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Follow this up with a discussion of how only small molecules can get through the wall of the alimentary canal and enter the body tissues. This can lead in to the ideas of mechanical and chemical digestion. Figure 7.10 looks very daunting, but you may like to work through this slowly with students, showing them how first mechanical digestion and then chemical digestion produce small molecules of nutrients that can get through the wall of the intestine. ◆ Teeth make a much easier topic for most students to deal with, and indeed many may already have covered this material in earlier years. Students usually enjoy looking at their own teeth, so you could begin the lesson with Worksheet 7.2 Checking your teeth. However, this involves eating, so it may be better not to do this in a laboratory. ◆ You may have some human teeth that students can look at, alongside Figures 7.12 and 7.13. It is worth collecting some milk teeth and, if possible, sawing some of them in half so that their internal structure can be seen. Dental X-rays are another potential source of information, if you can obtain any. ◆ If available, use a three-dimensional model of the human body to show students the different regions of the alimentary canal and their positions in the body. ◆ You may like to dissect a small mammal to show the alimentary canal, liver and pancreas. ◆ Use Worksheet 7.3 The structure of the digestive system to help students to learn the positions and names of the organs in the digestive system. ◆ Use a piece of rubber tubing and a marble to illustrate how muscles squeezing inwards cause a bolus of food to move through the alimentary canal. ◆ A piece of velvet or pile carpet can give students a good picture of how the villi in the small intestine increase surface area. ◆ Activity 7.3 A model of absorption, could be carried out. Common misunderstandings and misconceptions ◆
It is very common for students to have difficulty with the concepts of digestion and absorption. They fail to understand that molecules need to move from inside the alimentary canal, through its walls, and into the blood (or lymph) before they can considered to be truly inside the body. ◆ They may confuse ingestion with indigestion. ◆ It is very common for students to fail to understand that the alimentary canal is a closed tube which runs, uninterrupted, from mouth to anus. They may think that food goes through the pancreas and liver, or even directly to the kidneys. This misunderstanding can lead to students making statements about the kidneys ‘excreting liquids from the foods that we eat’. ◆ Some old textbooks describe enzymes being present in ‘intestinal juice’ in the small intestine, but this is not correct. The enzymes in the small intestine come either from pancreatic juice or from the cells covering the villi. ◆ Many textbooks imply that water is absorbed only in the colon. Most absorption of water takes place in the small intestine, with only a relatively small quantity being absorbed in the colon. Homework ideas ◆ ◆ ◆ ◆
Coursebook questions 7.7 to 7.23 End-of-chapter questions 1, 3, 4, 6 Workbook exercise 7.2 Functions of the digestive system Workbook exercise 7.3 Tooth decay data analysis Original material © Cambridge University Press 2014
© Cambridge University Press 2014
IGCSE Biology
Chapter 7: Teaching ideas
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