Digestion (Pt 2)
Adrian Whyte and Joanna Johnson, Contributor

Normal Digestive Tract

IN 'DIGESTION Pt. 1', we spoke about the two stages of digestion and the role each part played in helping to bring about nutrition in humans. Today we are going to the process by which a meal is broken down by digestion to make it accessible for utilisation by the body.

Explain what happens to a cheeseburger meal from the time it is ingested until the nutrients it contains enter the blood stream.

Recall: Nutrition in humans involves the following processes:

Ingestion Digestion Absorption Assimilation Egestion.

In this question you are being asked to explain what happens to the cheese burger meal from ingestion to absorption. We will be using the diagram of the digestive tract to aid in our explanation.

Now the first thing that should be noted is that this meal contains the following nutrients:

Protein
Carbohydrate in the form of starch
Lipid (fat)

Food is ingested into the mouth (buccal cavity) where it is crushed by the teeth and moistened with saliva to form a bolus. Now saliva contains with it the enzyme Amylase which is responsible to break down starch into maltose.

The bolus that is formed is pushed to the back of the mouth towards the oesophagus1 (swallowing). The bolus is then pushed towards the stomach by a series of contractions in the walls of the oesophagus (peristalsis).

Upon reaching the stomach, the bolus is now mixed with secretions from the stomach known as gastric juice, which:

1. Makes the content of the stomach acidic due to the hydrochloric acid it contains. This helps kill bacteria in the food and stops the action of the enzyme amylase.

2. Contains the enzyme pepsin which breaks down proteins to polypeptides.

The food spends a few hours in the stomach mixing and churning the bolus of food with the gastric juice to form chyme.

When the chyme leaves the stomach, it enters the first part of the small intestine (duodenum). When the food particles, now as a mixture of chyme, enter the duodenum, there is a secretion of pancreatic juice into the small intestine to:

1. Neutralise the acidic content of the chyme to provide an optimum pH of 7-8 for the pancreatic enzymes.

2. Contains

a. bile salts - emulsify2 fat

b. amylase - to continue to break down the remaining starch trypsin ­ to continue the breakdown of proteins

c. lipase to break down fats into fatty acids and glycerol.

As the food travels further along the small intestine (jejunum and ileum) enzymes continue the breakdown of the various food types into their simplest components:

starch maltose glucose

protein polypeptides amino acids

lipids fatty acids and glycerol

The absorption of food nutrients occurs in the ileum which is adapted for this process due to the presence of villi3 on the walls of the small intestine.

1. esophagus or oesophagus ­ the spelling varies depending on whether the book was published in the United States or English system.

2. emulsify - breakdown of large pieces of fat into smaller pieces with larger surface area for chemical digestion.

3. villi - tiny finger-like projections that increase the surface area of the ileum.

* Adrian Whyte and Joanna Johnson teach Biology at Ardenne High School
masterbio@gmail.com.