Digestive System

11. If by an unknown accident the acid secreting cells of the stomach wall

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 ?

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Protein only
Lipid
Carbohydrate only
Protein and Carbohydrate
This question was previously asked in
UPSC NDA-1 – 2018
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The correct answer is (A) Protein only. The acid-secreting cells (parietal cells) in the stomach produce hydrochloric acid (HCl). HCl creates a highly acidic environment (pH 1.5-3.5) necessary for two main functions related to protein digestion: (1) It activates pepsinogen, an inactive enzyme precursor, into its active form, pepsin. Pepsin is the primary enzyme for protein digestion in the stomach. (2) The acid helps denature proteins, unfolding their complex structures and making the peptide bonds more accessible to pepsin.
Stomach acid is crucial for activating pepsin, the main enzyme for protein digestion in the stomach, and for denaturing proteins.
Digestion of carbohydrates primarily begins in the mouth (salivary amylase) and continues in the small intestine. Digestion of lipids primarily occurs in the small intestine with the help of bile and pancreatic lipase. While the acidic environment of the stomach might have minor effects on other biomolecules, its most significant role is in initiating protein digestion.

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12. Match List I with List II and select the correct answer using the code

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

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List I
(Molecule)		List II
(Product of digestion)
A. Proteins1. Nitrogenous bases and pentose sugars
B. Carbohydrates2. Fatty acids and glycerol
C. Nucleic acids3. Monosaccharides
D. Lipids4. Amino acids
2 3 1 4
2 1 3 4
4 1 3 2
4 3 1 2
This question was previously asked in
UPSC NDA-1 – 2016
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The correct matching is A-4, B-3, C-1, D-2.
Digestion breaks down complex molecules into simpler units that can be absorbed by the body.
– Proteins are polymers of amino acids, and digestion breaks them down into individual amino acids. (A -> 4)
– Carbohydrates (like starch, disaccharides) are broken down into monosaccharides (simple sugars like glucose, fructose, galactose). (B -> 3)
– Nucleic acids (DNA, RNA) are broken down into nucleotides, and further breakdown yields nitrogenous bases, pentose sugars, and phosphate groups. Option 1 lists nitrogenous bases and pentose sugars, which are components resulting from nucleic acid digestion. (C -> 1)
– Lipids (fats and oils) are broken down into fatty acids and glycerol. (D -> 2)
Digestion is a catabolic process carried out by enzymes. For example, proteases digest proteins, amylases digest carbohydrates, nucleases digest nucleic acids, and lipases digest lipids. These broken-down monomers (amino acids, monosaccharides, fatty acids, glycerol, nitrogenous bases, pentose sugars, phosphate) are then absorbed across the intestinal lining into the bloodstream or lymphatic system for use by the body’s cells.

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13. Hydrochloric acid is produced in which one of the following organs of

Hydrochloric acid is produced in which one of the following organs of human body?

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Oesophagus
Small intestine
Stomach
Large intestine
This question was previously asked in
UPSC Geoscientist – 2021
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Hydrochloric acid (HCl) is produced in the stomach by specialized cells called parietal cells (also known as oxyntic cells) found in the gastric glands of the stomach lining. This acid creates a highly acidic environment necessary for protein digestion by the enzyme pepsin and for killing ingested microorganisms.
The stomach is the primary organ in the digestive system responsible for producing strong acid.
The oesophagus is a tube connecting the mouth to the stomach. The small intestine is primarily involved in enzymatic digestion and nutrient absorption. The large intestine is mainly involved in water absorption and forming waste.

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14. Which one of the following is the correct sequence of parts in human a

Which one of the following is the correct sequence of parts in human alimentary canal?

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Buccal cavity, oesophagus, stomach, small intestine, large intestine and anus
Buccal cavity, oesophagus, stomach, large intestine, small intestine and anus
Buccal cavity, stomach, oesophagus, small intestine, large intestine and anus
Buccal cavity, stomach, oesophagus, large intestine, small intestine and anus
This question was previously asked in
UPSC Geoscientist – 2020
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Option A represents the correct sequence of parts in the human alimentary canal.
Food enters the human digestive system through the mouth (buccal cavity), then passes down the pharynx and oesophagus to the stomach. From the stomach, it moves into the small intestine, then the large intestine, and finally indigestible material is eliminated through the anus. This represents a unidirectional flow.
The alimentary canal is a continuous tube extending from the mouth to the anus. The small intestine is where most digestion and absorption occur, and it is located between the stomach and the large intestine. The large intestine primarily absorbs water and electrolytes and forms faeces before excretion. The sequence in option A accurately reflects this anatomical pathway.

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15. Which one of the following parts of human alimentary canal can be of m

Which one of the following parts of human alimentary canal can be of maximum length ?

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Stomach
Small intestine
Large intestine
Rectum
This question was previously asked in
UPSC CDS-2 – 2022
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Among the given options, the small intestine is typically the longest part of the human alimentary canal.
– The length of the small intestine in an adult human is usually about 6 to 7.5 meters (20-25 feet) when relaxed, although it measures shorter in a living body due to muscle tone.
– The stomach is a J-shaped organ about 25-30 cm (10-12 inches) long.
– The large intestine is about 1.5 to 1.8 meters (5-6 feet) long.
– The rectum is the final section of the large intestine, about 15-20 cm (6-8 inches) long.
– The small intestine is where most of the digestion and absorption of nutrients takes place, and its great length provides a large surface area for these processes, enhanced by folds, villi, and microvilli.

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16. Which one of the following pairs of enzymes do not work at acidic pH ?

Which one of the following pairs of enzymes do not work at acidic pH ?

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Trypsin and Pepsin
Chymotrypsin and Pepsin
Trypsin and Amylase
Pepsin and Amylase
This question was previously asked in
UPSC CDS-2 – 2022
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Enzymes have optimal pH ranges at which they function effectively. Different enzymes in the digestive system operate at different pH levels corresponding to the environment in which they are found. We need to identify the pair of enzymes that do not function in an acidic environment.
– Pepsin is a protease found in the stomach, which has a highly acidic pH (around 1.5-2.5) due to the secretion of hydrochloric acid. Pepsin has an optimal pH in this acidic range.
– Trypsin and Chymotrypsin are proteases found in the small intestine, where the environment is alkaline (pH around 8) due to the secretion of bicarbonate from the pancreas. They do not work at acidic pH.
– Amylase (like salivary amylase and pancreatic amylase) works best at neutral to slightly alkaline pH (around 6.7-7.0 for salivary, around 8 for pancreatic). It is largely inactive at acidic stomach pH.
The question asks for the pair of enzymes that *do not* work at acidic pH. Trypsin and Amylase both have optimal pH values in the neutral or alkaline range and are inactive in acidic conditions. Pepsin, however, works at acidic pH.
The pH in different parts of the human digestive system varies significantly. The mouth is near neutral, the stomach is highly acidic, the small intestine is alkaline, and the large intestine is slightly acidic. This variation allows for optimal activity of different digestive enzymes.

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17. Which one of the following statements regarding bile secreted by liver

Which one of the following statements regarding bile secreted by liver is not correct?

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Bile contains enzymes for digestion of lipids.
Bile facilitates emulsification of fats.
Bile neutralizes the acidic pH of the food coming from stomach.
Bile makes the pH of the food alkaline and facilitates action of pancreatic enzymes.
This question was previously asked in
UPSC CDS-1 – 2023
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Bile is secreted by the liver and stored in the gallbladder. It plays a crucial role in digestion, particularly of fats, but it does not contain digestive enzymes itself. Statement A is incorrect because bile lacks the enzymes necessary for the chemical breakdown of lipids or any other macromolecules.
The functions of bile primarily include emulsification of fats and neutralization of acidic chyme.
Bile contains bile salts, bile pigments, cholesterol, and phospholipids. Bile salts act as emulsifying agents, breaking large fat globules into smaller droplets, increasing the surface area for enzymatic action by lipases (which are present in pancreatic juice). Bile is alkaline (pH ~7.5-8.8) and helps neutralize the acidic food entering the duodenum from the stomach, creating an optimal environment for the action of pancreatic enzymes.

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18. Which of the following mineral acids is found in human stomach ?

Which of the following mineral acids is found in human stomach ?

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Hydrochloric acid
Lactic acid
Uric acid
Methanoic acid
This question was previously asked in
UPSC CDS-1 – 2022
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Hydrochloric acid (HCl) is the primary mineral acid found in the human stomach.
The parietal cells in the stomach lining secrete hydrochloric acid. This acid creates a highly acidic environment (pH 1.5 to 3.5) which is essential for killing ingested microorganisms, denaturing proteins to facilitate digestion, and activating pepsinogen into pepsin, the main digestive enzyme in the stomach.
Lactic acid is produced during anaerobic respiration in muscles and some microorganisms; it is not typically found free in the stomach. Uric acid is a waste product of purine metabolism and is excreted by the kidneys. Methanoic acid (formic acid) is not found in the human stomach.

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19. Which one of the following is the first enzyme to mix with food in the

Which one of the following is the first enzyme to mix with food in the digestive tract?

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Trypsin
Cellulose
Pepsin
Amylase
This question was previously asked in
UPSC CDS-1 – 2020
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Digestion begins in the mouth, where food is chewed and mixed with saliva. Saliva contains the enzyme salivary amylase (also known as ptyalin), which starts the enzymatic breakdown of carbohydrates (starch) into simpler sugars. Other enzymes listed, such as pepsin (in the stomach) and trypsin (in the small intestine), act later in the digestive tract. Cellulose is a type of carbohydrate, not a human digestive enzyme.
– The mouth is the beginning of the digestive tract where food mixes with saliva.
– Saliva contains salivary amylase (ptyalin).
– Salivary amylase starts the digestion of starch.
– Pepsin acts in the stomach, and trypsin acts in the small intestine.
Salivary amylase works best in a slightly alkaline to neutral pH (around 6.7 to 7.0) present in the mouth. Its activity is inhibited by the acidic environment of the stomach. Although carbohydrate digestion starts in the mouth, it is completed in the small intestine by pancreatic amylase.

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20. Which one of the following acids is produced in human stomach?

Which one of the following acids is produced in human stomach?

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Formic acid
Sulphuric acid
Nitric acid
Hydrochloric acid
This question was previously asked in
UPSC CDS-1 – 2020
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The human stomach produces gastric acid, which is primarily composed of hydrochloric acid (HCl). This acid creates a highly acidic environment in the stomach (pH typically between 1.5 and 3.5), which is essential for activating the enzyme pepsin for protein digestion and killing ingested pathogens.
– Hydrochloric acid (HCl) is the main acid present in gastric juice.
– Gastric acid helps in digestion and provides protection against microorganisms.
Other components of gastric juice include pepsinogen, mucus, and intrinsic factor. Formic acid, Sulphuric acid, and Nitric acid are not naturally produced in the human stomach.

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