Science 8th grade
UNIT 2: Food and digestion 2.6 Enzymes
Science 8th grade
UNIT 2: Food and digestion 2.6 Enzymes
We have seen that large molecules of nutrients must be broken down into small molecules, so that they can be absorbed.
Teeth break down big pieces of food into small pieces. But they don't have any effect on the molecules of the different nutrients that make up the food.
The large nutrient molecules are broken down into small molecules by chemicals called enzymes.
Digestion as a chemical reaction
Digestion changes a substance made up of large molecules into a new substance made up of small molecules.
When one substance is changed into a different substance, we say that a chemical reaction has taken place. The diagram shows a chemical reaction that happens during digestion.
Questions
1) Explain why digestion by enzymes is a chemical reaction.
$A + I$: 2) Do teeth cause a chemical reaction to take place? Explain your answer.
Catalysts
The enzymes inside the alimentary canal make these chemical reactions happen.
The enzymes are not changed themselves. They just help to speed up the reactions.
A substance that speeds up a chemical reaction, but is not changed itself, is called a catalyst. So enzymes are biological catalysts.
Questions
3) Teeth help to break down large pieces of food into lots of smaller pieces.
Suggest how this can help enzymes to digest the food faster.
4) A protein molecule is made up of a long chain of smaller molecules called amino acids. Draw a series of diagrams showing how an enzyme digests protein molecules to amino acid molecules. Use the diagram at the bottom of the previous page as a starting point.
Activity 2.6 (Using an enzyme to digest starch)
SE: The enzyme that digests starch is called amylase.
1) Label two test tubes A and B.
2) Put $5cm$ of starch solution into each of tubes A and B.
3) Add $5cm$' of amylase solution to tube A. Add 5cm of water to tube B. Leave both tubes in a warm place for about 20 minutes.
4) Using a clean dropper pipette, take a small sample of liquid from tube A. Put it onto a white tile, and add some iodine solution. Write down the result.
5) Repeat step 4 with a sample from tube B.
6) Using a clean dropper pipette, take another sample from tube A. Put it into a boiling tube, and add some Benedict's solution. Place the boiling tube into a water bath at about ${80^ \circ }C$. Leave it for two or three minutes, then write down the result.
7) Repeat step 6 with a sample from tube B.
8) Record all of your results clearly, and try to explain them.
