HUMAN DIGESTIVE SYSTEM – NEET EXAM KEY NOTES BIOLOGY FOR CBSE, ICSE, STATE STUDENTS

 

1. Human Digestive System Overview

PARTS OF HUMAN DIGESTIVE SYSTEM

• Digestive tract (Alimentary canal): Mouth → Pharynx → Oesophagus → Stomach → Small Intestine → Large Intestine → Rectum → Anus

• Associated glands: Salivary glands, Liver, Pancreas

2. Parts and Functions

a. Mouth & Buccal Cavity

• Digestion begins in the mouth.

• Saliva → secreted by 3 pairs of salivary glands (parotid, submandibular, sublingual).

• Saliva composition: water, salts, mucus, lysozyme, salivary amylase (ptyalin).

• Enzyme action:

  • Starch → Maltose by salivary amylase (pH ≈ 6.8).

• Teeth: 32 permanent (2-1-2-3 per quadrant, i.e., 2123 dental formula).

• Tongue: taste and manipulation of food.

b. Pharynx & Oesophagus

• Pharynx: connects mouth to oesophagus; common passage for food and air.

• Epiglottis: prevents food entry into windpipe.

• Oesophagus: muscular tube → peristaltic movement pushes food into stomach.

• No enzyme secretion here.

c. Stomach

• Regions: cardiac, fundic, pyloric.

• Gastric glands secrete:

  • HCl → converts pepsinogen → pepsin, kills microbes, maintains acidic pH (~1.8).

  • Pepsinogen → active pepsin (proteins → peptones & proteoses).

  • Mucus → protects stomach lining.

  • Intrinsic factor → helps in Vitamin B₁₂ absorption.

• Chyme: semi-digested food leaving stomach.

d. Small Intestine

• Longest part (~6.5 m).

• Divided into duodenum, jejunum, ileum.

• Major digestion and absorption occur here.

Digestive juices:

1. Bile (from liver)

   • No enzymes.

   • Contains bile salts, bile pigments, cholesterol.

   • Emulsifies fats (breaks into small droplets).

2. Pancreatic juice (from pancreas)

   • Trypsinogen → Trypsin (by enterokinase): proteins → peptides

   • Chymotrypsinogen → Chymotrypsin: proteins → peptides

   • Carboxypeptidase: peptides → amino acids

   • Amylase: starch → maltose

   • Lipase: fats → fatty acids + glycerol

3. Intestinal juice (Succus entericus)

   • Maltase: maltose → glucose

   • Lactase: lactose → glucose + galactose

   • Sucrase: sucrose → glucose + fructose

   • Peptidase: peptides → amino acids

   • Lipase: fats → fatty acids + glycerol

   • Nucleosidase: nucleotides → sugar + base

e. Absorption of Digested Products

Nutrient	Site of Absorption	Method

Glucose, amino acids

Small intestine (villi)

Active transport

Fatty acids, glycerol

Small intestine (lacteals)

Diffusion

Water, minerals

Large intestine

Diffusion

Alcohol, some drugs

Stomach

Simple diffusion

f. Large Intestine

• Parts: caecum, colon, rectum.

• Functions:

  Absorption of water, minerals, vitamins.
• Formation of feces.
• Houses symbiotic bacteria (e.g., E. coli produces vitamin K and B-complex).

g. Disorders

Disease	Cause	Symptoms
Jaundice	Liver disorder (bile pigments in blood)	Yellowing of skin & eyes
Vomiting	Reflex action (medulla oblongata)	Expulsion of gastric contents
Diarrhea	Intestinal infection	Watery stools, dehydration
Constipation	Reduced bowel movement	Hard stools
Indigestion	Improper enzyme secretion	Abdominal discomfort

3. Enzymes Summary Table (Most Important)

Enzyme	Source	Substrate	Product
Salivary amylase	Saliva	Starch	Maltose
Pepsin	Stomach	Proteins	Peptones
Lipase (gastric)	Stomach	Fats	Fatty acids + Glycerol
Trypsin	Pancreas	Proteins/Peptides	Peptides/Amino acids
Amylase (pancreatic)	Pancreas	Starch	Maltose
Lipase (pancreatic)	Pancreas	Fats	Fatty acids + Glycerol
Maltase	Intestine	Maltose	Glucose
Sucrase	Intestine	Sucrose	Glucose + Fructose
Lactase	Intestine	Lactose	Glucose + Galactose
Peptidase	Intestine	Peptides	Amino acids