Science 8th grade
UNIT 2: Food and digestion 2.3 Digestion and absorption
Science 8th grade
UNIT 2: Food and digestion 2.3 Digestion and absorption
The alimentary canal
Your mouth is the entrance to a long tube called the alimentary canal. The other end of the tube is called the anus.
The diagram summarises what happens to the food that an animal cats, as it travels through this tube.
Absorption and digestion
The food inside the alimentary canal can only reach your body cells if it can get out through the walls of the tube. This process is called absorption.
Protein, starch and fat are important nutrients. Each of these nutrients is made up of large molecules. A molecule is the tiniest particle of a substance that can exist.
Molecules of protein, starch and fat are so big that they cannot get through the walls of the alimentary canal. So, in order to get these nutrients to your cells, the big molecules have to be broken down into much smaller ones. Then the small molecules can be absorbed.
This is what digestion is. Digestion is the breakdown of large molecules into small ones, so that they can be absorbed.
Activity 2.3 (A model of absorption)
SE: Visking tubing is similar to the walls of the alimentary canal. It has tiny holes in it - much too small for you to see - that will let small molecules go through, but not big molecules.
1) Collect a piece of Visking tubing. Moisten it with water. Rub it between your fingers until it opens up into a tube.
2) Tie a knot in one end of the tube.
3) Very carefully, using a dropper pipette, fill your Visking tubing with a 'meal' of starch solution and sugar solution. When it is nearly full, use cotton to tie it very tightly around the top.
4) Rinse your tubing in water, to wash off any starch or sugar that got onto the outside of it.
5) Put your tubing into a beaker. Add enough water to the beaker to cover the tubing Leave it for about 15 or 20 minutes.
6) Now take a sample of water from the beaker and test it for starch. Record your results.
7) Take a second sample of water from the beaker and test it for sugar. Record your results.
Questions
${A_1}$: Explain why it was important to tic both ends of the tubing very tightly.
${A_2}$: Explain why it was important to wash the outside of the tubing.
${A_3}$: Which nutrients - starch or sugar - were able to get out of the tubing?
${A_4}$: Use what you know about starch molecules and sugar molecules to suggest an explanation for your results.
${A_5}$: Imagine you have eaten a meal containing starch and sugar. Do both of these nutrients need to be digested inside your alimentary canal?
Explain your answer.