NUTRITION
STUDENTS' ACTIVITIES
Carbohydrates are the products of photosynthesis. Simple sugars are the first to be formed but are immediately converted into starch, the first visible product of photosynthesis. It is therefore possible to find out if a plant has carried out photosynthesis by testing for starch. Presence of starch implies that photosynthesis has occurred.Several practical activities should be carried out in order to learn more about the nature and factors necessary for photosynthesis. These activities include the following:
EXPERIMENT 1: TESTING FOR STARCH
Aim: - To test a leaf for starch.
Apparatus and Materials
Beaker, boiling tube, heat source, test tube holder
Iodine solution, ethanol, water and a leaf that has been exposed to light.
Procedure
Half fill a beaker with water and boil the water. Place the leaf in boiling water for one or two minutes. This denatures the enzymes and stops chemical reactions. Put the leaf into a boiling tube and pour enough ethanol to cover the leaf. Place the boiling tube into the beaker of boiling water. A water bath is used to prevent accidents that may arise from alcohol being inflammable. Remove the flame when the ethanol boils.
Re-introduce the flame when the ethanol stops boiling. Repeat until the leaf is decolourised.
Diagram to illustrate boiling a leaf in alcohol using a water bath.
(Testing a leaf for starch)
- Dip the leaf in water to soften it and make it permeable.
- Using a pipette, add dilute iodine solution to the leaf surface.
Student’s discussion:
1. When boiling the leaf in ethanol, what happened to the colour of ethanol? Explain your observation.
2.What colour did the leaf turn when you added iodine?
3. What do these results show?
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