Resources: The handout, geranium or basil plant, boiling tube, ethanol, 250ml beaker, bunsen/stove & ancillaries, stirring rod, white tile, iodine and a pipette.
Handout: Testing a leaf for starch.
Revise the content from the lesson ‘Explain the role of aerobic respiration’ in the completed ‘Cells & Organisation’ unit.
Remind the students that glucose is a sugar and ask them to list what people use sugar for. Ask them to work with their partners to come up with a list of uses.
Use the presentation slides to firstly explain the production and use of glucose in a plant. Cover the processes of photosynthesis and respiration and explain the role of glucose in each. Then, explore the relationship between glucose and starch, as well as glucose and fats. Prompt the students to connect the slides to the learning completed within the ‘Nutrition’ unit. Answer questions and take part in activities during the presentation, then carry out the test for starch in leaves.
Career Film: Jonathan Newell works as the Hilltop Live Public Engagement Officer with the RHS.
Expert Film: This is Jonathan Newell, who works as the Hilltop Live Public Engagement Officer at the RHS. Jonathan talks about how plants use glucose.
The students will test a leaf kept in the dark and one kept in the light for starch. They should follow this specific method as they complete the experiment.
Firstly, take a leaf from a plant that has been allowed to grow in direct sunlight.
*The leaf will be delicate at this stage and damage easily so handle with care.
Students should then complete the questions on the handout.
Support: Provide a table for the students to complete:
| Leaf | Condition | Photosyntheis? | Glucose + starch | Colour with iodine |
| Green | sunlight | |||
| Green | dark | |||
| White | sunlight |
Challenge: Provide a range of pictures of different leaves, including variegated leaves. The students must predict the results of the iodine test when leaves have been kept in different conditions.
E.g.:
As a class, review the students’ ideas from the challenge task. What were their predictions? They can also review the keywords and add them to their glossary using the unit keywords sheet provided.
The process of photosynthesis in plants is useful in making organic compounds out of carbon dioxide. This is doable through carbon-fixation reactions. The process produces both oxygen and organic compounds out of water, carbon dioxide and energy.
Carbon dioxide is the source of carbon. It enters the photosynthetic process via the carbon-fixation reactions. These reactions take place in the dark. Photosynthesis produces organic products. Organic products must contain carbon.
The main organic compounds that come from plant photosynthesis are carbohydrates. The simplest carbohydrate is glucose. Glucose is made from carbon dioxide, water and light energy in the process of photosynthesis. Glucose can be linked with other molecules to form more complex carbohydrates. It is the starting material for cellular respiration and the main product of photosynthesis. Glucose is the food source and can be used in different ways.
Glucose is stored as starch and fats and oils. It is used for respiration to release energy too. It is used to produce cellulose which helps to strengthen the plant cell wall and to produce proteins like chlorophyll for light absorption and enzymes for chemical reactions.