All about digestion
Joanna George-Johnson, Contributor

Students at the Waterford High School in Portmore get into the action during one of the No Violence tours. - Ricardo Makyn/Staff Photographer

In a previous lesson we did a detailed study of the food nutrients. Today we begin to take a look at how the human body obtains these nutrients from the food we eat.

The nutrition of man can be described as heterotrophic and holozoic. It is heterotrophic, as man doesn't produce its own food by feeding on organic substances produced by other organisms. It is holozoic, as man takes in undigested food, breaks it down into usable units, then assimilates these units (nutrients). Before we go any further, let us outline the steps involved in holozoic nutrition.

• Ingestion: Simply the taking in of food into the body.
  
  
• Digestion: The process of breaking down the large and sometimes insoluble substances or food eaten into simpler, more soluble ones.
  
  
• Assimilation (absorption): Movement of soluble particles from the alimentary canal into the bloodstream.
  
  
• Egestion: Getting rid of undigested food.

Types of Digestion

Two main types of digestion exist: mechanical and chemical.

• Mechanical digestion is simply breaking down food into smaller pieces. The food remains chemically unchanged. In man, mastication (chewing) is the way by which this is done.
  
  
• Chemical digestion involves breaking complex molecules into simpler ones. In biological systems this process is catalyzed by enzymes.

The teeth and mechanical digestion

The structure of the human tooth is shown in the diagram below.

The tooth has two main parts: the part embedded in the gum is called the root, and the part above is called the crown.

• Enamel covers the crown of the tooth. It is described as the hardest substance produced by biological systems. It is easy to see why this is necessary, as the teeth are used to chop, grind and crush food, thus are susceptible to a great deal of wearing.
  
  
• Beneath the enamel is the dentine. It isn't as hard as the enamel, and contains channels filled with living protoplasm.
  
  
• The pulp cavity is in the middle of the tooth, and contains nerves and blood vessels to nourish the protoplasm embedded in the dentine.

Of course there are different types of teeth, each with its function. Briefly:

• Incisors, which are sharp, chisel-like teeth at the front of the mouth. They are used to bite off pieces of food.
  
  
• Canines, which are pointed teeth between the incisors and premolars, used to shred food.
  
  
• Premolars and molars are the backmost teeth, used to crush/grind food.

Questions

1. Label the following diagram of the tooth.

2. Explain the difference between egestion and digestion.

3. Both chemical and mechanical digestion occur in the mouth. Provide a balanced argument of three points explaining why this is so.

4. Tooth enamel is the hardest substance produced by biological systems. Teeth are, however, susceptible to corrosion, that is, decay. Fully explain the process of tooth decay.

Joanna George-Johnson teaches at Ardenne High School.
Email: Masterbio@gmail.com.