Human Digestion and Absorption - Complete NEET Biology Notes

Human Physiology is the highest weightage unit in NEET Biology, and Digestion contributes 4-6 questions annually. This comprehensive guide covers all NCERT concepts.

Overview of Digestive System

Definition: Process of converting complex food into simple absorbable forms through mechanical and chemical breakdown.

Components

1. Alimentary Canal: Mouth → Pharynx → Esophagus → Stomach → Small intestine → Large intestine → Rectum → Anus

2. Associated Glands: Salivary glands, Liver, Pancreas

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Alimentary Canal

1. Mouth (Buccal Cavity)

Structures:

• Teeth (32 in adults): I 2/2, C 1/1, PM 2/2, M 3/3 = 32
• Tongue: Taste, mixing, swallowing
• Salivary glands

Dental Formula:

• Adults: 2123/2123 = 32
• Children: 2102/2102 = 20 (milk teeth)

Types of Teeth:

2. Pharynx

• Common passage for food and air
• Epiglottis prevents food entry into trachea

3. Esophagus (Food Pipe)

• ~25 cm long muscular tube
• No digestion occurs
• Peristalsis: Wave-like muscular contractions push food

4. Stomach

Shape: J-shaped muscular organ

Regions:

Gastric Glands (Types of Cells):

Gastric Juice Composition:

• HCl (pH 1.5-2.5)
• Pepsinogen → Pepsin (active)
• Gastric lipase (minor fat digestion)
• Rennin/Chymosin (milk protein, mainly in infants)

5. Small Intestine

Length: ~6 meters (longest part of alimentary canal)

Three Parts:

Intestinal Glands:

• Crypts of Lieberkühn: Secrete intestinal juice (succus entericus)
• Brunner's glands: Secrete mucus (only in duodenum)

6. Large Intestine

Length: ~1.5 meters

Parts: Caecum → Colon (ascending, transverse, descending, sigmoid) → Rectum → Anus

Functions:

• Water absorption
• Electrolyte absorption
• Formation of feces
• Houses gut bacteria (vitamin synthesis)

Vermiform Appendix:

• Vestigial organ
• Attached to caecum
• Contains lymphoid tissue

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Digestive Glands

1. Salivary Glands

Saliva Composition:

• Water (99.5%)
• Salivary amylase (ptyalin): Starch → Maltose
• Lysozyme: Antibacterial
• Mucin: Lubrication
• pH: 6.8 (slightly acidic)

2. Liver (Largest Internal Organ)

Weight: ~1.5 kg

Functions:

• Bile production
• Detoxification
• Glycogen storage
• Plasma protein synthesis
• Cholesterol metabolism

Bile:

• Produced by hepatocytes
• Stored in gallbladder
• Released into duodenum via bile duct
• Contains NO digestive enzymes

Bile Composition:

NEET Important: Bile emulsifies fats (breaks into smaller droplets) but does NOT digest them. Lipase digests fats.

3. Pancreas

Type: Both exocrine and endocrine

Exocrine Function (Digestive):

• Secretes pancreatic juice into duodenum
• Alkaline pH (7.8-8.2) neutralizes gastric acid

Pancreatic Enzymes:

Activation: Trypsinogen → Trypsin (by enterokinase from intestine) Then trypsin activates other zymogens.

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Digestion Process

In Mouth

In Stomach

In Small Intestine

Complete digestion occurs here.

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Absorption

Site of Absorption

Absorption Mechanisms

Absorption of Different Nutrients

Carbohydrates:

• Absorbed as monosaccharides
• Glucose and galactose: Active transport (with Na⁺)
• Fructose: Facilitated diffusion

Proteins:

• Absorbed as amino acids
• Active transport (with Na⁺)

Fats:

• Form micelles with bile salts
• Fatty acids and glycerol absorbed into enterocytes
• Re-esterified to triglycerides
• Packaged as chylomicrons
• Enter lymphatic system (lacteals)

Water and Electrolytes:

• Most in small intestine
• Remaining in large intestine
• Osmosis and active transport

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Intestinal Adaptations for Absorption

Structural Adaptations

Villi Structure:

• Finger-like projections
• Covered by columnar epithelium
• Central lacteal (lymph vessel)
• Blood capillaries
• Brush border (microvilli)

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Regulation of Digestion

Hormonal Regulation

Neural Regulation

• Parasympathetic (Vagus): Stimulates secretion
• Sympathetic: Inhibits digestion
• Enteric nervous system: Local reflexes

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Digestive Disorders

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Previous Year NEET Questions

Q1 (NEET 2023): Which of the following enzymes is secreted by stomach?

• (a) Trypsin
• (b) Lipase
• (c) Pepsin ✓
• (d) Amylase

Q2 (NEET 2022): Emulsification of fat is done by:

• (a) Lipase
• (b) Bile salts ✓
• (c) Trypsin
• (d) Pepsin

Q3 (NEET 2021): Intrinsic factor for B12 absorption is secreted by:

• (a) Chief cells
• (b) Parietal cells ✓
• (c) Goblet cells
• (d) G cells

Q4 (NEET 2020): Enterokinase converts:

• (a) Pepsinogen to pepsin
• (b) Trypsinogen to trypsin ✓
• (c) Proteins to proteoses
• (d) Fats to fatty acids

Q5 (NEET 2019): The absorbed fatty acids are transported as:

• (a) Glucose
• (b) Amino acids
• (c) Chylomicrons ✓
• (d) Simple lipids

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Quick Revision Points

1. Dental formula adults: 2123/2123 = 32
2. Largest gland: Liver
3. Largest internal organ: Liver
4. Longest part of alimentary canal: Small intestine (~6 m)
5. Bile contains NO enzymes
6. Emulsification ≠ Digestion
7. Pepsinogen activated by: HCl
8. Trypsinogen activated by: Enterokinase
9. Rennin found in: Infants (milk digestion)
10. Salivary pH: 6.8
11. Gastric pH: 1.5-2.5
12. Fats absorbed into: Lacteals (lymphatics)
13. Vitamin B12 absorbed in: Terminal ileum
14. Appendix location: Caecum

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FAQs

Q: Why doesn't the stomach digest itself? A: The stomach is protected by: (1) thick mucus layer secreted by mucous cells, (2) bicarbonate ions in mucus neutralizing acid, and (3) rapid cell turnover replacing damaged cells.

Q: Why is bile important even without enzymes? A: Bile salts emulsify fats into smaller droplets, increasing surface area for lipase action. Without bile, fat digestion is severely impaired.

Q: What is the role of HCl in digestion? A: HCl (1) activates pepsinogen to pepsin, (2) provides optimal pH for pepsin (1.5-2.5), (3) kills most bacteria, and (4) denatures proteins making them easier to digest.

Q: Why are fats absorbed differently from carbohydrates and proteins? A: Fatty acids and glycerol are hydrophobic. They form chylomicrons (lipoprotein particles) that are too large to enter blood capillaries directly, so they enter lacteals (lymphatic vessels) first.

Q: What causes lactose intolerance? A: Lactose intolerance occurs due to deficiency of lactase enzyme. Undigested lactose passes to the large intestine where bacteria ferment it, producing gas and causing bloating, cramps, and diarrhea.