Digestion and Absorption Class 11

What is the process of Digestion and Absorption?

Digestion and Absorption

Digestion is the process by which complex food substances are broken down into simpler, absorbable forms. This process involves mechanical and chemical actions starting from the mouth and proceeding through various parts of the alimentary canal like the stomach, small intestine, and so on.

The main aim of digestion is to convert macromolecules like:

Carbohydrates → into glucose
Proteins → into amino acids
Fats (lipids) → into fatty acids and glycerol

Once digestion is complete, these simpler molecules are absorbed into the bloodstream or lymph through the lining of the small intestine – a process known as absorption.

Points:

Mechanical digestion: Physical breakdown (e.g., chewing)
Chemical digestion: Enzymatic breakdown (e.g., amylase, pepsin)
Major sites: Buccal cavity, stomach, small intestine
Absorption mainly occurs in the jejunum and ileum
Specialized structures like villi and microvilli help in efficient absorption

Difference Between Digestion and Absorption

Feature	Digestion	Absorption
Definition	The process of breaking down complex food into simpler, soluble forms.	The process of taking in digested nutrients into the bloodstream or lymph.
Location	Mouth, stomach, and small intestine	Mainly in the small intestine (especially ileum)
Involves	Enzymes, acids, and mechanical processes	Villi and microvilli in the intestinal wall
Purpose	To convert large food molecules into absorbable units	To transport nutrients to body cells for energy and growth
End Product	Simple sugars, amino acids, fatty acids, etc.	Nutrients like glucose, amino acids, vitamins, water
Example	Protein → Peptides → Amino acids	Amino acids absorbed into blood from the intestine

Digestive System – Components

The digestive system is a complex group of organs responsible for the ingestion, digestion, absorption, and elimination of food. It is divided into two main parts:

1. Alimentary Canal (Digestive Tract):

A long, muscular tube starting from the mouth and ending at the anus.
The alimentary canal is a long, muscular, and coiled tube that begins at the mouth and ends at the anus. It forms the main pathway for food digestion, absorption, and elimination in the human body.
This canal is responsible for:

Moving food along the tract
Digesting it into simpler molecules
Absorbing nutrients
Removing undigested waste

Major parts:

Mouth
Pharynx
Oesophagus
Stomach
Small Intestine (Duodenum, Jejunum, Ileum)
Large Intestine (Colon, Rectum, Anus)

Major Organs of the Alimentary Canal

The alimentary canal is made up of several major organs that work in a coordinated manner to carry out the digestion, absorption, and elimination of food.
Each organ has a specific role in the step-by-step transformation of food into absorbable nutrients.

1. Mouth (Oral Cavity)

The mouth, also called the oral cavity, is the first part of the alimentary canal where the process of digestion begins.
It plays a vital role in:

Ingestion of food
Mechanical digestion through chewing (mastication)
Chemical digestion by saliva (which contains the enzyme salivary amylase)

Components of the Oral Cavity:

Teeth – Crush and grind food
Tongue – Helps in mixing food, taste sensation, and swallowing
Salivary glands – Secrete saliva containing mucus and enzymes
Palate (Hard and Soft) – Forms the roof of the mouth and helps in swallowing

2. Pharynx

The pharynx is a short muscular tube that serves as a common passageway for both food and air. It connects the oral cavity to the oesophagus (for food) and the nasal cavity to the larynx (for air).
It plays a key role in the swallowing process (deglutition) by helping push food from the mouth into the oesophagus.
Divisions of the Pharynx:

Nasopharynx – Behind the nasal cavity (only air passes)
Oropharynx – Behind the oral cavity (food and air pass)
Laryngopharynx – Opens into both larynx (voice box) and oesophagus

3. Esophagus

The esophagus (or food pipe) is a muscular, straight tube about 25 cm long that connects the pharynx to the stomach. It lies behind the trachea and passes through the diaphragm to enter the abdominal cavity.
It plays no role in digestion but is essential for transporting food from the mouth to the stomach.
Features:

Made up of smooth muscles
Food moves through the esophagus by peristaltic movements (wave-like muscular contractions)
The gastroesophageal sphincter at its end controls the entry of food into the stomach and prevents backflow (reflux)

4. Stomach

The stomach is a J-shaped, muscular sac-like organ located on the left side of the upper abdominal cavity, just below the diaphragm. It serves as a temporary storage and mixing chamber for food.
It plays a major role in the mechanical and chemical digestion, especially the digestion of proteins.
Main Parts of the Stomach:

Cardiac region – Where food enters from the esophagus
Fundus – Upper, dome-shaped part
Body – Central, major portion
Pyloric region – Lower part leading to the duodenum (small intestine)

5. Small Intestine

The small intestine is a long, coiled tube (about 6 meters) in length, and it is the main site for digestion and absorption of nutrients. It extends from the pyloric end of the stomach to the large intestine.
It is divided into three parts:
1. Duodenum (~25 cm)

First and shortest part
Receives bile from the liver and pancreatic juice from the pancreas
Major site of chemical digestion

2. Jejunum (~2.5 m)

Middle portion
Site of most nutrient absorption
Has numerous villi for increased surface area

3. Ileum (~3.5 m)

Last and longest part
Absorbs vitamin B12, bile salts, and remaining nutrients
Connects to the large intestine via the ileocecal valve

Features:

Lined with villi and microvilli → increase surface area for absorption
Secretes intestinal juice (succus entericus)
Enzymes complete digestion of carbs, proteins, and fats

6. Large Intestine

The large intestine is the final part of the alimentary canal, responsible mainly for absorbing water, electrolytes, and forming solid feces from undigested food. It is shorter but wider than the small intestine (about 1.5 meters long).
It connects the ileum of the small intestine to the anus, and is divided into the colon, rectum, and anal canal.
Main Parts:
Colon:

Ascending colon
Transverse colon
Descending colon
Sigmoid colon
Major site for water and mineral absorption
Hosts gut bacteria that produce vitamins like vitamin K

Rectum:

Short terminal part of the large intestine
Stores feces temporarily before defecation

Anal canal and Anus:

Regulates the elimination of feces through the process of egestion

Functions:

Absorption of water and salts
Formation and temporary storage of feces
Housing symbiotic bacteria that aid in vitamin production

7. Anus

The anus is the terminal opening of the alimentary canal, through which undigested waste (feces) is eliminated from the body.
It plays a vital role in the process of egestion, also known as defecation.

Features:

Surrounded by anal sphincter muscles (internal and external)
These muscles control the voluntary and involuntary release of feces
Responds to signals from the nervous system when the rectum is full

2. Digestive Glands

Digestive glands are specialized glands that secrete enzymes, mucus, and digestive juices to help in the chemical breakdown of complex food molecules into absorbable forms.
These glands do not directly handle food but play a supportive and essential role in the digestive process.

Major Digestive Glands:

1. Salivary Glands

Salivary glands are exocrine glands located in and around the mouth that secrete saliva, which plays a key role in moistening food and beginning the digestion of carbohydrates.
They are the first digestive glands to act on food during ingestion.

Types of Salivary Glands:

There are three pairs of major salivary glands:

Parotid glands – Located near the ears
Submandibular glands – Beneath the lower jaw
Sublingual glands – Under the tongue

Functions of Saliva:

Contains salivary amylase (ptyalin) – starts digestion of starch into maltose
Lubricates food for easier swallowing
Maintains oral hygiene
Provides a medium for taste sensation

2. Liver

The liver is the largest gland in the human body, located in the upper right side of the abdominal cavity, just below the diaphragm. It plays a central role in digestion, metabolism, detoxification, and storage.
As a digestive gland, its main contribution is the production of bile, which aids in fat digestion.

Features:

Weighs around 1.2–1.5 kg
Made up of structural units called hepatic lobules
Covered by a connective tissue capsule called Glisson’s capsule

Digestive Functions of the Liver:

Secretes bile, a greenish-yellow fluid stored in the gallbladder
Bile contains bile salts and pigments (e.g., bilirubin)
Bile salts help in emulsification of fats (breaking fat into small droplets)
No digestive enzyme is present in bile

3. Pancreas

The pancreas is a leaf-shaped, mixed gland (both endocrine and exocrine) located behind the stomach, in the curve of the duodenum. It plays a crucial role in digestion as well as in blood sugar regulation.
As a digestive gland, the pancreas secretes pancreatic juice, which contains several important digestive enzymes.

Exocrine Function (Digestive Role):

Pancreatic juice is secreted into the duodenum via the pancreatic duct
Contains digestive enzymes:
Amylase → digests carbohydrates
Trypsin & chymotrypsin → digest proteins
Lipase → digests fats
Nucleases → digest nucleic acids

Also contains bicarbonate ions (HCO₃⁻) to neutralize stomach acid

Endocrine Function (Brief):

Islets of Langerhans secrete:
Insulin (lowers blood sugar)
Glucagon (raises blood sugar)

4. Intestinal Glands

Intestinal glands are microscopic exocrine glands located in the lining of the small intestine. They secrete intestinal juice (also called succus entericus), which plays a key role in the final steps of digestion.
These glands ensure that complex food molecules are broken down into absorbable units right before absorption.

Types of Intestinal Glands:

1. Crypts of Lieberkühn

Found between the villi in the intestinal mucosa
Secrete enzymes and mucus
Also contain goblet cells and Paneth cells (for antimicrobial defense)

2. Brunner’s Glands

Located in the submucosa of the duodenum
Secrete alkaline mucus to protect intestinal lining from acidic chyme

Enzymes in Intestinal Juice (Succus Entericus):

Maltase, lactase, sucrase – digest disaccharides
Peptidases – complete protein digestion
Lipases – assist in lipid digestion
Nucleotidases – digest nucleic acids

Digestion of Food

Digestion is the process by which complex, insoluble food substances are broken down into simpler, absorbable forms that can be utilized by the body.
It involves mechanical, chemical, and enzymatic actions that begin in the mouth and continue through the alimentary canal.

1. In the Mouth

Mechanical digestion by chewing.
Chemical digestion by salivary amylase (breaks starch into maltose).

2. In the Stomach

Gastric juice contains:
Pepsin: Breaks proteins into peptones.
HCl: Activates enzymes, kills germs.
Mucus: Protects stomach lining.

3. In the Small Intestine

Most digestion occurs here.
Bile from liver (stored in gallbladder) emulsifies fats.
Pancreatic juice has:
Trypsin – proteins → peptides
Amylase – starch → maltose
Lipase – fats → fatty acids + glycerol

Intestinal juice (Succus entericus) contains:

Maltase, Lactase, Sucrase (disaccharides → monosaccharides)
Peptidases (peptides → amino acids)

Absorption of Nutrients

Absorption is the process by which the end products of digestion (like glucose, amino acids, fatty acids, vitamins, and minerals) are taken up by the cells lining the alimentary canal, mainly in the small intestine, and transported into the blood or lymph.
This step is essential for making the digested food available to body cells for energy, growth, and repair.

Main Site of Absorption:

Small Intestine – Especially the jejunum and ileum
Lined with villi and microvilli, which increase surface area for maximum absorption

Modes of Absorption:

Simple Diffusion – e.g., fatty acids, glycerol
Facilitated Transport – e.g., fructose
Active Transport – e.g., glucose, amino acids, minerals
Osmosis – e.g., water

Other Sites:

Mouth – Minor absorption (some drugs)
Stomach – Alcohol, some water
Large Intestine – Water, minerals, vitamins

Assimilation and Egestion

Once digestion and absorption are complete, the body performs two final steps: Assimilation and Egestion.

Assimilation

Assimilation is the process by which the absorbed nutrients are transported via blood to body cells, where they are used for:

Energy production (via cellular respiration),
Growth and repair of tissues,
Storage in the form of glycogen (in liver) or fat (in adipose tissue).

It is a vital step that ensures nutrients serve their physiological purpose in the body.

Egestion

Egestion is the elimination of undigested and unabsorbed food materials from the body through the anus in the form of faeces.
It involves:

Movement of waste through the large intestine,
Absorption of excess water,
Expulsion via defecation.

Digestion and Absorption Chart

Stage	Organ/Part	Process	Enzyme/Secretion	Action/Function
1.	Mouth (Buccal Cavity)	Ingestion, Mechanical & Chemical Digestion	Saliva (Salivary amylase / Ptyalin)	Breaks down starch into maltose; chewing breaks food down
2.	Pharynx & Esophagus	Swallowing, Transport (peristalsis)	—	Moves food to stomach
3.	Stomach	Protein digestion begins	Gastric juice: pepsin, HCl, mucus	HCl kills bacteria, pepsin digests proteins into peptides
4.	Liver	Produces bile	Bile (stored in gallbladder)	Emulsifies fats; prepares for digestion
5.	Pancreas	Secretes digestive enzymes	Pancreatic juice: trypsin, amylase, lipase	Trypsin: proteins, Amylase: carbs, Lipase: fats
6.	Small Intestine (Duodenum, Jejunum, Ileum)	Main site of digestion & absorption	Intestinal juice + pancreatic + bile	Completes digestion; nutrients absorbed by villi
7.	Villi and Microvilli (in Small Intestine)	Absorption	—	Absorb amino acids, sugars, fatty acids into blood/lymph
8.	Large Intestine (Colon)	Water & mineral absorption	—	Absorbs water, forms solid feces
9.	Rectum & Anus	Egestion (defecation)	—	Removes undigested waste from body

Nutrients Digestion Summary

Nutrient	Digestion Starts in	Enzymes Involved	End Products
Carbohydrates	Mouth → Small Intestine	Salivary amylase, Pancreatic amylase, Maltase	Glucose
Proteins	Stomach → Small Intestine	Pepsin, Trypsin, Peptidase	Amino acids
Fats	Small Intestine	Bile (emulsifier), Pancreatic lipase	Fatty acids + Glycerol