Digestion and Absorption

NCERT Line-by-Line Breakdown

Unit 5: Human Physiology

⚠️ Syllabus Alert: NEET 2026

According to the official 2026 notification provided, Human Digestion is NOT listed in Unit 5. Unit 5 starts directly with “Breathing and Exchange of Gases”.

However, “Digestive System of Insect (Frog)” IS included in Unit 2. This post covers the standard Human Physiology content for comprehensive knowledge, followed by the Syllabus-Compliant Frog Digestion section.

Digestion is the process of conversion of complex insoluble food substances into simple absorbable forms. It is carried out by the digestive system by mechanical and biochemical methods.

1. Human Digestive System

[Image of human digestive system diagram]

A. Alimentary Canal

Begins at the mouth (anterior) and opens out through the anus (posterior).

🦷 Dentition (Teeth)

Thecodont: Embedded in jaw socket.
Diphyodont: Two sets of teeth (Milk & Permanent).
Heterodont: 4 types: Incisors (I), Canines (C), Premolars (PM), Molars (M).
Dental Formula (Adult): 2123 / 2123

[Image of human dental formula]

B. Stomach & Intestine

Stomach: J-shaped bag. Parts: Cardiac, Fundic, Body, Pyloric.
Small Intestine: Duodenum (C-shaped), Jejunum (coiled), Ileum (highly coiled).
Large Intestine: Caecum (Symbiotic microbes), Colon (Ascending, Transverse, Descending, Sigmoid), Rectum.

C. Histology of Gut

Wall possesses 4 layers (outer to inner):

Serosa: Mesothelium.
Muscularis: Outer Longitudinal, Inner Circular (Oblique in stomach).
Sub-mucosa: Loose CT. Contains Brunner’s Glands (in Duodenum).
Mucosa: Forms Rugae (Stomach) and Villi (Intestine).

[Image of transverse section of gut layers]

2. Digestive Glands

1. Salivary Glands
3 Pairs: Parotids (Cheek), Sub-maxillary (Lower Jaw), Sub-linguals (Below tongue). Secrete Saliva (Amylase).

2. Liver (Largest Gland)
Weight: 1.2-1.5 kg. Structural unit: Hepatic Lobules covered by Glisson’s Capsule. Secretes Bile (No enzymes, only salts).

3. Pancreas
Heterocrine (Exocrine + Endocrine). Exocrine part secretes alkaline pancreatic juice containing enzymes.

3. Digestion of Food

A. In Mouth (Buccal Cavity)

Starch (30%) → (Salivary Amylase, pH 6.8) → Maltose.

B. In Stomach

Gastric glands have 3 cell types:

🔹 Mucus neck cells: Mucus.
🔹 Peptic/Chief cells: Pepsinogen (Pro-enzyme).
🔹 Parietal/Oxyntic cells: HCl & Intrinsic Factor (Vit B12 absorption).

Reaction: Proteins → (Pepsin, pH 1.8) → Proteoses + Peptones.

C. In Small Intestine

Succus Entericus (Intestinal Juice) completes digestion.

Pancreatic Enzymes:
• Trypsinogen → (Enterokinase) → Trypsin.
• Proteins → (Trypsin) → Dipeptides.
• Fats → (Lipases) → Diglycerides.
• Nucleic Acids → (Nucleases) → Nucleotides.

4. Absorption of Food

Nutrient	Mechanism
Glucose, Amino Acids	Active Transport (requires energy) or Facilitated Transport (Carrier Na+).
Fats	Cannot enter blood directly. Form Micelles → Chylomicrons (Protein-coated) → Lymph vessels (Lacteals).
Water	Osmosis (Large intestine absorbs significant water).

[Image of villi structure]

5. Disorders of Digestive System

Jaundice: Liver affected. Skin/eyes turn yellow (Bile pigments).
Vomiting: Ejection of stomach contents. Reflex in Medulla.
PEM (Protein Energy Malnutrition):
- Marasmus: Deficiency of Protein + Calories (< 1 year).
- Kwashiorkor: Deficiency of Protein only (> 1 year). Edema present.

🐸 Frog Digestive System (Included in Unit 2)

Since the syllabus explicitly mentions the Digestive System of Frog, memorize this:

Carnivorous: Alimentary canal is short.
Tongue: Bilobed and Sticky (to catch prey).
Stomach: Secretes HCl and Gastric juices.
Liver: Secretes bile (stored in Gall Bladder).
Pancreas: Secretes pancreatic juice.
Digestion: Occurs in Stomach (HCl + Gastric juice) and Intestine (Bile + Pancreatic juice).
Villi: Finger-like folds in the intestine for absorption.
Cloaca: Undigested waste is passed out through this common opening.

📝 Rapid Fire MCQs

Q1. Which cells of the stomach secrete HCl and Intrinsic Factor?

A) Peptic cells
B) Chief cells
C) Oxyntic cells
D) Goblet cells

Click to check Answer

Answer: C) Oxyntic cells (Parietal cells).

Q2. In Frogs, the common opening for digestive and excretory system is:

A) Anus
B) Cloaca
C) Rectum
D) Ureter