The Digestive system.

WEEK : 1


TOPIC:  The Digestive system.

CONTENT:   (i) Meaning of Digestion

(ii) Definition of simple metabolic terms e.g. Enzymes absorption.

(iii) Digestion and absorption of nutrients (Carbohydrates, proteins etc.) in the mouth and esophagus



Most of the food taken into the mouth requires some alteration before they can be absorbed into the blood stream. Digestion is the process by whichfood molecules are broken down into various small substances that can be absorbed into the bloodstream and assimilated into the cells for utilization.Digestion is mainly a chemical process brought about by the action of numerous enzymes. The whole process of digestion takes place in the alimentary canal.



Digestion and Absorption of Nutrients in the Mouth and Esophagus


The process of digestion begins in the mouth. The teeth are used to crush large pieces of food. This process is called mastication; mastication exposes the food to action of enzymes. The tongue mixed the food with the saliva whichcontains ptyalin, an enzyme that breakdown the starch.Saliva performs the following functions 1. It containsmucin which lubricates the food and makes swallow easier. It provide an alkaline medium necessary for the action of ptyalin,the enzymes that breakdown carbohydrate.  Digestion of starch starts from the mouth by the action of the saliva amylase otherwise known as ptyalin.The saliva provides an alkali medium that facilitates the action of ptyalin. This breakdown the complex carbohydrates into simple carbohydrates called maltose. No chemicalreaction happens to protein and fat in the mouth.

Sub topic 2: Digestion ofProtein nutrients in the stomach

Food passes into the stomach through the esophagus after mixing in the mouth. In the stomach, gastric glands secrete gastric juice, a mixture of water, enzymes, and hydrochloric acid that acts primarily on protein foods. It is as a result of present of hydrochloric acid that gastric juice is acidic. The acidic medium in the stomach (i) Stop the action ofptyalin (ii) Stop the growth and destroys the bacteria that comes along with the food from the mouth. Mucus is secreted in the stomach to prevent the stomach wall from the action the gastric juice and hydrochloric acid (HCL).


The digestion of carbohydrates stops in the stomach because salivary enzymes cannot work in acidic medium in the stomach. The chime is push into the small intestine through peristaltic movement. The gastric juice contains two enzymes   they are    rennin and pepsin, they are the enzymes that act on protein digestion. Rennin converts the soluble protein in milk, caseinogens into an insoluble form called casein –by a process called curdling of milk. Pepsin converts the other foods protein to smaller peptides and peptones. Thus in the stomach protein is the only nutrient acted upon. Digestion of fats does not take place in the stomach. The muscular contractions of the stomach assist in mixing the food with gastric secretions until a liquefied (chime) is formed.


  1. Explain the meaning of digestion
  2. Describe the digestive process of carbohydrate in the mouth or esophagus.

Objective Questions

  1. The milk protein which broken down by rennin in the stomach is ……………
    • Trypsin
    • Dipeptide
    • Casein
    • Peptide
  2. Pepsin is an important enzyme in the digestion of :
    • Protein
    • Carbohydrate
    • Starch
    • Fat
  3. Hydrochloric acid which essential for digestion is found is found in the …………
    • Stomach
    • Mouth
    • Pancreas
    • Large intestine

4. Digestion of food does not take in ……….

      • Mouth
      • Large intestine
      • Stomach
      • Duodenum

5. Lactases act on lactose and convert them to a unit of ………. and…………..

      • Glucose and a unit of lactose
      • Glucose and a unit of fructose
      • Lactose and a unit of glucose
      • Fructose and a unit of galactose



Food and Nutrition for Senior Secondary Schools Book 1 by F. A. Bakare, N. M. Eze, H. B. Bala

Exam focus, food and nutrition for Senior Secondary Schools, WASSCE and SSCE