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

Alimentary Canal: Mouth → Pharynx → Esophagus → Stomach → Small intestine → Large intestine → Rectum → Anus
Associated Glands: Salivary glands, Liver, Pancreas

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:

Type	Function	Shape
Incisors (8)	Cutting	Chisel-shaped
Canines (4)	Tearing	Pointed
Premolars (8)	Crushing	Flat crown
Molars (12)	Grinding	Broad crown

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:

Region	Location	Function
Cardiac	Near esophagus	Entry of food
Fundus	Upper dome	Temporary storage
Body	Main part	Mixing, churning
Pyloric	Near small intestine	Controlled release

Gastric Glands (Types of Cells):

Cell Type	Secretion	Function
Mucous cells	Mucus	Protects stomach lining
Parietal (Oxyntic)	HCl, Intrinsic factor	Activates pepsinogen, kills bacteria
Chief (Peptic/Zymogenic)	Pepsinogen	Protein digestion (as pepsin)
G cells	Gastrin	Stimulates gastric secretion

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:

Part	Length	Function
Duodenum	~25 cm	Receives bile and pancreatic juice
Jejunum	~2.5 m	Major digestion and absorption
Ileum	~3.5 m	Absorption, especially B12

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

Digestive Glands

1. Salivary Glands

Gland	Location	Secretion
Parotid	Below ear	Serous (watery, amylase-rich)
Submandibular	Below jaw	Mixed
Sublingual	Below tongue	Mucous

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:

Component	Function
Bile salts	Emulsification of fats
Bile pigments	Bilirubin, biliverdin (waste)
Cholesterol	Excretion
Phospholipids	Emulsification

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:

Enzyme	Substrate	Product
Trypsinogen (→ Trypsin)	Proteins	Peptides
Chymotrypsinogen (→ Chymotrypsin)	Proteins	Peptides
Procarboxypeptidase (→ Carboxypeptidase)	Peptides	Amino acids
Pancreatic amylase	Starch	Maltose
Pancreatic lipase	Fats	Fatty acids + Glycerol
Nucleases	Nucleic acids	Nucleotides

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

Digestion Process

In Mouth

Nutrient	Enzyme	Product
Starch	Salivary amylase	Maltose + Dextrins
(30% digested in mouth)

In Stomach

Nutrient	Enzyme	Product
Proteins	Pepsin	Proteoses, Peptones
Milk casein	Rennin	Calcium paracaseinate
Fats	Gastric lipase	Fatty acids (minor)

In Small Intestine

Complete digestion occurs here.

Nutrient	Source	Enzyme	Final Product
Starch	Pancreatic amylase	Amylase	Maltose
Maltose	Intestinal juice	Maltase	Glucose
Sucrose	Intestinal juice	Sucrase	Glucose + Fructose
Lactose	Intestinal juice	Lactase	Glucose + Galactose
Proteins	Pancreas	Trypsin, Chymotrypsin	Peptides
Peptides	Intestine	Peptidases	Amino acids
Fats	Pancreas	Lipase	Fatty acids + Glycerol
Nucleic acids	Pancreas	Nucleases	Nucleotides
Nucleotides	Intestine	Nucleotidases	Sugars + Bases + PO₄

Absorption

Site of Absorption

Substance	Primary Site
Glucose, amino acids	Small intestine (jejunum, ileum)
Fatty acids	Small intestine (via lacteals)
Vitamin B12	Terminal ileum
Water	Large intestine
Alcohol	Stomach
Drugs	Stomach

Absorption Mechanisms

Mechanism	Substances	Energy Required
Passive diffusion	Water, small molecules	No
Facilitated diffusion	Fructose	No
Active transport	Glucose, amino acids, Na⁺	Yes (ATP)

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

Intestinal Adaptations for Absorption

Structural Adaptations

Feature	Increase in Surface Area
Length (~6 m)	Base surface area
Circular folds (plicae)	3×
Villi	10×
Microvilli	20×
Total	~600× (~250 m²)

Villi Structure:

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

Regulation of Digestion

Hormonal Regulation

Hormone	Source	Stimulus	Action
Gastrin	Stomach (G cells)	Food in stomach	↑ HCl secretion
Secretin	Duodenum	Acid chyme	↑ Pancreatic bicarbonate
CCK (Cholecystokinin)	Duodenum	Fat, protein	↑ Bile release, ↑ Pancreatic enzymes
GIP	Duodenum	Glucose, fat	↓ Gastric secretion, ↑ Insulin

Neural Regulation

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

Digestive Disorders

Disorder	Cause	Symptoms
GERD (Acid reflux)	Weak LES	Heartburn
Peptic ulcer	H. pylori, NSAIDs	Stomach pain, bleeding
Jaundice	Liver dysfunction, bile duct obstruction	Yellow skin, dark urine
Constipation	Low fiber, dehydration	Infrequent bowel movements
Diarrhea	Infection, food poisoning	Frequent loose stools
Lactose intolerance	Lactase deficiency	Bloating, gas after milk

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

Quick Revision Points

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

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.

Key Takeaways

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