glands as an inactive precursor called pepsinogen. HCl in the stomach activates pepsinogen to pepsin. Pepsin breaks proteins into smaller peptide fragments. Trypsin is a pancreatic protease, not a stomach enzyme. Amylase digests starch. Concept Tested: Pepsin: the protease secreted by the stomach wall; activated from pepsinogen by HCl

NDA 2021-II

Q. 5. What is the total number of chambers in the stomach of domestic animals like cattle, buffalo, goat and sheep?

(a) Four

(b) Two

(c) One

(d) Three

Answer: (a) Four

Explanation: Cattle, buffalo, goats, and sheep are ruminants. Their stomach has four chambers: the rumen, reticulum, omasum, and abomasum. The rumen is the largest chamber and is where microbial fermentation of cellulose begins. This four-chambered stomach allows ruminants to digest tough plant material efficiently. Concept Tested: Ruminant stomach: four chambers: rumen, reticulum, omasum, abomasum

NDA 2019-I

Q. 6. Tooth enamel is made up of which one of the following calcium compounds?

(a) Calcium carbonate

(b) Calcium sulphate

(c) Calcium hydroxide

(d) Calcium phosphate

Answer: (d) Calcium phosphate

Explanation: Tooth enamel is the hardest biological substance in the human body. It is composed primarily of hydroxyapatite, which is a crystalline calcium phosphate compound (Ca₁₀(PO₄)₆(OH)₂). The high mineral content of calcium phosphate gives enamel its extreme hardness and resistance to wear. Calcium carbonate is found in chalk and shells, not in enamel. Concept Tested: Tooth enamel composition: calcium phosphate (hydroxyapatite)

NDA 2018-II

Q. 7. The acidic semidigested food coming out of the stomach is neutralized by

(a) pancreatic juice

(b) duodenal secretion

(c) large intestine secretion

(d) bile juice

Answer: (a) pancreatic juice

Explanation: When the acidic chyme (partially digested food) from the stomach enters the duodenum, it must be neutralised for the intestinal enzymes to work. Pancreatic juice contains sodium bicarbonate (NaHCO₃), which neutralises the acid. This raises the pH of the contents of the duodenum to an alkaline level, suitable for pancreatic and intestinal enzymes to function. Concept Tested: Pancreatic juice (sodium bicarbonate) neutralises acidic chyme in the duodenum

NDA 2018-I

Q. 8. If by an unknown accident the acid secreting cells of the stomach wall of an individual are damaged, digestion of which one of the following biomolecule will be affected to a greater extent?

(a) Protein only

(b) Lipid

(c) Carbohydrate only

(d) Protein and Carbohydrate

Answer: (a) Protein only

Explanation: The acid-secreting cells (parietal cells) of the stomach produce HCl. This acid serves two key functions: it activates pepsinogen to pepsin, and it provides the acidic environment needed for pepsin to digest proteins. Without HCl, pepsinogen cannot be activated, and protein digestion in the stomach cannot occur. Carbohydrate digestion (by salivary amylase and pancreatic amylase) and lipid digestion (by bile and lipase in the intestine) are not primarily dependent on stomach acid. Concept Tested: HCl (from parietal cells) activates pepsinogen to pepsin; loss of acid primarily affects protein digestion

NDA 2016-I

Q. 9. Match List I with List II and select the correct answer using the code given below the Lists:

List I (Molecule)	List II (Product of digestion)
A. Proteins	1. Nitrogenous bases and pentose sugars
B. Carbohydrates	2. Fatty acids and glycerol
C. Nucleic acids	3. Monosaccharides
D. Lipids	4. Amino acids

Code:

(a) A-2, B-3, C-1, D-4

(b) A-2, B-1, C-3, D-4

(c) A-4, B-1, C-3, D-2

(d) A-4, B-3, C-1, D-2

Answer: (d) A-4, B-3, C-1, D-2

Explanation: Proteins are digested into amino acids (A–4). Carbohydrates are digested into monosaccharides (B–3). Nucleic acids are digested into nitrogenous bases and pentose sugars (C–1). Lipids are digested into fatty acids and glycerol (D–2). Option (d) correctly matches all four pairs. Concept Tested: Final products of digestion: proteins → amino acids; carbohydrates → monosaccharides; nucleic acids → nitrogenous bases + pentose sugars; lipids → fatty acids + glycerol

★ JOVIK Exam Insight The final products of digestion are a recurring NDA topic. Each macromolecule has a specific end product: proteins → amino acids; starch/carbohydrates → monosaccharides (glucose, fructose, galactose); lipids → fatty acids + glycerol; nucleic acids → nitrogenous bases + pentose sugars.

NDA 2014-II

Q. 10. Which one of the following is not a function of liver?

(a) Conversion of glucose into glycogen

(b) Production of urea

(c) Destruction of dead and worn-out red blood cells

(d) Absorption of food and excess water from the undigested food

Answer: (d) Absorption of food and excess water from the undigested food

Explanation: The liver performs many functions: it converts glucose to glycogen (glycogenesis), produces urea from ammonia (urea cycle), destroys old red blood cells, produces bile, and synthesises plasma proteins. However, absorption of food and water from undigested food is a function of the large intestine, not the liver. The colon absorbs water and electrolytes from undigested material. Concept Tested: Functions of the liver vs. large intestine: water/food absorption belongs to the large intestine, not the liver

NDA 2013-I

Q. 11. The complete digestion of carbohydrates, proteins and fats occurs in

(a) stomach

(b) liver

(c) small intestine

(d) large intestine

Answer: (c) small intestine

Explanation: Complete digestion of all three major macronutrients: carbohydrates, proteins, and fats: occurs in the small intestine. Pancreatic enzymes (amylase, trypsin, chymotrypsin, lipase) and intestinal enzymes complete the digestion. The stomach only partially digests proteins (pepsin). The large intestine absorbs water and passes undigested material. The liver does not directly digest food. Concept Tested: Complete digestion of all macronutrients occurs in the small intestine

NDA 2012-II

Q. 12. Which one among the following is the hardest part of our body?

(a) Skull bones of head

(b) Thumb nails

(c) Enamel of teeth

(d) Spinal vertebra

Answer: (c) Enamel of teeth

Explanation: Tooth enamel is the hardest substance in the human body. It is composed of about 96% hydroxyapatite (calcium phosphate mineral), making it harder than bone. Bones contain organic material (collagen) and are not as hard as enamel. Nails are made of keratin, which is much softer. Concept Tested: Tooth enamel is the hardest substance in the human body (96% hydroxyapatite)

NDA 2011-I

Q. 13. Human stomach produces acid ‘X’ which helps in digestion of food. Acid ‘X’ is:

(a) acetic acid.

(b) methanoic acid.

(c) hydrochloric acid.

(d) citric acid.

Answer: (c) hydrochloric acid.

Explanation: The stomach produces hydrochloric acid (HCl), secreted by parietal (oxyntic) cells of the gastric mucosa. HCl serves several functions: it activates pepsinogen to pepsin, provides an acidic environment for pepsin to digest proteins, kills many ingested bacteria, and denatures proteins to make them more accessible to enzymes. Concept Tested: Hydrochloric acid (HCl) produced by parietal cells of the stomach

Q. 14. Which one among the following statements about stomach is not correct?

(a) Stomach acts as a temporary reservoir

(b) Stomach mixes food with gastric juice

(c) Stomach secretes lipase and amylase in gastric juice

(d) Rate of stomach emptying depends on the type of food

Answer: (c) Stomach secretes lipase and amylase in gastric juice

Explanation: Option (c) is incorrect. The stomach does not secrete significant amounts of lipase or amylase in its gastric juice. Gastric juice primarily contains pepsin (for protein digestion) and HCl. Amylase is secreted by the salivary glands and pancreas. Lipase is mainly a pancreatic enzyme. Options (a), (b), and (d) are all correct statements about stomach function. Concept Tested: Stomach secretes pepsin and HCl: not lipase or amylase; those come from salivary glands and pancreas

Q. 15. Which one of the following organs breaks fat to produce cholesterol?

(a) Intestine

(b) Liver

(c) Lungs

(d) Kidneys

Answer: (b) Liver

Explanation: The liver is the primary site of cholesterol synthesis in the body. It produces cholesterol from acetyl-CoA via the mevalonate pathway. The liver also breaks down fats (lipolysis) and packages lipids into lipoproteins for transport. Most of the body’s cholesterol is produced endogenously by the liver, not obtained from food alone. Concept Tested: Liver as the primary site of cholesterol synthesis and fat metabolism

NDA 2009-II

Q. 16. Jaundice in human beings is the result of

(a) incomplete metabolism of biliverdin

(b) complete metabolism of biliverdin

(c) incomplete metabolism of bilirubin

(d) complete metabolism of bilirubin

Answer: (c) incomplete metabolism of bilirubin

Explanation: Bilirubin is a yellow pigment produced from the breakdown of haem (from old red blood cells). Normally, the liver processes bilirubin and excretes it in bile. When the liver cannot process bilirubin adequately: due to liver disease, bile duct obstruction, or excessive RBC destruction: bilirubin accumulates in the blood. This causes the yellow discolouration of skin and eyes known as jaundice. Biliverdin is an earlier green pigment that is converted to bilirubin. Concept Tested: Jaundice caused by incomplete metabolism (accumulation) of bilirubin

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