Digestive System and Enzymes
~9 min read
- Pathway: Mouth → oesophagus → stomach → small intestine → large intestine → rectum.
- Key enzymes: Salivary amylase (starch), pepsin (protein, acidic), trypsin (protein, alkaline), lipase (fat).
- Absorption: Small intestine — longest part — has villi for nutrient absorption. Water absorbed in large intestine.
The digestive system is a CDS/OTA favourite — over 50 PYQs route here, mainly on the alimentary canal, enzymes and bile.
The Alimentary Canal
- Mouth (buccal cavity): digestion begins here. Salivary amylase (ptyalin) starts starch breakdown into maltose.
- Oesophagus: pipe carrying food by peristalsis — no digestion.
- Stomach: HCl creates acidic pH (~1.5–2); pepsin begins protein digestion.
- Small intestine (~6–7 m): the longest part. Bile from liver emulsifies fats; pancreatic juice (trypsin, amylase, lipase) + intestinal juice complete digestion. Villi absorb nutrients into blood.
- Large intestine: absorbs water and electrolytes; forms faeces.
Major Digestive Enzymes
| Enzyme | Source | Substrate → Product | Optimal pH |
|---|---|---|---|
| Salivary amylase (ptyalin) | Salivary glands | Starch → Maltose | ~6.8 (neutral) |
| Pepsin | Stomach | Protein → Peptides | 1.5–2 (acidic) |
| Trypsin / Chymotrypsin | Pancreas | Protein → Peptides | 8–9 (alkaline) |
| Pancreatic amylase | Pancreas | Starch → Maltose | Alkaline |
| Pancreatic lipase | Pancreas | Fats → Fatty acids + Glycerol | Alkaline |
| Lysozyme | Saliva, tears | Antibacterial — breaks bacterial cell wall | — |
Bile and Liver
- Bile is produced by the liver and stored in the gall bladder. It contains no digestive enzymes.
- Functions of bile: emulsifies fats (breaks them into tiny droplets), neutralises the acid coming from the stomach, and makes intestinal contents alkaline so pancreatic enzymes can work.
- Urea (waste product of protein metabolism) is produced in the liver via the urea cycle, transported in blood, and excreted by kidneys.
- Trypsin and chymotrypsin do not work at acidic pH — they need the alkaline environment of the small intestine.
Where Things Are Absorbed
- Small intestine — main site of absorption of glucose, amino acids, vitamins, fatty acids. Villi and microvilli hugely increase surface area.
- Large intestine — absorbs water and electrolytes; forms semi-solid faeces.
- Liver — hepatic cells re-absorb metabolites from portal blood.
- Renal tubules — re-absorb water and useful solutes (in the kidney, not digestive tract).
CDS/OTA PYQ Examples
Q: In human digestive system, the process of digestion starts in:
(a) Oesophagus (b) Buccal cavity (c) Duodenum (d) Stomach
Answer: (b) Buccal cavity — salivary amylase begins starch digestion. [CDS-I 2015]
Q: Which one of the following is the first enzyme to mix with food in the digestive tract?
(a) Trypsin (b) Cellulose (c) Pepsin (d) Amylase
Answer: (d) Amylase (salivary amylase / ptyalin). [CDS-I 2020]
Q: Which pair of enzymes do NOT work at acidic pH?
(a) Trypsin and Pepsin (b) Chymotrypsin and Pepsin (c) Trypsin and Amylase (d) Pepsin and Amylase
Answer: (c) Trypsin and Amylase — both prefer alkaline pH. [CDS-II 2022]
Q: Which one of the following statements regarding bile is NOT correct?
(a) Bile contains enzymes for digestion of lipids (b) Bile emulsifies fats (c) Bile neutralises acidic pH from stomach (d) Bile makes food alkaline
Answer: (a) Bile contains NO digestive enzymes — it only emulsifies fat. [CDS-I 2023]
Q: Which one of the following parts of human alimentary canal can be of maximum length?
(a) Stomach (b) Small intestine (c) Large intestine (d) Rectum
Answer: (b) Small intestine (~6–7 m). [CDS-II 2022]
Q: An antibacterial enzyme present in saliva and tears is:
(a) ribozyme (b) lipase (c) lysozyme (d) isomerase
Answer: (c) Lysozyme — breaks down bacterial cell walls. [CDS-I 2024]
Drill Digestive System and Enzymes for CDS/OTA
CDS/OTA-pattern items on Digestive System and Enzymes with answer keys and explanations.
Start Free Mock TestFrequently Asked Questions
Where is urea produced in the body?
Urea is produced in the liver during deamination of amino acids (the urea cycle). It is then carried in blood to the kidneys where it is filtered out and excreted in urine.
Why doesn't pepsin work in the small intestine?
Pepsin needs acidic pH (1.5–2) found in the stomach. Bile and pancreatic juice make the small intestine alkaline, so pepsin is inactivated and trypsin/chymotrypsin take over.
What is the role of HCl in the stomach?
Activates pepsinogen to pepsin, kills swallowed bacteria, and provides the acidic pH that pepsin needs to work.