Digestion in man, Buccal digestion and Gastric digestion (digestion in stomach)

The heterotrophic organism ( consumer ) obtains its food in the form of ready organism materials which are large and complex molecules , These molecules can not diffuse through the cell membrane of living organism , So , the living organism must digest these molecules to become useful materials .


Digestion is the conversion of large food molecules ( polymers ) into smaller ones ( monomers ) by means of hydrolysis , This process is catalyzed by an enzymatic action .

The importance of digestion : The breaking down of large and complex food substances into a simpler and smaller molecules which are easily absorbed by the cells ( either by the diffusion or active transport ) , The cell will then use these simple compounds as a source of energy in the building of new tissues .

Examples :

  • ProteinsAmino acids    ( breaking down )
  • StarchGlucose ( Monosaccharide )
  • Fats Fatty acids + Glycerol


Enzymes is a protein substance which has the properties of catalyst and has the ability to activate a particular chemical reaction .

The enzyme‘s ability to activate a particular chemical reaction depends on the structure of reacting molecules ( reactants ) & the name of enzyme .

When the reaction is completed , the resulting molecules break away from the enzymes , leaving it in the same form as it was before the reaction .

Characteristics of enzymes

  1. Specific , as each enzyme can accelerate only one type of reactions .
  2. Do not affect the products of reaction , as they work as catalyst , increasing the rate of reaction , until it reaches the equilibrium .
  3. Most have a reversible effect , as the same enzyme may catalyze the decomposition of a complex molecule into two simpler ones and may recombine these two small molecules to give rise to the same complex molecule .
  4. Their activity depends on the temperature and pH of the medium .
  5. Some are secreted from the cells in an inactive state , so , they need certain substances to activate them .

Examples : Pepsin enzyme is secreted by the stomach as an inactive pepsinogen which is changed into the active pepsin in the presence of HCl in the stomach .

pepsinogen ( Inactive )pepsin ( Active )     ,   ( HCl acid )

Structure of the digestive system in man 

Digestive ( Alimentary ) canal which consists of : mouth , pharynx , oesophagus , stomach , small intestine , large intestine , rectum and anus opening .

Accessory ( Associated ) glands which are salivary glands , liver and pancreas .



Buccal digestion

Mouth : The digestive canal starts with the mouth which contains teeth , tongue and salivary glands .

Teeth : that are differentiated into :

  • Incisors : in the front of jaw for cutting food .
  • Canines : follow the incisors to tear food .
  • Premolars and molars : at the back for crushing and grinding food .

Tongue : helps to manipulate the food to be chewed by the teeth and it serves as an organ of taste .

Salivary glands : There are three pairs of salivary glands which open into the mouth cavity through ducts , The saliva secreted by the salivary glands contains :

  1. Mucus that softens the food to be easily swallowed .
  2. Amylase enzyme ( Ptyalin enzyme ) which works in a weak alkaline medium of pH = 7.4 .

Function of amylase : it catalyzes the hydrolysis of starch to disaccharide maltose .

Starch + Water Maltose sugar ( disaccharide )     , ( Amylase enzyme & weak alkaline medium  )

Pharynx : It is a cavity at the back of mouth which leads to two tubes :

  1. Oesophagus .
  2. Trachea ( which is a part of the respiratory system ) .

Swallowing process is an organized reflex action which pushes the food from the mouth to the oesophagus and during this , the top of trachea with the larynx is elevated together , causing the epiglottis to close over the glottis ( the entrance to air passage ) .

Oesophagus : It extends from the pharynx downward through the neck and into the chest cavity , It is about 25 cm long , It lies parallel to the vertebral column , It is lined with glands to secrete mucus , Food is carried through the oesophagus to the stomach by a phenomenon known as peristalsis .

Peristalsis : A series of rhythmical muscular contractions and relaxations of the circular muscles of alimentary canal to sweep any food contained within the canal , It is also responsible for churning the food and mixing it with the digestive juices .

Gastric digestion ( digestion in stomach )

Stomach is a dilated muscular sac which lies in the abdominal cavity , It is joined with the oesophagus by a constricted circular muscle which is called the cardiac sphincter , It is connected to the small intestine by a muscular valve of circular smooth muscle which is called the pyloric sphincter .

Stomach secretes a gastric juice ( a colourless acidic liquid ) which consists of 90 % water , HCl acid , Pepsin enzyme which is secreted in an inactive form called pepsinogen .

Function of HCl :

It creates an acidic medium ( 1.5 – 2.5 pH ) which :

  1. Stops the action of ptyalin enzyme .
  2. Kills the harmful bacteria that may enter with the food .
  3. Activates the pepsinogen enzyme into active pepsin .

 Pepsinogen ( Inactive ) Pepsin ( Active ) ,  (  HCl acid )

Protein digestion : Pepsin catalyzes the hydrolysis of protein by breaking a certain peptide linkages in the long chain of protein to yield smaller fragments called polypeptides .

Protein + WaterPolypeptides   ( Pepsin & HCl )

Proteins are the only food substances which are affected by the gastric juice , Although the stomach is made up of protein , the gastric juice does not affect the cells which line it , this due to the presence of mucus secretions which protect the cells against the effect of digestive enzymes , The presence of pepsinogen in an inactive form which is activated only when it is mixed with HCl in the cavity of stomach .

Chemical reactions in organism’s bodies ( Metabolism & Enzymes )

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