The digestive system consists of the alimentary canal, which is a long tube from the mouth, attached to various organs and ends at the anus. The alimentary canal is a long tube through which food substances pass, become digested (broken down), partly absorbed and the rest passed out. The alimentary canal has five main parts: (i) The mouth (ii) The gullet (or Oesophagus)  (iii) The stomach  (iv) The small intestine  (v) The large intestine.


It comprises three prominent structures for digestion namely the teeth, salivary glands and the tongue.THE MOUTH

TEETH: Structures for food mastication.

SALIVARY GLANDS: Secrete SALIVA (A digestive juice) 

TONGUE: Organ of taste. 


It is also known as the Oesophagus. It is lined with smooth involuntary muscles by which contraction of food from the mouth is regulated into the stomach by PERISTALSIS. Peristalsis is the motion by which food is pushed along the gullet and the rest of the digestive tract. 


This is a muscular organ, which serves in the digestion of food and also acts as temporary food store. A digestive juice called the GASTRIC JUICE is secreted into the stomach by both the PEPTIC and OXYNTIC CELLS. 


The longest portion of the digestive tract (about 9m in length if fully stretched out) which begins with the C-shaped DUODENUM on which the LIVER and PANCREAS are attached. Both discharge their secretions – THE BILE and the PANCREATIC JUICE respectively to the duodenum.


Also known as COLON. It processes undigested food portion as FAECES for export outside the body through the ANUS. No digestion occurs in it. Excess water is also removed through the feaces by large intestine.


A balanced diet consists of carbohydrates, fats & oils, proteins, mineral salts, vitamins and water. The carbohydrates, proteins and fats are large complex molecules. They must be broken down into small simpler molecules before the body can use them.

The conversion of large complex molecules to small simpler molecules is the process of digestion.

Digestion is defined as the breakdown of large complex molecules in foods into small simpler absorbable molecules by the enzymes. Digestion takes place in the mouth, stomach and the small intestine. There are four digestive juices along the digestive tract. These are:

(i)        SALIVA

(ii)       GASTRIC JUICE



Each of them contains one or more digestive enzymes, which converts specific food complex to simpler forms.

The following are the components of each digestive juice.

Parts of Alimentary Canal Digestive Juice Components
Mouth Saliva Ptyalin + Water
Stomach Gastric juice Pepsin + Rennin
S.I (Duodenum) Pancreatic Juice Amylase + Lactase + Sucrase + Trypsin
S.I (Ileum) Intestinal Juice Maltase + Lactase + Sucrase + Erepsin + Lipase



Credit: Britannica



Ptyalin – converts cooked starch to maltose.

Pepsin – converts protein to peptones and polypeptides.

Rennin- coagulates liquid proteins (milk) into thick digestive form.

Trypsin – breaks down protein to peptones and polypeptides.

Amylase – breaks down starch into maltose.

Lipase – breaks down fats into fatty acid and glycerol.

Maltase – breaks maltose to glucose.

Lactase – breaks lactose to glucose & fructose.

Sucrase – breaks sucrose to glucose & galactose.

Erepsin – breaks polypeptides into amino acid.

Mouth: Digestion of starch begins in the mouth. Ptyalin converts starch to maltose sugar in the mouth.

Stomach: Only proteins are digested in the stomach. Pepsin converts solid proteins into peptones and polypeptides while Rennin curdles milk proteins. 

Small Intestine (Duodenum): The three major classes of foods are all digested here.

Trypsin – converts protein to peptones and polypeptides.

Amylopsin – converts starch to maltose.

Lipase – converts fats& oil to fatty acid & glycerol.

Small Intestine (Ileum) – Digestion is completed here as all partially digested food substance are completely digested.

Maltase – breaks down maltose to glucose.

Erepsin – breaks down polypeptides to amino acid.

Lipase – breaks down fats & oil to fatty acid and glycerol.

These final products of digestion in the Ileum are the small simple molecules which can be absorbed into the body. 



When food has been finally digested to simple absorbable forms, they are simply absorbed in the small intestine into the blood stream by diffusion through several fingers like projections on the internal surface area of the ileum.

These projections are known as VILLI. Each villus comprises a network of capillaries and the lateal vessels. Glucose and Amino acid diffuse into the network of capillaries of villi into the blood stream while the lateal vessel takes up fatty acid and glycerol.

Indigestible portion of the food is regulated into the larger intestine where it is dehydrated and processed into semi solid feaces for egestion.



  1. Test for starch

Starch + Iodine solutions

Result: Blue – black colour confirms the presence of starch

  1. Test for reducing sugars e.g. glucose

Glucose + Fehling solutions

Boil the mixture

Result: Brick – red precipitate confirms the presence of reducing sugar

  1. Test for Non-reducing sugars eg Sucrose

Sucrose + Dill Hcl acid

Boil for about 5 minutes

Cool the mixtures

Add some NaHCO3 until there is fitting

Now add Fehling’s solution

Warm the tubes

Result: Brick – red precipitate obtained

N.B: If Sucrose is not first heated as described, it would give no colour with Fehling solution for it is a non-reducing sugar. The HCl breaks down the sucrose into simpler sugar, which reacts with the Fehling solution.

  1. Test for protein

2cm3 of protein (milk) + 2-3 drops of Million’s reagent

Boil the mixture

Result: Orange colour or Brick red precipitate confirms presence of protein.`

  1. Test for lipids

Oil + Sudan III solution

Result: Red stains confirm presence of oil or fat

OR: oil + paper

Result: Paper turns translucent to show that lipid is present

  1. Test for water

Water + white anhydrous CuSO4

Result: Blue colour confirms presence of water

OR: Water Blue cobalt II chloride paper

Result: Pink colour confirm the presence of water


See also

Water and Chemical Pollution

Air and Noise Pollution

Environmental Health

Classes of food


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