Topic: How is the food digested?

Author: Elżbieta Szedzianis

Target group

7th grade students of an eight‑year elementary school.

Core curriculum

4. Digestive system and nutrition. Student:

4) explains where proteins, fats and sugars are digested; defines the products of these processes and points to their place of absorption; plans and conducts experiments investigating the influence of substances contained in saliva on digestion of starch.

Lesson aim

The students describe digestion that takes place in the digestive system.

Key Success Criteria

• you will explain what the enzymes are;

• you will explain three examples of enzymes;

• you will describe how the small intestine is adapted to absorb the digested food;

• you will explain, what happens to the undigested rests.

Key competences

• communicating in the mother tongue;

• communicating in a foreign language;

• Mathematical competence and basic competences in science and technology; 

• digital competence;

• learning to learn;

• Social and civic competences.

Methods/forms of work:

Work with film, experiment, a talk.

Individual and group work.

Teaching measures:

• abstract;

• interactive whiteboard or traditional blackboard;

• tablets/computers;

• test‑tubes;

• beakers;

• potato starch;

• ice;

• water at the temperature of 37°C and 50°C;

• Lugol's Iodine;

• saliva.

Lesson plan overview (Process)

Introduction

The teacher informs students that during today’s class, they will learn about the history of a wholemeal bread roll with butter and cheese in the digestive system. The teacher gives the topic of the lesson, then, using accessible language, explains to the students the aim of the lesson, and shows them the success criteria.

Realization

1. The teacher explains what enzymes are and analyses the illustration “The working of enzymes”. He talks about the digestive process in specific parts of the digestive system.

2. The teacher divides students into groups. The students plan an experiment and check the working of amylase on starch in 0°C, 37°C and 50°C. They formulate hypotheses and define the method of checking them, then they carry out an experiment.

3. The teacher asks the students to show the results of the experiment and draw conclusions.

4. The teacher discusses the absorption of nutrients.

5. The teacher again shows them the illustration “Digestive system”. The students explain what is going to happen to a wholemeal bread roll with butter and cheese in the digestive system. The teacher directs their attention to the digestion of starch, protein and fat. He reminds about undigested food,
   from which faeces are formed.

Summary

7. The students prepare a table that describes digestive enzymes and their functions.

8. The students answer the question: “Can you explain how starch, protein and fat are digested?”.

Homework

Students complete interactive exercises.

The following terms and recordings will be used during this lesson

Terms

Texts and recordings

How is the food digested?

In the digestive tract the food is digested to the form that can easily be absorbed by cells, and then the food is absorbed. Mechanical processing of food begins in the oral cavity and is based on processing the portions of food. Thanks to this, during the next stage the enzymes that participate in the chemical digestion have easier access to them. Chemical digestion is an enzymatic (chemical) process of breaking down the ingredients of food, during which complex organic compounds are broken down into simple compounds that can be absorbed by the body. Digestive enzymes are secreted by secreting cells:

• salivary glands,

• wall of the stomach,

• pancreas,

• small intestine walls.

Digested food in form of chemical compounds that can be used by the cells (e.g. amino acids, monosaccharides, glycerol and fatty acids) is absorbed by the intestinal cells into blood vessels and lymph vessels. Through them, food reaches the liver, and then all the tissues in the body. Undigested food is expelled out of the organism in the form of faeces. Working, the digestive system cooperates with other systems of our body.

Chemical digestion takes place because of digestive enzymes. These are the substances that work inside the digestive system, and which, with the help of water, allow to break down complex chemical compounds into simpler elements.

In the oral cavity the process of breaking down polysaccharides (starch, glycogen) into monosaccharides is initiated. This is possible thanks to the fact that saliva includes an enzyme called salivary amylase. It works on food while it passes through the oesophagus. It works only for a short time, so the saccharides are not completely broken down.

Stomach wall cells secrete gastric juice, which, besides hydrochloric acid includes also an enzyme that breaks down proteins - pepsin. Pepsin cuts long strings of proteins into smaller fragments. Both hydrochloric acid and pepsin can be dangerous for the stomach, whose tissues are composed of proteins. Because of this, the hydrochloric acid is secreted only when the stomach receives food that irritates the stomach walls, whereas digestive enzymes work only in the presence of the hydrochloric acid.

Proteins, saccharides and fats are digested in the duodenum. Liver secretes bile which breaks down fats into small drops that are easily accessible for the enzymes that digest fats.

Pancreas secretes a number of enzymes, such as pancreatic amylase (that digests saccharides), lipase (that digests fats), nuclease (that digest nucleic acids), trypsin (that digests proteins). Trypsin cuts long strings of proteins into shorter ones, peptides, and then other digestive enzymes break them down into even smaller particles. The final product of the digestion of proteins are amino acids.

Food remains in the duodenum long enough so that most of the compounds with large particles are digested to the point when they become monosaccharides, amino acids, glycerol and fatty acids.

Liquid contents of the digestive system are transported to the next of the small intestine which is surrounded by a very dense net of blood vessels and lymph vessels. That is where the products of digestion are absorbed through the walls of the small intestine into the blood and the lymph.

Blood vessels of the intestinal villi absorb glucose and amino acids, which then are transported to liver and then to body cells as energy source. Excess glucose is converted by the liver and the muscles into glycogen, spare sugar, and stored there. Mineral salts and vitamins are dissolved in water and transported into blood vessels. The result of the digestion of fats, fatty acids and glycerol, pass from the intestine into lymph vessels and then into blood, with which they flow to all the cells of the body. There, they will be used to build the cell membrane and as energy. Excess of fat is stored in adipocytes. Together with fatty acids, lymph received vitamins that are dissolved in fats.

Digestive enzymes cannot digest certain ingredients of the food. One of such ingredients is dietary fiber, an ingredient of vegetables. It is moved to the large intestine. The large intestine is inhabited by many types of bacteria, which feed of undigested food and break them down in fermentation processes. Bacteria are about 80% of the mass of the faeces that is expelled from the body through rectum with a constrictor. Water, mineral salts and vitamins are absorbed from the faecal mass in the large intestine.

• The digestive tract is where the food is processed and where chemical digestion and absorption take place.

• Preliminary digestion of polysaccharides take place in the oral cavity.

• The digestion of proteins is initiated in the acidic environment of the stomach.

• Intense digestion of sugars, proteins and fats takes place in the small intestine.

• Liver secretes bile which destroys large particles of fat into small drops.

• The products of digestion of sugars, proteins and fats are absorbed in the small intestine.

• The large intestine is where the water is absorbed and where faecal mass is thickened.

• Undigested food is expelled from the body in form of faeces.