Chapter Overview & Weightage
Digestion and Absorption covers the human digestive system — from mouth to anus — along with the enzymes, secretions, and absorption mechanisms at each stage. NEET tests this through enzyme-substrate matching, organ-function pairing, and disorder identification.
This chapter carries 3-4% weightage in NEET, with 2-3 questions per paper. Questions are typically factual — enzyme names, pH requirements, absorption sites.
| Year | NEET (Q count) | Key Topics Tested |
|---|---|---|
| 2024 | 2 | Bile function, absorption in ileum |
| 2023 | 3 | Pepsin pH, pancreatic enzymes, protein digestion sequence |
| 2022 | 2 | HCl function in stomach, deficiency disorders |
Key Concepts You Must Know
Tier 1 (Core concepts)
- Complete digestive tract: mouth → pharynx → oesophagus → stomach → small intestine (duodenum, jejunum, ileum) → large intestine → rectum → anus
- Enzymes of digestion: salivary amylase, pepsin, trypsin, lipase, maltase, lactase, sucrase
- Role of HCl: activates pepsinogen → pepsin, kills bacteria, provides acidic pH
- Bile salts: emulsification of fats (not enzymatic digestion)
- Absorption: mainly in small intestine (jejunum and ileum); water absorption in large intestine
Tier 2 (Frequently tested)
- Peristalsis and segmentation movements
- Pancreatic juice: trypsinogen, chymotrypsinogen, procarboxypeptidase, lipase, amylase
- Enterokinase converts trypsinogen to trypsin (this activates other zymogens)
- Absorption mechanisms: active transport (amino acids, glucose), passive (fructose), fatty acids via lacteals
Tier 3 (Occasionally tested)
- Disorders: PEM (kwashiorkor, marasmus), indigestion, constipation, vomiting, jaundice, diarrhoea
- Dental formula of humans:
- Caecum and appendix (vestigial in humans)
Important Formulas
| Organ | Enzyme/Secretion | Substrate | Product |
|---|---|---|---|
| Mouth | Salivary amylase (ptyalin) | Starch | Maltose |
| Stomach | Pepsin (from pepsinogen) | Proteins | Peptones, proteoses |
| Stomach | HCl | — | Activates pepsinogen, kills bacteria |
| Duodenum | Pancreatic amylase | Starch | Maltose |
| Duodenum | Trypsin | Proteins | Peptides |
| Duodenum | Lipase | Fats (emulsified) | Fatty acids + glycerol |
| Small intestine | Maltase | Maltose | Glucose + glucose |
| Small intestine | Lactase | Lactose | Glucose + galactose |
| Small intestine | Sucrase | Sucrose | Glucose + fructose |
| Nutrient | Site of Absorption | Mechanism |
|---|---|---|
| Glucose, amino acids | Small intestine (jejunum) | Active transport |
| Fructose | Small intestine | Facilitated diffusion |
| Fatty acids, glycerol | Small intestine (ileum) | Via lacteals (lymph) |
| Water | Large intestine | Osmosis |
| Vitamin B12 | Ileum | Active transport (intrinsic factor needed) |
| Alcohol | Stomach (partial) | Simple diffusion |
Remember the enzyme activation cascade: Enterokinase (from intestinal mucosa) activates trypsinogen → trypsin. Then trypsin activates chymotrypsinogen → chymotrypsin and procarboxypeptidase → carboxypeptidase. Enterokinase is the master switch. NEET tests this cascade regularly.
Solved Previous Year Questions
PYQ 1 — NEET 2024
Problem: Bile juice contains no enzymes but is essential for digestion because:
(A) It activates trypsinogen (B) It emulsifies fats (C) It digests proteins (D) It neutralises HCl only
Solution:
Bile contains bile salts and bile pigments (bilirubin, biliverdin) but no digestive enzymes. Its primary digestive role is emulsification of fats — breaking large fat globules into smaller droplets, increasing the surface area for lipase action.
Bile also neutralises the acidic chyme from the stomach, but the key digestive function is emulsification.
Answer: (B) It emulsifies fats
PYQ 2 — NEET 2023
Problem: The enzyme enterokinase converts:
(A) Pepsinogen to pepsin (B) Trypsinogen to trypsin (C) Proteins to proteoses (D) Fats to fatty acids
Solution:
Enterokinase (also called enteropeptidase) is secreted by the intestinal mucosa. It specifically activates trypsinogen to trypsin. This is the key activation step — trypsin then activates other pancreatic zymogens.
Pepsinogen is activated by HCl in the stomach, not by enterokinase.
Answer: (B) Trypsinogen to trypsin
PYQ 3 — NEET 2022
Problem: Kwashiorkor is caused by deficiency of:
(A) Calories (B) Protein (C) Both protein and calories (D) Vitamins
Solution:
Protein-energy malnutrition (PEM) includes two conditions:
- Kwashiorkor: protein deficiency (but adequate calories) — symptoms include edema, pot belly, thin limbs
- Marasmus: deficiency of both protein and calories — symptoms include extreme wasting
Answer: (B) Protein
Kwashiorkor = protein deficiency. Marasmus = both protein and calorie deficiency. Don’t swap them. The child with kwashiorkor may actually have a swollen belly (edema from low albumin), which looks paradoxically “well-fed” — a classic NEET trap.
Difficulty Distribution
| Difficulty | % of Questions | What to Expect |
|---|---|---|
| Easy | 50% | Enzyme-organ matching, direct definitions |
| Medium | 35% | Enzyme activation cascade, absorption mechanism |
| Hard | 15% | Hormonal regulation of digestion, disorder specifics |
Expert Strategy
Day 1: Trace the digestive journey from mouth to anus. At each organ, note: what secretion is produced, what enzyme it contains, what substrate it acts on, what product is formed. Build a single table covering the entire tract.
Day 2: Focus on the small intestine — it does the most work. Brush border enzymes, pancreatic enzymes, bile function. Understand the difference between enzymatic digestion (breaking bonds) and emulsification (physical process).
Day 3: Absorption mechanisms and disorders. Know which nutrients are absorbed where and by which mechanism. For disorders, 2-3 lines per disorder is sufficient for NEET.
NCERT diagrams of the digestive system with labelled glands are directly used in NEET. Study the diagrams on pages of NCERT Chapter 16. Being able to identify structures in a diagram is worth 1-2 easy marks.
Common Traps
Trap 1 — Bile has no enzymes. Despite being essential for fat digestion, bile contains NO enzymes. It performs emulsification (a physical process). Many students incorrectly call bile a “digestive enzyme.”
Trap 2 — HCl activates pepsinogen, not enterokinase. Enterokinase activates trypsinogen. HCl activates pepsinogen. These are two different activation events in two different organs (stomach vs intestine). Mixing them up is the most common error in this chapter.
Trap 3 — Fats are absorbed via lacteals, not blood capillaries. Fatty acids and glycerol are re-esterified in intestinal cells to form chylomicrons, which enter the lymphatic system (lacteals) first, then reach the blood. Other nutrients go directly into blood capillaries.
Trap 4 — Salivary amylase works at neutral/slightly acidic pH. It is inactivated in the stomach’s acidic environment. So starch digestion starts in the mouth but stops in the stomach. Pancreatic amylase resumes starch digestion in the duodenum (alkaline pH).