Types of Digestion

What is digestion?

Digestion is the process of breakdown of large food molecules into small water-soluble food molecules so that they can be absorbed into the watery of blood plasma. In special organisms, these smaller substances are absorbed with the help of the small intestine into the blood stream.


Mechanical digestion of mouth

The process of digestion begins in the mouth. Teeth in mouth grind the food. This makes the food ready for the action of enzymes.

Chemical digestion in mouth

There are three pairs of salivary glands in the mouth. These secrets saliva. Saliva contains an enzyme called “ptyalin”

Action of ptyalin

Ptyalin enzyme acts upon starch of the food, and converts it partly into maltose.

Conversion food into a bolus

This partially digested and masticated food then takes the form of a ball known as “bolus”, which is easily pushed the esophagus


The stomach is a sac-structure its wall is made up of a thick membrane. Food from the esophagus enters the stomach and stored in it.

Mechanism of digestion in stomach

In stomach food is digested mechanically. The walls of stomach contain “gastric glands” which secrets gastric juice. This gastric juice contains HCL and the enzymes named as “renin” and “pepsin”

Action of HCL

  1. HCL kills the germ and bacteria present in food.
  2. It softens the food.

Action of pepsin

  1. It acts proteins and convert them into peptones.

Action of renin

  1. Resin acts on milk to convert it into curdle in infants.
  2. It converts soluble milk proteins into insoluble proteins.

Conversion of food into chyme

The thick walls of stomach churn up the food and by the action of HCL and other enzyme. Food is converted into a thick fluid like substance called “chyme”. This chime gradually releases into the small intestine.


Food is moved from the esophagus onward up to the anus by the help of an automatic movement, called peristalsis.


Next to the stomach is small intestine. This is a long narrow tube in the form of coiled loops. First part of small intestine is duodenum.

  1. Absorption of food in small intestine

The next part of small intestine after duodenum is “ileum”. The rest of the digestion is completed here. The walls of small intestine have glands which produce intestinal juices; this juice has enzymes aminopeptidase and disaccharides.


It converts the peptides into amino acids.


Maltose, lactose or sucrose is converted into glucose by the action of disaccharides.

  1. Absorption of digested food into the body through villi

On the internal walls of ileum there are very fine finder like projection called villi. Inside each villus there is a rich supply of blood capillaries forming a network. There is also a single lymph vessel called “lacteal”. Digested food is absorbed in it. Besides fatty acids all the nutrients diffuse through. The surface cells of villi and are taken into the blood flowing in blood capillaries. These capillaries join together to from a large blood vessel called “hepatic portal vein”, which carries the absorbed   food to the liver. While fatty which are passed into lacteal rather than blood capillaries. lacteals of villi join together to form lymph vessels, which form a lymphatic system to deliver these fats into blood stream.


Enzymes are the chemical compound that work as a catalyst in chemical reactions of a cell. They are usually globule protein molecules. They are helpful in speeding up the chemical reactions without being involved by themselves.

Hydrolytic enzymes

Enzymes which are involved in the process of digestion are called hydrolytic enzymes. They are secreted by digestive glands. These digestive glands are

  1. liver   b) pancreas.
  2. LIVER


Liver is the largest gland in the body. It is a reddish brown gland that is located in the abdomen behind the diaphrphragm more towards the right side.


  1. It secretes an alkaline greenish yellow juice called bile. Bile is stored in a bag-like organ gall-bladder which lies under the surface of liver.
  2. Bile has no enzymes in it but contains some salts.
  3. Bile helps in breaking down the larger molecules of fast into smaller particles.
  4. These smaller fat particles are easy to digest in small intestine.
  5. b) Pancreas


It is a long leaf-like organ lies behind the stomach horizontally within the curve of duodenum.


  1. The secretion of pancreas is known as pancreatic juice, which is poured through a duct called pancreatic duct.
  2. Pancreatic juice contains sodium bicarbonate and many enzymes.

Following are the three important enzymes.


It acts on stomach to break it down into maltose.


  • It converts the proteins into smaller peptides.


  • It breaks fat droplets into fatly acids and glycerol.

Carbon dioxide is necessary for photosynthesis


  1. Two ported plant are destarched by placing them in a dark room. They are watered properly during this period.
  2. Each pot is enclosed in a transparent polythene bag as shown in figure.
  3. A petri dish containing soda lime (potassium hydroxide) is placed in the other pot. Absorb any carbon dioxide present in the polythene bag.
  4. A petri dish containing sodium bicarbonate solution is placed in the other pot.
  5. Plants are then left in light for several hours.
  6. A leaf from each pot is detached and tested for starch with iodine solution.


  1. The leaf from the pot containing soda lime does not turn blue, because soda absorbed carbon dioxide present in the polythene bag.
  2. The leaf from the pot containing sodium bicarbonate turns blue indicating the presence of starch.


It is proved that carbon dioxide is essential for photosynthesis.


 The individual sub-units of large organic molecules are often called monomers. It’s a Greek word which means one part.

Example: monosaccharides like glucose, amino acids, fatty acid and glycerol.


Monomers combine together to form long chains, which are known as polymers.

Example:  polysaccharides like starch, glycogen,, proteins, fats and oils.

Dehydration synthesis

A process in which two monomers are joined together by the removal of a water molecule and, as a result a bond is formed between two sub-units is known as dehydration synthesis.


The process used for the synthesis of polymers is called condemnation.

Oxygen is produced during photosynthesis


  1. Take a shoot of an aquatic plant (hydrilla) in a beaker of water.
  2. A short stemmed funnel is placed over the shoot in the take beaker over supports.
  3. A test tube filled with water is inverted over the stem of the funnel. The apparatus is kept in light for several hours.
  4. When sufficient gas is produced the test tube is removed.
  5. A glowing match stick is inserted in the tube containing oxygen gas.


  • The match stick burns with a flame showing that the gas is rich in oxygen.


  • It is proved that oxygen is evolved during photosynthesis.

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