Digestion: where does it occur and why does it happen?
Joanna George-Johnson, Contributor

This Waterford High School student keeps cool by drinking a bag juice during a recent Manning Cup match at her school. - Photo by Anthony Minott

Digestion is a very important process that occurs in the human body. This process takes place in a tube that extends from the mouth to the anus. It is called the alimentary canal.

For today's lesson, we will be looking at what happens to a meal of fried chicken and fries from the time it is ingested, until the nutrients it contains enter the blood stream. (For our purpose, we will assume that the chicken and fries meal contains the following nutrients: lipids, proteins and starch.) Bear in mind that these are the only nutrients that can be digested. Vitamins and minerals are small enough and do not need to be digested. They can pass through the walls of the alimentary canal as is.

Recall: Nutrition in humans involves the following processes:

Ingestion Digestion Absorption Assimilation Egestion

1. 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 in it the enzyme Amylase, which is responsible for breaking down starch into maltose.

2. 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).

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

a. makes the contents 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.

b. 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.

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

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

b. Pancreatic juice contains:

i. bile salts - emulsify2 fat
ii. amylase - to continue the breakdown of remaining starch
iii. trypsin - to continue the breakdown of proteins
iv. 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. (These enzymes are found on the walls of the small intestines.)

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.

The part of the meal that is indigestible will continue into the colon then to the rectum for storage until it is passed out as faeces through the anus.

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. This ensures maximum absorption.

Questions:

1. For each of the nutrients identified above, state which part of the meal is the source.

2. What type of digestion is emulsification?

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