Digestive System – NDA Biology PYQs

Question [NDA 2006-I]
In human beings, the digestion of proteins starts in which part of the alimentary canal?
(a) Mouth
(b) Stomach
(c) Duodenum
(d) Ileum

Answer: (b)
Protein digestion begins in the stomach. The enzyme pepsin is produced there. It acts on proteins in the acidic environment of the stomach. The mouth handles carbohydrate digestion (starch). The duodenum receives already-started digestion products from the stomach. Ileum is the last part of the small intestine where absorption happens.

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Question [NDA 2006-I]
The enzyme present in the saliva of human beings breaks —
(a) starch into maltose
(b) proteins into peptides
(c) peptides into amino acids
(d) fats into fatty acids

Answer: (a)
Saliva contains ptyalin (salivary amylase). It acts on starch and breaks it into maltose. Pepsin (not saliva) breaks proteins into peptides. Trypsin breaks peptides into amino acids. Lipase breaks fats into fatty acids. Saliva has no enzyme for proteins or fats.

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Question [NDA 2006-II]
Which one of the following enzymes is found in human saliva?
(a) Pepsin
(b) Ptyalin
(c) Rennin
(d) Erepsin

Answer: (b)
Ptyalin is the enzyme in saliva. It is also called salivary amylase. Pepsin is in the stomach. Rennin curdles milk in infants (not humans). Erepsin is an intestinal enzyme.

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Question [NDA 2007-II]
Which one of the following is an enzyme?
(a) Gastrin
(b) Keratin
(c) Trypsin
(d) Vasopressin

Answer: (c)
Trypsin is a digestive enzyme produced by the pancreas. It breaks proteins and peptides into amino acids. Gastrin is a hormone that stimulates stomach acid production. Keratin is a structural protein found in hair, skin, and nails. Vasopressin is an antidiuretic hormone produced by the pituitary.

Question [NDA 2007-II]
In human body, which one of the following secretes hormones as well as digestive enzymes?
(a) Oesophagus
(b) Spleen
(c) Large intestine
(d) Pancreas

Answer: (d)
The pancreas is the only organ that does both jobs. As an exocrine gland, it releases lipase, amylase, and trypsin into the small intestine. As an endocrine gland, it releases insulin and glucagon into the blood. The oesophagus only transports food. The spleen is part of the immune and blood system. The large intestine absorbs water.

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Question [NDA 2011-I]
Which one among the following statements about the 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)
The stomach does NOT secrete lipase or amylase. These enzymes come from the pancreas and salivary glands respectively. The stomach secretes pepsin and HCl. Options (a), (b), and (d) are all correct statements about the stomach.

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Question [NDA 2013-I]
The complete digestion of carbohydrates, proteins and fats occurs in —
(a) Stomach
(b) Liver
(c) Small intestine
(d) Large intestine

Answer: (c)
The small intestine is the site of complete digestion. The pancreatic enzymes (lipase, amylase, trypsin) and bile all work here. The stomach only starts protein digestion. The liver produces bile but does not digest food itself. The large intestine only absorbs water.

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Question [NDA 2016-I]
Match List I with List II — Products of digestion:

• A. Proteins → ?
• B. Carbohydrates → ?
• C. Nucleic acids → ?
• D. Lipids → ?

List II:

1. Nitrogenous bases and pentose sugars
2. Fatty acids and glycerol
3. Monosaccharides
4. Amino acids

Answer: A-4, B-3, C-1, D-2
Proteins (A) → Amino acids (4). Carbohydrates (B) → Monosaccharides (3). Nucleic acids (C) → Nitrogenous bases and pentose sugars (1). Lipids (D) → Fatty acids and glycerol (2). The nucleic acid end product is the least intuitive in this question. Students often confuse the lipid end product with the carbohydrate one.

Question [NDA 2018-I]
If the acid-secreting cells of the stomach wall are damaged, digestion of which biomolecule is most affected?
(a) Protein only
(b) Lipid
(c) Carbohydrate only
(d) Protein and Carbohydrate

Answer: (a)
Stomach acid (HCl) creates the acidic environment that pepsin needs to digest proteins. Without acid, pepsin cannot work. So protein digestion is specifically affected. Fat digestion happens in the small intestine with lipase — it does not need stomach acid. Carbohydrate digestion uses amylase — also not acid-dependent.

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Question [NDA 2022-III]
The digestion of fat in the human intestine is performed by —
(a) trypsin
(b) bile and lipase
(c) bile and amylase
(d) bile and pepsin

Answer: (b)
Fat digestion requires two things working together: bile (to emulsify fat into small droplets) and lipase (to chemically break those droplets into fatty acids and glycerol). Bile alone cannot digest fat. Lipase alone would be slow without emulsification first. Trypsin digests proteins. Amylase digests carbohydrates. Pepsin digests proteins in the stomach.

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Question [NDA 2025-I]
The optimum pH at which pepsin and trypsin work 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)
Pepsin works in the stomach’s strongly acidic environment at pH ~2.0. Trypsin works in the alkaline small intestine at pH ~7.9. These two enzymes work at opposite ends of the pH scale. Remembering that pepsin = acidic and trypsin = alkaline is sufficient for all pH-related questions in this topic.

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