In human body, the digestion of protein begins in which of the following organs ?
- (a)Liver
- (b)Mouth
- (c)Small intestine
- (d)Stomach
Correct — D, the stomach. Protein digestion begins in the stomach. Gastric glands secrete hydrochloric acid, which activates the inactive enzyme pepsinogen into pepsin; pepsin, working in the acidic medium, breaks proteins down into smaller peptides. No protein-digesting enzyme acts earlier in the tract — saliva in the mouth has no protease. Protein digestion is then completed in the small intestine (by pancreatic trypsin/chymotrypsin and intestinal peptidases), but it BEGINS in the stomach.
- (a)Liver — The liver secretes bile, which emulsifies fats — it has no role in starting protein digestion and secretes no protein-digesting enzyme.
- (b)Mouth — In the mouth, saliva contains ptyalin (salivary amylase), which begins the digestion of starch (carbohydrate), not protein — there is no protease in saliva.
- (c)Small intestine — The small intestine COMPLETES protein digestion (trypsin, chymotrypsin, peptidases) but does not begin it — digestion of protein has already started in the stomach with pepsin.
Digestion of the three macronutrients begins in different places. Carbohydrate (starch) digestion begins in the mouth (salivary amylase/ptyalin). Protein digestion begins in the stomach, where HCl converts pepsinogen to pepsin and pepsin breaks proteins into peptides. Fat digestion mainly occurs in the small intestine (bile emulsifies, pancreatic lipase digests). The small intestine is where all three are finally completed and absorbed.
The trap is that the small intestine is the main site of digestion, so it looks right — but the stem asks where protein digestion BEGINS. Track the enzymes: mouth = amylase (starch), stomach = pepsin (protein), small intestine = trypsin/chymotrypsin + others (completion). The first protein-acting enzyme is pepsin in the stomach, so the answer is the stomach.
- Stomach: HCl activates pepsinogen → pepsin, which starts breaking proteins into peptides
- Mouth: salivary amylase (ptyalin) begins starch (carbohydrate) digestion — no protease
- Small intestine: completes protein digestion (trypsin, chymotrypsin, peptidases)
- Liver: secretes bile for fat emulsification, not protein digestion
- Mouth: salivary amylase — starch only (no protein digestion)
- Stomach: HCl + pepsin — PROTEIN DIGESTION BEGINS
- Small intestine: trypsin/chymotrypsin — protein digestion completed
Pepsin in the acidic stomach is the first enzyme to act on proteins — so protein digestion begins in the stomach; the small intestine only completes it.
- Choosing 'small intestine' because it is the main digestive site — but protein digestion only completes there, it begins in the stomach
- Thinking the mouth starts protein digestion — saliva digests starch, not protein
UPPSC/UPSC ask where a specific nutrient's digestion begins or which enzyme/organ acts on it — the key discrimination is BEGIN vs COMPLETE, and matching enzyme to nutrient (pepsin ⇒ protein ⇒ stomach).
Match List I (Substance) with List II (Physiological role): I. Ptyalin II. Pepsin III. Renin IV. Oxytocin — A) Converts angiotensinogen in blood into angiotensin B) Digests starch C) Digests proteins D) Hydrolyses fats E) Induces contraction of smooth muscles
- (a) I-B, II-C, III-A, IV-E
- (b) I-C, II-D, III-B, IV-E
- (c) I-B, II-C, III-E, IV-A
- (d) I-C, II-A, III-B, IV-D
Answer(a) I-B, II-C, III-A, IV-E
Same concept — digestive enzymes and their roles. It confirms pepsin digests proteins (the gastric enzyme that starts protein digestion in the stomach) while ptyalin digests starch, exactly the discrimination this UPPSC question rewards.
Match List I (Vitamin / Enzyme / Hormone / Protein) with List II: I. Vitamin II. Enzyme III. Hormone IV. Protein — A. Pepsin B. Carotene C. Keratin D. Progesterone
- (a) I – A, II – B, III – C, IV – D
- (b) I – B, II – A, III – D, IV – C
- (c) I – B, II – A, III – C, IV – D
- (d) I – A, II – B, III – D, IV – C
Answer(b) I-B, II-A, III-D, IV-C
Related concept — it classifies pepsin as a digestive enzyme, the protease that acts in the stomach; recognising pepsin as the protein-digesting enzyme underlies why protein digestion begins in the stomach.
- practice — not a real PYQ
Which enzyme is responsible for beginning protein digestion in the human stomach?
- (a)Ptyalin
- (b)Pepsin
- (c)Trypsin
- (d)Lipase
Answer(b) Pepsin — activated from pepsinogen by stomach HCl, it breaks proteins into peptides.
- practice — not a real PYQ
Digestion of starch (carbohydrate) begins in which organ?
- (a)Stomach
- (b)Mouth
- (c)Small intestine
- (d)Liver
Answer(b) Mouth — salivary amylase (ptyalin) begins starch digestion in the mouth.