Process of Digestion

Learning Objectives:

At the end of this session, you will be able to know about

  • Digestion process
  • Steps involved in absorption process
  • Steps involved in metabolic pathway
  • Site of storage

Lesson Plan:


          Carbohydrates is an important sources energy to the body. Dietary carbohydrates consist of Polysaccharides, Disaccharides and monosaccharides. Only monosaccharides could be absorbed into the blood stream. Through digestion process complex carbohydrates are converted into simple sugars.

The Process of Digestion:

          Digestion of food is the break down of larger molecules into smaller molecules through mechanical and chemical process.

          Different organs in the Gastrointestinal tract work together in the process of digestion.


          While we talk about carbohydrates, the first stage of digestion begins in the mouth. During mastication or chewing the food, saliva gets secreted along with an enzyme called salivary amylase otherwise called as Ptyalin, which converts starch into dextrin and maltose.  

Though it is an initial process which has begun in the mouth, salivary amylase does not have much impact to the complex starch to get digested fully.


After mastication, carbohydrates along with salivary amylase passes to the stomach through esophagus, where the salivary amylase gets inactivated due to the gastric acid.

In the stomach, the digestion process continues with acidic gastric juice and enzyme. Semi-solid mass of food along with the secretions from pancreas, liver and gall bladder are known as chyme.

Next process happens in the duodenum, the first segment of the small intestine, where pancreatic juices contains pancreatic amylase mixes with the chyme which completes the break down of starch and glycogen into disaccharides called maltose, sucrose and lactose.  

 Brush border enzymes found on the microvilli of the small intestine such as maltase, iso-maltase, sucrase and lactase catalyze the reaction, converts disaccharides into monosaccharides i.e. glucose, fructose and galactose.

          Indigestible fibers and starch e.g. cellulose, hemicellulose, pentosans and galactans etc., which are not digested in the small intestine passes into the large intestine, where the indigestible fibers are digested by colonic bacteria releases short chain fatty acids (SCFA) such as acetate, butyrate and Propionate.

The short chain fatty acids are absorbed across the gut epithelium and pass through the portal vein to the liver for metabolism.


Absorption of Carbohydrates

Absorption of monosaccharides takes place in the small intestine. The cells of the small intestine called enterocytes only absorb the simplest form of sugars.


  Glucose and galactose enter the enterocyte by secondary active transport, here ATP is used and higher concentration of sodium pulls glucose and galactose into the cell.

Fructose enters the cell through passive transport which is does not requires ATP. The concentration should be higher in the intestinal lumen than in the absorptive cells which enquires permeability.

          After glucose, fructose and galactose enters the villi, they are transported by passive diffusion into the blood stream and reaches the liver for metabolism.  

Metabolism of Carbohydrates

The absorbed glucose has the following pathways

  • Oxidation – Glycolysis & Kreb’s Cycle


  • Synthesis of non-essential amino acid
  • Glycogenesis – excess glucose is converted into glycogen and stored in liver and muscles
  • Lipogenesis – excess glucose is converted into lipid and stored in adipose tissue.


Glycogen is the main storage form of carbohydrates. It is stored mainly in liver and muscles.